Highlights


Modeling non-uniform Li transport in solid-state 3D Li-ion batteries

Modeling non-uniform Li transport in solid-state 3D Li-ion batteries

Scientific Achievement

Finite-element multiphysics simulations guided by experiments reveal both spatial & temporal non-uniform cathode utilization due to 3D nano-geometry and low LiPON conductivity, leading to capacity loss at higher power & local degradation sites.

Significance and Impact

Combining simulations & experiments provides guidance for nanostructure design principles, emphasizing the role of 3D geometry in determining power performance and geometry-dependent degradation hotspots.

Research Details

  • Small structural and compositional inhomogeneities lead to large spatial inhomogeneities in capacity utilization and degradation.
  • This 3D finite element model includes troictropic and potential dependent diffusivity in the LiCoO2 cathode, capacity-dependent potential of the Si anode, in addition to the Butler-Volmer kinetics and diffusional/migrational mass transport.

References

A.A. Talin, D. Ruzmetov, A. Kolmakov, K. McKelvey, N. Ware, F. El Gabaly, B. Dunn, H.S. White, “Fabrication, Testing & Simulation of All Solid State 3D Li-ion Batteries”, submitted, 2016.

Summary

NEES scientists demonstrated fully-functional 3D Solid State Lithium Ion Batteries (SSLIB) with microscale internal dimensions fabricated using materials and thin film deposition methods. Analysis of the experimental results using finite element modeling indicates that the origin of the observed poor power performance is the structural inhomogeneity of the 3D SSLIB, coupled with low electrolyte ionic conductivity of LiPON, which lead to highly non-uniform internal current density distribution and poor cathode utilization with localized Li+ depletion at cathode interface. Simulation result with conformal electrolyte geometry demonstrated reduced potential losses, but did not affect the cathode diffusion effects, suggesting that future 3D SSLIB designs will necessarily have to account for limited Li-ion transport behavior.

Talin and coworkers at Sandia NL successfully demonstrated a working 3D SSLIB prototype consisting of conical Si microcolumns onto which layers corresponding to the current collector, LiCoO2 cathode, LiPON electrolyte and Si anode, are sequentially deposited using physical vapor deposition (PVD). However, the power performance of these 3D structures lags significantly (a capacity decrease of almost 90% with increasing C-rate) compared to the similarly prepared 2D planar batteries. The finite element COMSOL model was based on the conical shape elements of the 3D SSLIB experimental cross section image. The model simulation incorporates both diffusion and migration of Li+ within the LiPON electrolyte, while the transport of Li+ within the LiCoO2 cathode and Si anode was assumed to occur only by diffusion. For the anode/electrolyte and cathode/electrolyte interfaces, we used potential-dependent Butler-Volmer (BV) kinetic expressions to account for local Li+ concentration as well as the local activation overpotential.

Acknowledgements

This work was supported by EFRC NEES, NIST.

What is "voltage" at atomic lengthscale in battery electrodes?

What is "voltage" at atomic lengthscale in battery electrodes?

Scientific Achievement

Electric double layers EDL (dipole layers) are predicted to exist at both solid-solid & solid-liquid interfaces, where the solid interfaces can absorb voltage drops to widen electrolyte stability window.

Significance and Impact

Consideration of both e- & Li+ movement is essential in manipulating surface dipoles as a viable strategy to improve electrode passivation.

Research Details

  • This model distinguishes the computed instantaneous voltage governing electron movement from the slower-responding voltage governing Li content at interfaces, in order to achieve a comprehensive description of electrochemical activities on electrode surfaces.
  • Modeling included relatively inert thin films Li3PO4 & Li2CO3, & cathode oxide LMO. Result showed that the applied voltage is strongly correlated with the surface dipole density at the surface.
  • Anion-induced spatial inhomogeneity can yield parasite reaction hotspots as shown in the LMO system.

References

Kevin Leung & Andrew Leenheer, “How voltage drops are manifested by lithium ion configurations at interfaces & in thin films on battery electrodes”, J. Phys Chem C, 119, 10234-10246 (2015) DOI: 10.1021/acs/jpcc.5b01643.

Summary

Battery electrode surfaces are generally coated with electronically insulating solid films of thickness on the order of 1-50 nm. In lithium ion battery, both electrons and Li+ ions can moved in the surface films and in the electrodes themselves in response to applied voltage changes. Consequently, the structures of electric dipole layers (EDL) and the atomic length scale effect of voltages became much more complex than the pristine noble metal electrodes (e.g. Pt) used in classical electric double layer studies. Incorporating and distinguishing the electronic and the ionic voltage components are critical for a comprehensive description of electrochemical activities on electrode surfaces, such as Li+ insertion dynamics, SEI formation, and electrodeposition at overpotentials.

In this work, NEES scientist Kevin Leung applied density functional theory (DFT) to investigate how the applied voltage is manifested as changes in the EDL at atomic length scales, including charge separation and interfacial dipole moments. He explicitly distinguished between the instantaneous electronic voltage and the slower ionic Li+ voltage, clarifying what is the "voltage" in atomic simulations and constructed electrode interface models that exhibit consistent electrochemical activities for both Li-insertion reactions and parasitic processes (e.g., SEI formation). The electric double layer components are predicted to exist at both solid-liquid and solid-solid interfaces.

Acknowledgements

Work was performed at Sandia NL - NM and supported by EFRC NEES.

From electron tunneling model to anode protection

From electron tunneling model to anode protection

Scientific Achievement

A density function theory (DFT) - based electron tunneling model was used to assess the electronic insulating ability of inorganic SEI components (Li2CO3, LiF, Li3PO4), yielding useful quantifiable parameters - electron tunneling barrier, critical thickness & initial irreversible capacity loss caused by Li+ ion consumption.

Significance and Impact

Predictive modeling allows meaningful design principles for anode protection to circumvent cycling instability and capacity loss.

Research Details

  • The model-predicted irreversible capacity loss due to solid electrolyte interphase (SEI) agrees well with experiments. The initial irreversible capacity loss is due to Self-limiting Electron Tunneling Property of SEI.
  • Electron tunneling barrier decreases under tension and increases under compression, causing non-negligible effect on the critical thickness & irreversible capacity loss due to deformation.

References

Y. Lin, Z. Liu, K. Leung, L. Chen, P. Lu, Y. Qi, J. Power Sources 309, 221-230 (2016) DOI: 10.1016/j.jpowsour.2016.01.078.

K. Leung, Y. Qi, K. Zavadil, Y. Jung, A. Dillon, A. Cavanagh, S. Lee, S. George, J. Am. Chem. Soc. 133, 14741 (2011)

Summary

It is generally recognized that the formation and continuous growth of a solid electrolyte interphase (SEI) layers are responsible for the irreversible capacity loss of batteries in both the initial and subsequent stages of cycling. In a previous work, NEES Scientists Leung et al. used ab initio molecular dynamics (AIMD) and constrained density function theory (cDFT) methods to model the electrolyte decomposition dynamics of nm-thick artificial oxide SEI layer. It was shown that the thin SEI layers slow down the electron transfer rate because of its electron tunneling barrier.

 

Here, based on the tunneling barrier obtained from DFT calculations, they developed a simple analytical model to estimate the first cycle irreversible capacity loss due to the Li ions consumed in the formation of these SEI layers on the anode surface, and compared its predictions with experimental measurements with good agreement. It was shown that the initial irreversible capacity loss is due to the self-limiting electron tunneling property of the SEI. The critical thickness d* needed to block electron tunneling on the electrode surface for inorganic SEI components (LI2O3, LIF and Li3PO4) were calculated to be between 2 and 3 nm. Other electron transport mechanisms are still needed in order to explain the electron leakage through thicker SEI components (10-100 nm).

Qi and Leung also looked at the coupling effect of stress, e.g. tension and compression, induced by electrode volume expansion. To demonstrate, the model indicated that a 10% volume expansion of graphite would cause a 4.5% increase in critical thickness of SEI and 11% increase in the irreversible capacity loss considering the deformed structure of LiF.

Acknowledgements

Simulation work was performed at MSU, supported by EFRC NEES & NSF GOALI.

Water-activated Mg2+ insertion into MnO2 nanostructure electrode

Water-activated Mg2+ insertion into MnO2 nanostructure electrode

Scientific Achievement

Enhanced Mg2+ insertion/deinsertion was observed in the presence of water molecules in conventional magnesium electrolyte. Consecutive cycling in water containing electrolyte "activates" MnO2 electrode by which improved Mg2+ insertion has been demonstrated in the dry magnesium electrolyte.

Significance and Impact

Observed Mg2+ insertion capability and cycling stability of MnO2 nanowire suggest potential of MnO2 as a promising cathode material for Mg-ion battery.

Research Details

  • Degree of Mg2+ insertion in the electrolytes with various water content was probed using ICP-AES.
  • Presence of water molecules in the conventional electrolyte significantly enhances Mg2+ insertion/deinsertion.
  • Portion of water molecules co-insert/deinsert along with Mg2+ ions into MnO2 as shown by EQCM.
  • Enhanced Mg2+ insertion was observed without presence of water molecules after consecutive cycling in the water containing electrolyte.

References

J. Song, M. Noked, E. Gillette , J. Duay, G. Rubloff & SB Lee, “Activation of a MnO2 Cathode by water-stimulated Mg2+ insertion for magnesium ion battery”, PCCP, 17, 5256-5264 (2015) DOI: 10.1039/c4cp05591h.

Summary

For divalent electrochemical storage systems such as Mg-ion battery, compared to the monovalent Li-ion counterpart, major developmental challenges remain which prevented a full realization of technological advancement. In particular, the slow kinetics associated with Mg2+ ion mobility has been tested with different host cathode materials without any significant breakthroughs.

In the present study, we used two strategies to facilitate Mg2+ insertion/deinsertion: 1) nanostructured cathode materials to reduce the ionic diffusion path, and 2) introduction of a small amount of strong dipole molecules, such as H2O molecules, to shield the divalent charges in order to reduce electrostatic interactions.

NEES scientists revealed that the magnitude of Mg2+ insertion into MnO2 nanostructured cathode highly depended on the ratio between water molecules and Mg2+ ions present in the electrolyte. Optimal ratio was shown to be 6:1 for H2O:Mg2+ in the electrolyte for highest Mg ion insertion capacity in our system. Furthermore, we demonstrated that sufficient electrochemical reaction in water-containing electrolyte can prime the nanostructured cathode material for enhanced insertion/deinsertion, even in the absence of water molecules.

Utilizing the synergistic effect of both strategies, this work demonstrated the potential of MnO2 nanowire cathode as a host materials for the divalent Mg-ion system.

Acknowledgements

Work was performed at University of Maryland and supported by EFRC NEES.

Direct electrical signatures of degradation & failure

Direct electrical signatures of degradation & failure

Scientific Achievement

This is the first direct in-situ signatures of the dc resistance of ultralong δ-MnO2 NW arrays as a function of the equilibrium electrical potential and Li+ content. Extremely high degree of dispersion for MnO2 nanosheets within the NW associated with high porosity as shown by TEM, is needed to achieve state-of-the-art charge storage performance.

Significance and Impact

This unique experimental setup provides a universal tool to study degradation in a massive array that mimic highly dense nanostructured electrodes in practice.

Research Details

  • As the equilibrium electrode potential is decreased from 0.60V to -0.80 V vs. MSE and Li+ is intercalated, the electrical conductivity of δ-MnO2 nanowires (NW) increases by up to one order of magnitude.
  • TEM analysis of LixMnO2 NW density implicated the NW porosity for the observed conductivity.
  • Ultralong arrays of Au@δ-MnO2 core-shell NWs (mm’s) allow spatial location of local defect sites and subsequent experiments to validate degradation mechanisms.

References

My Le, Yu Liu, Hui Wang, Rajen K. Dutta, Wenbo Yan, John C. Hemminger, Ruqian Q. Wu, Reginald M. Penner, Chem Mater (2015) 27(9), 3494-3504. DOI a href="https://dx.doi.org/10.1021/acs.chemmater.5b00912">10.1021/acs.chemmater.5b00912.

Wenbo Yan, Mya Le Thai, Rajen Dutta, Xiaowei Li, Wendong Xing, Reginald M. Penner, ACS Appl Mater Interfaces (2014) 6(7), 5018. DOI 10.1021/am500051d.

Summary

In-situ electrical conductivity of LixMnO2 nanowires.

Earlier Penner et al. pioneered a lithographic synthesis technique (LPNE) for forming very long nanowires of controlled cross section, with impact on understanding the behavior of oxide nanostructures, and with promise for elucidating degradation and failure mechanisms. Fabrication of 200 MnO2 nanowires in parallel (40-60nm high x 275-870nm wide) spanning a 10 µm gap between two Au contacts has enabled determination of nanowire electrical conductivity as a function of lithiation, revealing pronounced disparities in electrical conductivity between LiMnO2 and MnO2. TEM showed distinct differences in porosity as a function of nanowire width, and charge storage capacity increased with porosity.

The detailed results illustrate for us the potential value of the LPNE approach for degradation studies - to understand dynamic changes in transport, structure, and storage capacity as a function of nanostructure design.

Acknowledgements

This work was supported as part of EFRC NEES; the XPS data was funded by DOE BES; DFE calculations by CaSTL, NSF.

100K cycles & beyond: extraordinary cycle stability using gel electrolyte

100K cycles & beyond: extraordinary cycle stability using gel electrolyte

Scientific Achievement

Incorporating PMMA gel stabilizes cycling of ultra-long Au@δ-MnO2 core-shell nanowire capacitors, allowing Li+ insertion/deinsertion up to 200K cycles at 95% Coulombic efficiency (> 10X longer cycling than without PMMA).

Significance and Impact

Nanostructures stabilization by gel is promising for liquid and solid state electrolyte systems. This and precision nanostructure design offer valuable testbed to understand degradation.

Research Details

  • Parallel experiments were performed using two electrolytes – 1.0 M LiClO4 in propylene carbonate (PC) without & with added 20 (w/w)% PMMA.
  • MnO2 oxide shell of the nanowire continue to access insertion-based capacity for more than 100K cycles, as shown by the scan rate dependence of Csp as a function of potential.
  • SEM analysis showed nanoscaled MnO2 loss and subsequent microscale loss features.

References

Mya Le, Girija Thesma Chandran, Rajen K. Dutta, Xiaowei Li, and Reginald Penner, “100k Cycles and Beyond: Extraordinary Cycle Stability for MnO2 Nanowires Imparted by a Gel Electrolyte”, ACS Energy Lett., (2016), 1, 57-63. DOI: 10.1021/acsenergylett.6b00029.

Summary

Researchers funded by Nanostructures for Electrical Energy Storage (NEES), a DOE Energy Frontier Research Center (EFRC), have demonstrated reversible cycle stability for up to 200,000 cycles with 94-96% Coulombic efficiency for symmetrical δ-MnO2 nanowire capacitors. By using a unique Degradation & Failure Discovery Platform consisting of two interpenetrating arrays of 375 ultralong (5 mm) core@shell Au@δ-MnO2 nanowires, a design feature intended to accentuate the influence of defects caused by degradation, Penner’s group at UCI accomplished extraordinary cycling performance by replacing the liquid electrolyte with a poly(methyl methacrylate) (PMMA) gel electrolyte. In the normal PC electrolyte without PMMA, the system showed dramatically reduced cycle stabilities.

These experiments demonstrate for the first time that nanowire-based battery and capacitor electrodes are capable of providing extremely long cycle lifetimes. In addition, evidence shows that MnO2 oxide shell of these nanowires continues to access insertion-based capacity. Based on the SEM analysis, one mechanism for the extended cycle life could be by mechanical confinement of the MnO2 shell materials on the gold nanowire current collector. Further investigation of the mechanical properties will be required.

Acknowledgements

This work was solely supported by EFRC NEES.

Carbonized leaf membrane as Na+ battery anode

Carbonized leaf membrane as Na+ battery anode

Scientific Achievement

We used a single-step thermal pyrolysis procedure to transform a leaf into a carbon membrane to utilize the plant’s hierarchical porosity network for electrochemical storage. Here for the first time we created a (sodium ion) battery (SIB) with a natural leaf structure for the carbon anode, without binder, additive, or additional current collector.

Significance and Impact

Hierarchical capillary transport in the leaf presents a nature-designed ion transport network for electrochemical energy storage.

Research Details

  • The carbonized leaf (1000°C under Ar) maintains a high charge capacity of 360 mAh/g and an initial Coulombic efficacy at 74.8% which remains stable over the next 200 cycles.
  • For structural characterization, XRD pattern & Raman spectra were used along with SEM/TEM.
  • Based on the resistance measurements, the electrical conductivity of the welded carbonized leaves was 710 s/m compared to 1250 s/m for the single leaf.

References

Hongbian Li, Fei Shen, Wei Luo, Jiaqi Dai, Xiaogan Han, Yanan Chen, Yonggang Yao, Hongli Zhu, Kun Fu, Emily Hitz, and Liangbing Hu “Carbonized-leaf membrane with anisotropic surfaces for sodium-ion battery” ACS Appl. Mater. Interfaces, 2016, 8(3), 2204-2210. DOI 10.1021/acsami.5b10875.

Summary

Nanostructures for Electrical Energy Storage (NEES) scientist Liangbing Hu and collogues at University of Maryland have cleverly taken advantage of the structural designs pre-selected by nature’s functional requirements in leaf, the most abundant biomass on Earth. As the anode for sodium ion batteries, carbon membranes derived from leaves have existing anisotropic surfaces with ideal porous mesostructures in between – the nonporous upper surface as a dense current collector, the sponge layer of carbon nanosheets for storage, and the back surface with stromata channels for sodium insertion. Hu et al. converted leaf’s meso- and macro-structures into utilizable hard-carbon precursor for SIB anodes through high temperature carbonization.

For the first time, they demonstrated a free-standing (binder-free, current-collector-free) anode for rechargeable SIBs, exhibiting a high specific capacity of 360 mAh/g and a high initial Coulombic efficiency of 75%. Moreover, as a proof of concept with scalable potentials, they showed the working of welded leaf carbon network. Future work remains to investigate possible welding mechanism(s) and/or the electron and ionic transport pathways.

Morphological characterization of the carbonized leaf was obtained using SEM/TEM, and electrochemical measurements were conducted using 2032 coin cells with Na plates as reference electrode.

Acknowledgements

Work was performed at UMD & supported by EFRC NEES.

Press Release

by Martha Heil, Science Communicator, UMD NanoCenter

Balancing meso-structural stability & ionic transport

Balancing meso-structural stability & ionic transport

Scientific Achievement

The soft-hard dual-template strategy provides a high pore volume and surface area OMCNW (ordered mesoporous carbon nanowires) with stable meso-structure upon Fe2O3 loading. The unique nanowire morphology and mesoporous structure of the OMCNW/Fe2O3 facilitate high ionic mobility in the composite, leading to >260 Fg-1 specific capacitance with good rate capability and cycling stability.

Significance and Impact

The discovering of a carbon framework with balanced meso-structure and morphology can serve to optimize both electronic and ionic transports in electrochemical energy storage and conversion.

Research Details

  • AAO and copolymer F127 are used together as hard and soft templates to fabricate OMCNW.
  • Compared to the soft-templated FDU-15, dual-templated composites showed X2 the total surface area and X6 the total pore volume, while maintaining mesostructural stability.
  • The synergistic effects of dual-template strategy on meso-structure were investigated by HRTEM & N2 sorption. The meso-structure and morphology influence on electrochemical properties were studied by EIS and CV.

References

Junkai Hu, Malachi Noked, Eleanor Gillette, Fudong Han, Zhe Gui, Chunsheng Wang and Sang Bok Lee, “Dual-template synthesis of ordered mesoporous carbon/Fe2O3 nanowires: high porosity and structural stability for supercapacitors”, J. Mater. Chem. A, 2015, 3, 21501, DOI 10.1039/C5TA06372H.

Junkai Hu, Malachi Noked, Eleanor Gillette, Zhe Gui and Sang Bok Lee, “Capacitance behavior of ordered mesoporous carbon/Fe2O3 composites: Camparison between 1D cylindrical, 2D hexagonal, and 3D bicontinuous mesostructures”, Carbon, 2015, 93, 903, DOI 10.1016/j.carbon.2015.06.019.

Summary

As shown in a previous study, Nanostructures for Electrical Energy Storage (NEES) PI Sang Bok Lee compared soft- and hard-template synthesis strategies to gain insights into mesoporous morphology and structural stability of ordered mesoporous carbon as promising active metal oxide storage scaffolds. It was shown that mesostructure stability is a critical parameter that can limit the usefulness of the commonly used 2D CMK-3 structure, synthesized using hard-templated method. In this work, NEES PIs SB Lee and C Wang developed a dual-template synthesis strategy in order to take advantage of both methodologies: 1) the stable mesostructure from the soft-template method, and 2) the high pore volume and surface area from the hard-template method.

Authors incorporated Fe2O3 nanoparticles into a dual-templated OMCNWs structure to fabricate a composite with optimized structural characteristics and electrochemical performances not achievable using either of the single-templated methods alone. Compared to the soft-templated (FDU-15) and hard-templated (CMK-8) mesoporous structures, the new dual-templated OMCNW/Fe2O3 composites demonstrated large surface area and pore volume size, large specific capacitance (264 Fg-1) with easy ion accessibility (EIS analysis), high rate capacitance retention and stable cycling performance over 1000 cycles.

This work provided us with a comprehensive comparison of ordered mesoporous carbon structures from different synthesis strategies as host materials for Fe2O3, and as a guideline for future design principles of mesostructure framework.

Acknowledgements

Work was performed at UMD & supported by EFRC NEES.

Improving Lithium Battery Performance by Characterizing Native Processes

Improving Lithium Battery Performance by Characterizing Native Processes

Scientific Achievement

Actual lithium-ion-battery operating conditions now can be replicated inside a transmission electron microscope (TEM). Compared to other liquid cells, this new platform uniquely enables nanoscale observations of lithium cycling at controlled rates.

Significance and Impact

High-capacity lithium battery electrodes can be improved by preventing dendritic lithium deposits. By understanding how these deposits form, we can reduce safety concerns and dramatically increase electrode capacity, potentially by 10x.

Research Details

  • TCycled battery-relevant electrolytes in our chemically compatible, multi-electrode device.
  • Observed lithium deposit nucleation and growth.
  • Manipulated the lithium deposit structures using electrochemical and electron beam control.

References

A. J. Leenheer, K. L. Jungjohann, K. R. Zavadil, J. P. Sullivan, C. T. Harris, ACS Nano (2015) 9(4), 4379. DOI: 10.1021/acsnano.5b00876.

A. J. Leenheer, J. P. Sullivan, M. J. Shaw, C. T. Harris, JMEMS (2015) 24(4), 1061. DOI: 10.1109/jmems.2014.2380771.

Summary

Electrodeposited metallic lithium is an ideal negative battery electrode, but non-uniform microstructure evolution during cycling leads to degradation and safety issues.

NEES scientists at Sandia National Laboratories designed a standalone, custom microfabricated, sealed liquid cell for in situ scanning transmission electron microscopy (STEM) to image the first few cycles of lithium electrodeposition and dissolution in liquid aprotic electrolyte at submicron resolution. Each cell contain 10 relatively small (0.26 μm2) working electrodes, allowing the ability to perform multiple experiments under identical chemical conditions. To simulate conditions in common Li-ion batteries with aprotic solvent-based electrolytes, the TEM liquid cell was filled with equal parts by volume ethylene carbonate and dimethyl carbonate containing 1 M lithium hexafluorophosphate salt (1:1 EC:DMC / 1 M LiPF6 ).

The electrodeposition and stripping of Li is shown to be spatially localized, rendering it susceptible to degradation and safety issues exemplified in dendrite formation. This localization is modulated by formation of solid elecrtrolyte interphase (SEI) layers and defects. Even at low dose, electron beam was found to affect the morphology of dendrite formation and growth dynamics, with implications for operando STEM liquid cell imaging and Li battery applications. Our results show that the SEI plays a critical role in Li dendrite initiation.

Acknowledgements

This work was supported in part by an LDRD project at Sandia National Laboratories and in part by Nanostructures for Electrical Energy Storage, an EFRC funded by DOE. This work was performed at CINT.

ALD Protection of Next-Generation Metal Anodes

ALD Protection of Next-Generation Metal Anodes

Scientific Achievement

Using atomic layer deposition (ALD), we deposited thin (14nm) Al2O3 layers directly onto Li metal anodes, providing corrosion protection against atmosphere, organic solvent & other chemical species, and enhancing cyclability in Li-S cells.

Significance and Impact

Metal anodes are recognized as promising candidates for the next-generation beyond-Li-ion batteries using advanced oxide, sulfur, or oxygen cathodes, but their high reactivity requires corrosion protection while maintaining good ion transport and electronic insulation at the anode surface.

Research Details

  • Integrated ALD, XPS, and battery testing without ambient exposure enable observation of post-cycling surface conditions.
  • Li metal protected with 14 nm ALD Al2O3 drastically reduces first cycle capacity loss in the Li-S system by preventing anode corrosion in the presence of S species in the electrolyte.

References

Kozen, A.C.; Lin, C.F.; Pearse, A.J.; Schroeder, M.A.; Han, X.; Hu, L.; Lee, S.B.; Rubloff, G.W.; Noked, M. Next-generation Lithium Metal Anode Engineering via Atomic Layer Deposition. ACS Nano 2015, 9(6), 5884.

Kozen, A.C.; Pearse, A.J.; Lin, C.F.; Schroeder, M.A.; Noked, M.; Lee, S.B.; Rubloff, G.W. Atomic Layer Deposition and in Situ characterization of Ultraclean Lithium Oxide and Lithium Hydroxide. J. Phys. Chem. 2014, 118, 27749.

Leung, K.; Qi, Y.; Zavadil, K.R.; Jung, Y.Sl; Dillon, A.C.; Cavanagh, A.S.; Lee, S.; George, S.M. Using Atomic Layer Deposition to Hinder Solvent Decomposition in Lithium Ion Batteries: Frist-principles Modeling and Experimental Studies. J. Am. Chem. Soc 2011, 133, 14741.

Summary

Due to the high energy density of 1840 mAh/g for Li metal, the battery community is increasingly considering the lithium metal anode systems - including sulfur, oxygen/air, and advanced oxide cathode systems, as a promising next-generation battery technology beyond Li-ion. However, the extreme reactivity of the Li surface results in parasitic reactions with solvents, contamination, and shuttled active species in the electrolyte, posing serious limitations particularly in capacity retention with charge/discharge cycling.

Using a unique ultrahigh vacuum system developed previously, NEES scientists demonstrated as a proof-of-concept that application of thin chemical protection layers to the Li metal surface can dramatically improve capacity retention. Detailed work was carried out documenting Li metal surface protection in three different corrosive environments – ambient atmosphere, organic solvent, and sulfur/electrolyte (dimethyoxylane) solution.

We tested both the initial and long term cycling performance of our anode passivation procedure using Li-S CR2032 coin cells with activated carbon cloth/sulfur composite cathodes. The ALD Al2O3 protection layer on Li prevented the profound self-discharge and capacity loss (from ∼1200 to ∼800 mAh/g) otherwise observed during the first 10 cycles. After 100 cycles, the protected anodes lost only ∼10% (vs. 50% for the bare Li metal surface) of their initial capacity. ALD is shown to be an effective method for protecting Li metal anodes, and corresponding benefits may be possible for other reactive metal anode systems such as Na, Mg and Al.

Acknowledgements

This work was solely supported as part of EFRC NEES. Work was performed at University of Maryland, College Park.

Single Material All-Solid-State Li-ion Batteries

Single Material All-Solid-State Li-ion Batteries

Scientific Achievement

A proof of concept of a single material battery using LGPS (Li10GeP2S12) as the electrolyte, anode, & cathode. We demonstrate a remarkably low interfacial resistance due to improved interfacial contact, modification of interfacial interactions & suppression of strain/stress.

Significance and Impact

This novel single element approach can be broadly applied to other solid-state battery systems to benefits both safety and reliability in energy storage by addressing electrode-electrolyte interfacial resistance.

Research Details

  • Electrochemical measurement of LGPS-C (LGPS:carbon 3:1 by wt.) electrodes were tested in coin cell using liquid electrolyte, establishing the Li-S/C component at 41% theoretical capacity as Li2S cathode, the Ge-S/C component at 80% theoretical capacity as GeS2 anode.
  • The single-LGPS battery contains layer thickness of LGPS/C cathode, LGPS electrolyte, & LGPS/C anode as 222, 330 & 148 mm respectively.

References

F. Han, T. Gao, Y. Zhu, K.J. Gaskell, C. Wang, A Battery Made From a Single Material, Adv. Meter., 27 (23), 3475 (2015).

Summary

All-solid-state (ASS) Li-ion batteries are receiving increasing interest for energy storage systems because of the safety and reliability benefits by replacing volatile and flammable liquid electrolyte with nonflammable inorganic solid electrolyte. From the basic science perspectives, the use of ASS systems pose the very fundamental challenge of high interfacial resistance across the three distinctive components: an anode material that can reversibly lithiate/delithiate at a low potential, a cathode material that does the same at a high potential, and a solid electrolyte with high ionic but low electronic conductivity. Conventional bulk-like ASS LIB use three different materials due to these stringent different requirements.

Drawing from their collective past experiences and knowledge on sulfide chemistries as potential electrode and electrolyte materials for Li-ion batteries, NEES scientists explored a new revolutionary approach to the ASS energy storage challenge. They designed a proof-of-concept interface-free ASS battery by using a single sulfide-based solid electrolyte material, Li10GeP2S12 (LGPS). After mixing with carbon, the Li–S and Ge–S components of the LGPS could act as active centers for its cathode and anode performance in a way similar to the Li2S cathode and GeS2 anode, respectively. This single-LGPS exhibited a remarkably low interfacial resistance due to: (1) the improvement of interfacial contact, (2) the modification of the interfacial interactions, and (3) the suppression of the strain/stress at the interface.

The single-material battery concept provides a promising direction to address the most challenging interfacial problem in ASS Li-ion battery. It can also be broadly applied to other solid-state battery systems, beneficial to a high-power, high-energy, long-cycling all-solid-state battery. Additional implications of this concept include the fabrication of a nanobattery by introducing electronically conductive material on both the surfaces of the LGPS nanomaterials.

Acknowledgements

This work was supported by NSF & as part of EFRC NEES, and was performed at UMD.

Press Release

"For batteries, one material does it all" by Martha Heil, UMD NanoCenter

ALD Solid Electrolytes

ALD Solid Electrolytes

Scientific Achievement

We have developed atomic layer deposition (ALD) processes for Li2O, Li3PO4, and LiPON as controlled solid electrolyte films, using in-situ processing & analysis to identify surface chemistry.

Significance and Impact

Chemical routes to thin solid electrolytes are needed to control film morphology and transport properties beyond the demonstrated limitations of sputtered materials. Solid electrolytes are a primary obstacle to solid state batteries and could be crucial to adoption of metal anode for beyond-Li-ion storage.

Research Details

  • ALD precursors include Li tert-butoxide (LiOtBu), H2O, tri-methyl-phosphate (TMP), and plasma N2, depending on which solid electrolyte material is desired.
  • In-situ XPS after ALD shows reversible LiOH <-> Li2O reaction involving H2O desorption above 240C. CO2 exposure leads to Li2CO3 formation, but otherwise all three solid electrolytes are formed carbon-free.

References

A.C. Kozen, A.J. Pearse, C-F Lin, M. Noked, G.W. Rubloff, Atomic Layer Deposition of the Solid Electrolyte LiPON, Chem. Mater., 27 (15), 5324 (2015); doi 10.1021/acs.chemmater.5b01654.

A.C. Kozen, A.J. Pearse, C-F Lin, M.A. Schroeder, M. Noked, S.B. Lee, G.W. Rubloff, Atomic Layer Deposition and in-situ Characterization of Ultraclean Li Oxide and Li Hydroxide, JPPC, 118, 27749 (2014); doi 10.1021/jp509298r.

Summary

LiPON (lithium phosphorus oxynitride) has been one of the most popular solid state electrolyte used for planar lithium ion microbatteries. As a physical deposition technique, reactive sputtering to form LiPON thin films onto planar surfaces has been demonstrated; however, such technique was shown to be rather inadequate for thin film deposition onto high aspect ratio 3D nanostructures – indeed, uniformity and conformity were major challenges. Motivated by our prior work on 3D solid state Li-ion nanobatteries, NEES scientists recently developed and demonstrated the first reported atomic layer deposition (ALD) process for LiPON thin film. We were able to take advantage of the unique integrated high-vacuum deposition surface characterization and battery assembly systems available in the ANSLab at University of Maryland, College Park, under the direction of Gary Rubloff.

Four precursors were used for the ALD process for Li2O, Li3PO4 and LiPON - lithium tert-butoxide (LiOtBu), H2O, trimethylphosphate (TMP), and plasma N2 (PN2), under conditions that are favorable for only certain substrate-precursor chemical reactions. Additionally, we demonstrated tunable nitrogen doping by using N2 plasma doses as a critical parameter. It was found that the LiPON ALD films over approximately 4.5% nitrogen are amorphous, whereas films with less than 4.5% nitrogen are polycrystalline.

Acknowledgements

Work was performed at UMD & supported by EFRC NEES.

Press Release

News Release article "New battery demonstrates "Sweet Spot" of electrolyte thickness and composition" by Martha Heil, UMD NanoCenter

Nonuniform Si Loss in Nanowire Anodes during Li Cycling

Nonuniform Si Loss in Nanowire Anodes during Li Cycling

Scientific Achievement

We observed for the first time Si loss from nanowires with cycling. This occurs preferentially, close to the current collector & is correlated with the growth of the solid-electrolyte-interphase (SEI) and capacity fading with charge/discharge cycling.

Significance and Impact

Results suggest that Si loss during cycling may contribute significantly to capacity fade. Spatial nonuniformities in nanowire designs may lead to predictable locations for accelerated degradation and failure.

Research Details

  • Template growth was used to synthesize the SiNWs on a platinum silicide (PtSix) pedestal which does not undergo SEI formation during Li cycling, establishing a built-in reference diameter for the NWs.
  • Nonuniform morphological changes along NW length were observed, with accelerated Si loss in diameter (0.1-0.5 nm/cycle) near the NW base (150nm from current collector) and less Si loss further from the base.
  • Capacity reduction with cycling is correlated to both Si volume loss and SEI growth.

References

J. Song, J. Duay, E. Gillette & SB Lee, "The Reversible Anomalous High Lithium Capacity of MnO2 Nanowires", Chem. Communications, 50, 7352-7355 (2014) DOI: 10.1039/C4CC02001D.

Summary

Due to their high energy and power densities, silicon nanowires (SINWs) as anode materials in lithium-ion batteries are of great interest. However, their high surface-to-volume ratio is believed to contribute to fading in capacity retention during cycling due to solid-electrolyte-interphase (SEI) formation. Effort to establish a stable and continuous SEI layer as a protective layer is an active area of research.

For the first time, scientists at Nanostructures for Electrical Energy Storage (NEES), a DOE Energy Frontier Research Center (EFRC), were able to directly measure silicon loss from the SINWs during lithiation/delithiation cycles. By using platinum silicide pedestals template as a built-in reference, rapid initial reduction in SINW diameter width near the NW base was clearly visible & quantifiable. Researchers were able to accurately predict the measured decrease in specific capacity with SEI layer growth over the first 200 cycles. The results imply that prior to stabilization of the SEI layer, the size and evolution of individual SiNWs, near their interface with the metallic charge-collecting conductors, play a critical role in the long term capacity retention during cycling for SiNW-based anode materials.

Figure Inset (b): The mass of SEI layers is given for four samples and determined by measuring the mass of SiNW based anode materials before and after 40, 80, 120, and 160 cycles. The normalized mass of SEI was obtained by dividing the mass of SEI layers by pristine SiNW mass. Specific discharge capacity measurements were collected between 20 mV and 1.5 V at 0.5C in a SiNW-based half cell. Square symbols are the calculated results of a model of the specific capacity change with cycle number based on the loss of Si: solid yellow line is a curve fit to the model, C = 2987-1353 {1 - exp[-0.0369(cycle no. -1)]}. The 2987 mA h/g in the curve fit equation is the measured value of discharge specific capacity in the first cycle.

Acknowledgements

Work was performed at CINT, LANL & supported by EFRC NEES.

Precision Nanobatteries by the Billions

Precision Nanobatteries by the Billions

Scientific Achievement

Batteries formed inside nanopores provide a means to assess fundamentals of ion and electron transport in nanostructures for energy storage. By rational design for both transport components, these nanobatteries deliver their stored energy efficiently at high power (fast charge & discharge) and for extended cycling.

Significance and Impact

Results demonstrate that precise nanostructures can be constructed to test the limits of 3-D nanobatteries and that the smallest batteries to date show excellent power and cycling performance.

Research Details

  • Each electrode includes an outer Ru nanotube current collector and an inner nanotube of V2O5 storage material to form an anode (prelithiated) & a cathode (pristine) separated by electrolyte.
  • The capacity retention for is 95% at 5C and 46% at 150C compared to 1C values (24s charge-discharge cycle). At the 5C rate, 81% capacity is retained after 1,000 cycles between 0.2V and 2.8V.

References

C. Liu, E.I. Gillette, X. Chen, A.J. Pearse, A.C. Kozen, M.A. Schroeder, K.E. Gregorczyk, S.B. Lee, G.W.Rubloff, "An all-in-one nanopore battery array", Nature Nanotechnology Advance Online Publication 10 November 2014, doi: 10.1038/nnano.2014.247.

Summary

Nanostructured batteries, when properly designed and built, offer promise for delivering their energy at much higher power and longer life than conventional technology. To retain energy density, nanostructures (e.g. nanowires) must be paced into dense "nanostructure forests", producing 3-D nanogeometries in which ions and electrons must rapidly move. Researchers have built arrays of nanobatteries inside billions of ordered, identical nanopores in an alumina template to determine how well ions and electrons can do their job in ultrasmall environments. Up to a billion nanopore batteries could fit in a grain of sand. The nanobatteries were fabricated by atomic layer deposition to make oxide nanotubes (for ion storage) inside metal nanotubes for electron transport, all inside each end of the nanopores. The tiny nanobatteries work extremely well: they can transfer half their energy in just a 30 second charge or discharge time, and they lose only a few % of this after 1000 cycles. Researchers attribute this performance to rational design and well-controlled fabrication, achieving nanotubular electrodes to accommodate ion motion in and out, and close contact between the thin nested tubes to ensure fast transport for both ions and electrons.

Acknowledgements

Work was performed at CINT, LANL & supported by EFRC NEES.

News Article

A Billion Holes Can Make a Battery

Scaling limits for miniature all-solid-state Li-ion battery

Scaling limits for miniature all-solid-state Li-ion battery

Scientific Achievement

A miniature diagnostic model was fabricated using LiCoO2 cathode, ultrathin conformal LiPON electrolyte layer & α-Si anode shells in an all solid-state heterostructure nanowire Li-ion battery (NWLIB).

Significance and Impact

In-situ correlation of electrochemistry and structure during cycling reveals interfacial breakdown of solid electrolyte, providing key insights for future all-solid nanobatteries.

Research Details

  • At 0.5-1.2 μm in diameter and up to 7 μm in length, the NWLIBs are the smallest complete secondary batteries realized to date.
  • With electrolyte thickness in the 110 nm range, rapid self-discharge accompanied by void formation were observed, as onset of space-charge limited electronic conduction (SCLC) occurred.
  • Detailed characterizations were carried out using multiple imaging techniques, coupled with correlative multivariate statistical analysis and tomography.

References

V. Oleshko etc., Nanoscale, 6, 11756-11768 (2014) DOI: 10.1039/c4nr01666a.
D. Ruzmetov etc., Nano Lett., 1, 505-511 (2012) DOI: 10.1021/nl204047z.

Summary

Nanostructures for Electrical Energy Storage (NEES) scientists designed model miniature all solid-state radial heterostructure nanowire Li-ion batteries (NWLIBs) composed of LiCoO2 cathode, an ultrathin conformal solid electrolyte layer LiPON, and amorphous Si anode shells. Sized 0.5-1.2 μm in diameter and up to 7 μm in length, these nanobatteries constitute a powerful engineering platform for diagnostics of nanoscale electrochemical processes in direct correlation to structural changes.

For the thinnest solid electrolyte layer ≈ 110nm, a rapid self-discharge along with void formation at the electrode/electrolyte interface occurred, indicating both electrical and chemical breakdown. The nanobatteries with LiPON thickness at 180 nm were able to maintain a potential above 2V for over 2 hours, showing substantial improvement. At these nanoscale dimensions, the onset of space-charge limited electronic conduction (SCLC) can drastically compromise the electrolyte stability.

We paid particular attention on the in-situ characterization of the nanoscale interfacial electrochemical & structural evolution during cycling, employing 3-D scanning & transmission electron microscopy (S/TEM), field-emission SEM and tomography techniques together with correlative multivariate statistical analysis. A detailed 3-D structural model was proposed based on this work, such as textured platelet-like LiCoO2 nanocrystallites around the void. This nanostructure is the result of using sputter deposition onto high aspect ratio vertical microrods and likely also contributes to the electrolyte breakdown.

Acknowledgements

This work was supported as part of EFRC NEES, NIST, and SNL.

Reversible anomalous high Li+ insertion

Reversible anomalous high Li+ insertion

Scientific Achievement

For amorphous nanostructured materials in organic electrolyte, reversible Li insertion reaches a ratio 1:1.5 per MnO2, 50% more than the assumed theoretical capacity for Mn4+ to Mn3+ conversion.

Significance and Impact

Electrochemical reaction mechanism of pseudocapacitance in MnO2 nanowires may involve additional reduction reactions with expanded voltage window, leading to higher charge capacity.

Research Details

  • Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to probe counter-ion insertion.
  • In aqueous electrolyte, the insertion ion is shown to be a combination of protons and cations.
  • The electrochemical lithiation potential can go beyond 0V down to -0.6 V vs. Ag/AgCl.
  • The Li ion insertion-deinsertion process is reversible as supported by ICP-AES & Raman spectroscopy.

References

J. Song, J. Duay, E. Gillette & SB Lee, "The Reversible Anomalous High Lithium Capacity of MnO2 Nanowires", Chem. Communications, 50, 7352-7355 (2014) DOI: 10.1039/C4CC02001D.

Summary

Nanostructures for Electrical Energy Storage (NEES) Researcher Sang Bok Lee at University of Maryland investigated the charge storage mechanism of manganese oxides (MnO2), a promising storage material for rechargeable lithium ion batteries (LIB) and supercapacitors. Here an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) instrument was used to track the magnitude of counter-ion insertion as the voltage window is expanded for nanowire arrays of amorphous MnO2.

Contrary to the generally assumed 1:1 insertion ratio of Li ion per MnO2 unit, it was found that in organic electrolyte, the magnitude of lithium ion insertion was in fact greater than one, reaching a maximum ratio of 1.56 at -1 V. This result is 50% more than expected. Authors attributed it to the further reduction of manganese to the Mn2+ state beyond the reduction from the Mn4+ to Mn3+ states.

The process was also found to be reversible, evidenced by results of ICP-AES and Raman spectroscopy. Furthermore, lithium insertion goes below the traditional stopping point of 0 V vs Ag/AgCl for supercapacitors. Based on the findings here, the actual end point for the electrochemical lithiation in this system was around -0.6 V vs. Ag/AgCl that will lead to higher charge capacity of MnO2.

Acknowledgements

Work was performed at University of Maryland and supported by EFRC NEES.

Approaching limits of transparency and conductivity

Approaching limits of transparency and conductivity

Scientific Achievement

In situ electrical & optical measurements of Li intercalated ultrathin graphite demonstrated a figure of merit σdcopt = 1,400, approaching the limits of transparency & conductivity for a doped graphene system.

Significance and Impact

An in situ planar nanobattery design enables future understanding of electrochemical intercalation dynamics & correlated optoelectronic properties of 2-D nanomaterials.

Research Details

  • Li intercalation simultaneously increases the DC electrical conductivity & optical transmission in the visible - transmission of 91.7% and a sheet resistance of 3.0Ω/sq were achieved for 19-layer LiC6, corresponding to a figure of merit σdcopt = 1,400, comparable to good metals.
  • The observed optoelectronic properties are explained by the suppression of interband optical transitions & an intraband Drude conductivity near the interband edge.

References

W. Bao etc., Nature Comms., 5, 4224 (2014) DOI: 10.1038/ncomms5224.

Summary

A team of researchers from the University of Maryland, including Liangbing Hu from the Energy Frontier Research Center (EFRC), Nanostructures for Electrical Energy Storage (NEES), and Monash University in Australia has developed a nearly transparent, highly conductive ultrathin graphite sheet that can be used to create more efficient solar cells and highly sensitive touchscreens. The new material was introduced in the July 1, 2014 issue of Nature Communications.

While intercalating various species of molecules between few-layer graphene (FLG) to improve transparency has been done before, this is the first time an increase in both visible-range transmittance and conductivity has been observed simultaneously with the use of Lithium. Due to the unique band structure of graphene layer, intercalation in FLG shifts the Fermi level upward and suppresses interband optical transitions due to Pauli blocking.

The team was able to demonstrate a 91.7% transmission of visible light and a sheet resistance of only 3.0Ω/sq, which corresponds to a figure of merit σdcopt = 1,400, thereby achieving the highest combined performance among all continuous thin films.

Acknowledgements

Work was performed at UMD and supported in part by EFRC NEES, ONR MURI and NSF-CMMI.

News Article

Adding Lithium Boosts Transparency and Conductivity of Graphite

Capacity loss due to irreversible Li trapping in AlLi alloy

Capacity loss due to irreversible Li trapping in AlLi alloy

Scientific Achievement

Combining galvanostatic cycling with in situ microscopy, capacity loss in Al anode is shown to be caused by irreversible trapping of Li in the AlLi alloy formed on the exposed surface of Al.

Significance and Impact

Real time observation of charge-discharge cycling in a thin film solid state battery reveals degradation mechanism intrinsic to alloy formation & surface oxidation.

Research Details

  • Real-time scanning electron microscopy and Auger spectroscopy were used.
  • Electrochemical reaction Li + Al -> AlLi only occurs on anode surface due to strain.
  • For a thin film, all-solid-state Li-ion battery (TFLIB) with Al anode, LiCoO2 cathode & N-doped Li3PO4 (LiPON) electrolyte, 90% capacity loss after 100 cycles was observed.
  • The capacity fade is due to the blockage of Li & Al diffusion paths from the Li-Al-O alloy on the anode surface. The alloy formation is irreversible & is driven by surface reaction.

References

M.S. Leite, D. Ruzmetov, Z. Li, L.A. Bendersky, N..C. Bartelt, A. Kolmakov, A.A. Talin, J. Materials Chem A (2014) 2, 20552. DOI: 10.1039/c4ta03716b.

Summary

By combining real-time scanning electron microscopy in ultra-high vacuum with electrochemical cycling, scientists at Nanostructures for Electrical Energy Storage (NEES), a DOE Energy Frontier Research Center (EFRC), characterized and quantified the dynamic degradation of Al anodes in Li-ion all-solid-state batteries, a promising alternative to the Si anodes for ultra lightweight energy storage systems.

In the Si nanostructure systems, reducing the diameters or thickness to below 100nm can better accommodate volumetric strains due to lithiation/de-lithiation. In contrast, Al nanowires and films degrade more rapidly as dimensions become smaller, as observed by Hudak et al. This loss of capacity was attributed to the rapid loss of electrical conductivity due to fracturing.

Here, researchers found that the loss in battery capacity is directly related to the irreversible Li trapping in the AlLi alloy formations on the anode surface, which remain in electrical contact with Al during cycling. The mechanism is based on the surface oxidation of AlLi causing the blockage of Al and Li diffusion pathways, necessary for the breakdown of AlLi alloy at room temperature, resulting in Al-Li-O alloy irreversibility.

Acknowledgements

Work was performed at UMD, NIST & SNL-CA, partially supported by EFRC NEES.

Rectification of Surface Charge in Single Conical Nanopore

Rectification of Surface Charge in Single Conical Nanopore

Scientific Achievement

Monovalent cations can modulate the effective surface charge density & influence cation mobility within conical nanopores. Experimental results are explained by all-atom molecular dynamics simulations.

Significance and Impact

For a quantitative description of ionic transport within pores at the nanoscale, atomic details in the model systems is needed to capture ionic current properties by different monovalent cations.

Research Details

Current-voltage curves through single asymmetric nanopores were recorded in 3 electrolytes, LiCl, NaCl & KCl.

In contrast to a continuum model prediction based on bulk behavior, highest degree of rectification was shown for KCl & lowest for LiCl.

Experimental observations were explained by all-atom molecular dynamics simulations, showing differential cation bindings to charged groups on the pore walls.

Summary

Functioning of nanoforest architectures is dependent on the ionic distributions inside the nano-constrictions, determined by the access of ions as well as ionic interactions with charged surfaces. A model system of single conically shaped nanopores provided insight into which interactions of ions with surfaces are important for ionic transport at the nanoscale.

NEES researchers at University of California, Irvine, recorded the current-voltage curves through single asymmetric nanopores were recorded in three electrolytes, LiCl, NaCl, and KCl. In contrast to the predictions of a continuum model based on the Poisson-Nernst-Planck equations, rectification degree of the pores was the highest for KCl, and the lowest for LiCl. The currents recorded in LiCl as the bulk electrolyte were also several times lower than predicted from relative bulk conductivities of KCl and NaCl. The experimental observations were explained by all-atom molecular dynamics simulations, which revealed differential binding between Li+, Na+, and K+ ions and charged groups on the pore wall, resulting in changes to both the effective surface charge of the nanopore and cation mobility within the pore.

This study alerts researchers that monovalent cations can modulate surface charge density of surfaces and influence ionic transport in pores at the nanoscale. Consequently, quantitative description of ionic transport might require introducing atomistic details of some elements of the system.

References

Trevor Gamble, Karl Decker, Timothy S. Plett, Matthew Pevarnik, Jan-Frederik Pietschmann, Ivan Vlassiouk, Aleksei Aksimentiev, and Zuzanna S. Siwy, "Rectification of Ion Current in Nanopores Depends on the Type of Monovalent Cations: Experiments and Modeling", J. Phys Chem C, 118, 9809-9819 (2014) DOI: 10.1021/jp501492g

Conductive Metal-Organic Frameworks (MOFs)

Conductive Metal-Organic Frameworks (MOFs)

A. Talin (SNL) - Thrust E

Scientific Achievement

Tetracyanoquinodimethane (TCNQ) molecules create an electrically conducting path through the Cu3(BTC)2 MOF structure.

Significance and Impact

A strategy in which nanopores are infiltrated with redox-active, conjugated guest molecules was developed, achieving MOFs with tunable, air-stable electrical conductivity increase over six orders of magnitude.

Research Details

  • Conductivity arises from TCNQ guest molecules bridging the binuclear copper paddlewheels in the framework, leading to strong electronic coupling between the dimeric Cu subunits.
  • By altering the exposure time to TCNQ, the magnitude of the conductivity could be controlled.

Summary

Scientists from Nanostructures for Electrical Energy Storage, a DOE Energy Frontier Research Center, have added something new to a family of engineered, high-tech materials called metal-organic frameworks (MOFs): the ability to conduct electricity. This breakthrough-conductive MOFs-has the potential to make these already remarkable materials even more useful, particularly for applications that require charge transport, such as super capacitors, battery electrodes, and electrocatalysts.

MOFs are three-dimensional crystalline materials with nanoscale pores made up of metal ions linked by various organic molecules. These frameworks have huge surface areas but are typically poor electrical conductors (in this case, a well-known copper containing MOF known as HKUST-1). The guest molecules introduced is tetracyanoquinodimethane (TCNQ), another insulating material in itself. But in combination with other organic molecules or metal ions such as copper, TCNQ forms electron-conducting charge-transfer complexes.

NEES Scientists infused and bound electron-sharing molecules into MOF thin films to create a material that is stable in air and approximately a million times more conductive than the unaltered MOF. Based on several spectroscopic experiments, the guest molecules serve two important purposes: they create additional bridges between the metal ions-copper, in this case-and they accept electrical charges.

Lithographically Patterned Capacitor with Ultra-long Nanowires (NWs)

Lithographically Patterned Capacitor with Ultra-long Nanowires (NWs)

Scientific Achievement

Interdigitated capacitors were made of Au/MnO2 NWs fabricated using lithographically patterned nanowire electrodeposition (LPNE). Each of the 750 NWs is 2.5 mm in length in a core-shell configuration.

Significance and Impact

Hybrid supercapacitors consisting of ultra-long Au/MnO2 NWs were shown to retain 90% of the capacitymax for 6,000 cycles while simultaneously delivering high power and high capacity.

Research Details

Capacitor contains 750 NWs 2.5 mm in length, each with a Au NW core (40 nm X ?200 nm) encapsulated within a hemicylindrical shell of d-phase MnO2 of controllable thickness, were patterned onto a planar glass surface.

Max power density of 165 kW/kg coupled with max energy density of 24 Wh/kg was demonstrated.

Summary

Building on their previous work developing the horizontal Au@d-MnO2 nanowires core-shell capacitor system using lithographically patterned nanowire electrodeposition (LPNE), NEES researchers at University of California, Irvine, now expanded the system to arrays of 750 nanowires that are each 2.5 mm in total length. In their work with MnO2 shell thickness of 68 nm, they demonstrated the capability to produce high power density (Pmax at 165 kW/kg) and high energy density (Emax at 24Wh/kg). A cycle life up to 6000 cycles retaining 90% of the maximum capacity was demonstrated at 100 mV/2 across a 1.2 V window.

These nanowires each consists of a gold nanowire core encapsulated within a hemicylindrical shell of d-phase MnO2 with thickness 60-220 nm. They are lithographically patterned in an interdigitated horizontal pattern onto glass. The sensitivity of the capacity to degradation processes are cumulative with increasing nanowire length, caused by possible failure modes such as nanowire scission, delamination of MnO2 shell from the gold core, and dissolution of MnO2.

These data support the conclusion that durable, ultrahigh power density capacitors can be fabricated based upon millimeter-scale metal core?metal oxide shell nanowire arrays.

References

W. Yan, M. L. Thai, R. Dutta, X. Li, W. Xing and R. Penner, "A Lithographically Patterned Capacitor With Horizontal Nanowires of Length 2.5 mm" ACS Appl. Mater. Interfaces (2014) DOI:10.1021/am500051d

Charge-Transfer Resistance of Carbon Nanotube (CNT) Interfaces

Charge-Transfer Resistance of Carbon Nanotube (CNT) Interfaces

P.G. Collins Group (UCI) - Thrust A

Scientific Achievement

Interfacial charge-transfer resistivity of CNTs is found to be as low as 30 m? cm2 and approximately 50X smaller than Pt or HOPG surfaces. This ultra-low resistivity arises from CNTs efficient utilization of surface area within 3D electrochemical composites, an effect that had previously been under-estimated by a factor of 20 in models that assumed a simple proportionality to area A.

Significance and Impact

This work provides the first complete model for CNT charge-transfer and design rules for 3D composites that include them. Compared to other electrodes, ultra-small, filamentous CNT networks have unique benefits for high-power cycling of energy storage composites.

Research Details

  • Individual single-walled nanotubes (SWNTs) coated with MnO2 provide an ideal model system to probe charge/discharge interfacial charge transfer and impedances at the nanoscale.
  • SWNTs, MWNTs, and Pt all behave as ultramicroelectrodes with area-normalized interface resistance Ri = (90 +- 10 ?m ) A-1/2 that is independent of surface chemistry, defects, and bandstructure.
  • The unusual A-1/2 scaling indicates that SWNT electronic impedances are tightly coupled to ionic transport in the MnO2.

Summary

In principle, heterogeneous composites provide opportunities to enhance capacity and power in energy-storage devices by taking advantage of the best properties of different materials. Recently, many experimental papers have reported surprising improvements in power delivery from charge-storage materials mixed with nanocarbons like carbon nanotubes or graphene.

Using a unique model system of an individual single-walled carbon nanotube (SWNT) loaded with a thin film of MnO2, scientists at NEES EFRC have explained the main cause of the enhancements caused by these nanocarbons. SWNTs obey unusual areal scaling rules that are well established for ultramicroelectrodes but not usually applicable to 3D composites. When optimally dispersed, SWNTs are super-efficient in the utilization of their distributed area, whereas a conventional analysis would incorrectly underestimate SWNT charge-transfer impedances by factors of 20 or more. New design rules from the NEES EFRC work surprisingly show that the benefits of SWNTs are not related to intrinsic conductivity, bandstructure, or surface chemistry, and they re-emphasize the importance of coupling between electronic and ionic transport in nanoscale systems.

References

B.L. Corso, I. Perez, T. Sheps, P.C. Sims, O.T. Gu?l, and P.G. Collins (2014). "Electrochemical Charge-Transfer Resistance in Carbon Nanotube Composites" Nano Letters. DOI: 10.1021/nl404349g

Pioneering Ionic Current Probe of MnO2 Mesorods in Single Nanopore

Pioneering Ionic Current Probe of MnO2 Mesorods in Single Nanopore

Siwy and Lee Research Groups - Thrust A

Scientific Achievement

For the first time, researchers have applied ionic current to directly probe both structural and electrochemical characterizations of nanoporous manganese oxide mesorods deposited in a single polymer pore. The present results showed the existence of nanovoids with effective diameters <5nm, which carried negative surface charge with metal cation selectivity.

Significance and Impact

Understanding ion transport mechanisms in confined space of metal oxide materials can have a revolutionary impact in electrochemical energy storage technology, as applied to lithium ion batteries and supercapacitors. The reported novel method using ionic current provides a valuable probe for charge polarity and species-selective transport in electrode nanomaterials.

Research Details

  • Resistance and current measurements supported a meshlike porous character for the deposited MnO2.
  • Current saturation experiments at low electrolyte concentration showed the existence of nanovoids with effective diameters less than 5nm.
  • Reversal potential measurements under salt concentration gradient indicated negatively charged surface area and selectivity for metal cations such as Li+ but not protons.

Summary

Research groups at Nanostructures of Electrical Energy Storage (NEES) EFRC applied ionic current as a novel method to probe the nanoporous structures and physical properties of MnO2 deposited as mesorods inside a single pore in a polymer film of polyethylene terephthalate (PET). Similar approach had been applied previously to polymer nanopores and biological channels, but not in application to metal oxide materials such as MnO2. Although data on characterization of manganese oxide in bulk are available, little information is forth coming on its transport mechanism under confined structural configurations, as in the case of the present study. Resistance and current measurements supported a meshlike porous character for the deposited MnO2. Additional current saturation experiment at low electrolyte concentration confirmed that the effective diameters of the voids are in nanometer range. Under cross-membrane salt concentration gradient, reversal potential measurements indicated that the nanovoids are negatively charged and are selective for metal cations such as Li+, but not protons. Observed nonlinear current-voltage curves suggested an asymmetrical nanopore structures with directional selectivity. These results laid the foundation for future investigation of the MnO2 nanotructures during charging/discharge cycles.

References

T. Gamble, E. Gillette, S.B. Lee and Z.S. Siwy, "Probing Porous Structure of Single Manganese Oxide Mesorods with Ionic Current" The Journal of Physical Chemistry C (2013), 117 (47), pp 24836-24842. http://pubs.acs.org/doi/pdf/10.1021/jp408107z

Nanoglue for Hybrid Nanostructures in Na Ion Anodes

Nanoglue for Hybrid Nanostructures in Na Ion Anodes

Liangbing Hu Group (UMD)

Scientific Achievement

In situ TEM studies show that atomic layer deposition (ALD)-Al2O3 coating forms an artificial solid electrolyte interface (SEI) layer, which robustly anchors Sn nanoparticles (SnNPs) to carbon nanofibers (CNF) and undergoes volume changes synergistically with the SnNP core during sodiation cycling.

Significance and Impact

Highly conformal coatings offer mechanisms for robust binding of nanoparticle active storage material to current-collecting substrates, demonstrated here for ALD-Al2O3 to bind Sn nanoparticles to carbon fibers.

Research Details

  • Na-Al-O layer formed irreversibly from a 3-5nm SnO2 sub layer & ALD-Al2O3 coating in initial surface sodiation.
  • Coin-cell battery using liquid electrolyte showed stabilized specific capacity of 650 (vs 110) mAh/g after 40 cycles with (vs without) ALD-Al2O3 coating.
  • Finite element simulations: Na-Al-O layer (thickness-dep) delays debonding initiation & allows capacity retention beyond interfacial delamination at 67% volume expansion.

Summary

Combined with tin's high theoretical specific capacity, Na ion batteries (NIBs) have increasingly become an attractive candidate for grid-scale energy storage due to its low cost and abundance. But as Si is for Li ion batteries, Sn experiences huge volume expansion (420%) upon sodiation, a grand challenge for both structural and cycle stability during charging/discharging. Researchers at Nanostructures for Electrical Energy Storage (NEES) EFRC have conducted a real time mechanistic characterization of atomic-layer-deposition Al2O3 (ADL-Al2O3) coatings on the hybrid nanostructure anode (SnNPS@CNF) during sodiation/desodiation.

In situ TEM electrochemical tests demonstrated the irreversible formation of Na-Al-O layer as an artificial solid electrolyte interface (SEI), acting as an ion conductive nanoglue that deforms coherently in synch with volume expansion of the encased nanoparticle core after repeated cycling. In coin-cell battery made from ALD-Al2O3 anode materials, a charge capacity of 650 mAh/g was maintained without capacity fade for 40 cycles, compared to 110 mAh/g of the bare SnNPs@CNF. Finite Element Method simulations supported the observation that Na-Al-O layer mechanically protects the contact between SnNPs and CNF during volume expansion, even after initiation of debonding.

Reference

Xiaogang Han, Yang Liu, Zheng Jia, Yu-Chen Chen, Jiayu Wan, Nicholas Weadock, Karen J. Gaskell, Teng Li and Liangbing Hu, Nano Letters (2014) 14, 139-147. DOI: 10.1021/nl4035626

Si Nanowire Anodes with Long Cycle Lives

Si Nanowire Anodes with Long Cycle Lives

S. Tom Picraux Group (LANL)

Scientific Achievement

Nanoporous AAO template has been used to guide VLS growth of Si nanowires and prevent Si island formation by reaction with underlying metal collector, yielding excellent cycle life (> 1100) and specific capacity (1000 mAh/g) for high density SiNW arrays.

Significance and Impact

Achieving viable nanostructured electrode arrays requires careful attention to all relevant reaction pathways and 3-D design.

Research Details

  • High density SiNW forests were deposited on stainless steel using bottom-up vapor-liquid-solid growth by chemical vapor deposition through AAO templates containing high density of sub-100nm nanopores.
  • Specific capacity measurements & SEM images show that elimination of milli- & microscale Si underlayer islands allows stable SiNW-current collector contacts during cycling, retaining Si's high charge capacity.

Summary

Interests in silicon's high theoretical capacity (4200 mA-h/g) as an anode in lithium-ion batteries and the accompanied challenge of its huge volume expansion (~300%) during charge/discharge cycling, have driven research effort into high density one-dimensional Si nanomaterials. Although the use of nanoscale Si materials allows lateral relaxation and reduces mechanical stress of the Si electrodes, cycle stability >50 under high charging rates has yet to be realized, a crucial step toward industrial battery application. Scientists at Nanostructures for Electrical Energy Storage (NEES) EFRC have successfully improved cycle stability to >1100 while retaining 1000 mA-h/g specific capacities under high cycle rates (10C) for Si nanowires (SiNW) as Li-ion anode.

They discovered the existence of underlayer Si islands at the interface of the SiNW and the current collector, as an auxiliary deposits during chemical vapor deposition (CVD) synthesis process. SEM images showed cracks in these islands that prevented retention of specific capacity beyond 200 cycles, due to disruption of electrical contact between SiNW and current collector. Modifications in the bottom-up, vapor-liquid-solid (VLS) growth process employing anodized aluminum oxide (Al2O3) templates containing a high density of sub-100 nm nanopores eliminated the underlayer Si island formation.

Reference

Jeong-Hyun Cho and S. Tom Picraux, Nano Letters (2013) 13, 5740-5747. http://dx.doi.org/10.1021/nl4036498.

Hoop-Strong Nanotubes for Battery Electrodes

Hoop-Strong Nanotubes for Battery Electrodes

Scientific Achievement

Chemically inert Ni coating outside Si nanotubes (NTs) constrains Si to expand inward upon lithiation, enhancing structural stability and charge/discharge cyclability.

Significance and Impact

Functional coatings can prevent structural instability while enhancing electron transport to exploit Si as high capacity nanostructured anode.

Research Details

  • In-situ TEM shows outward expansion of Si NTs if uncoated or with native oxide, leading to structural instability of Si and solid-electrolyte-interphase (SEI) during repeated cycling.
  • Ni outer coating of 16 nm prevents outer surface expansion, forcing lithiation-induced expansion inward.
  • Ni-coated SiNTs show dramatically better capacity retention (85% at 100 cycles)

Reference

K. Karki, Y. Zhu, Y. Liu, C.-F. Sun, L. Hu, Y. Wang, C. Wang, and J. Cumings. "Hoop-Strong Nanotubes for Battery Electrodes." ACS Nano (2013) DOI: 10.1021/nn403895h

Interface and Bandgap Engineering at the Nanoscale

Interface and Bandgap Engineering at the Nanoscale

Scientific Achievement

First direct observation that radial hetero-structuring can completely suppress the commonly observed surface insertion of Li ions, and can exclusively induce axial lithiation.

Significance and Impact

Interface and bandgap engineering can be utilized to control ionic transport/insertion at the nanoscale, a potential new tool to control volume expansion of high-energy anode materials.

Research Details

  • Deposition of a conformal, epitaxial, ultrathin (~1 nm) Si shell on Ge NWs suppresses surface Li+ insertion and induces axial lithiation along the <111> direction in a layer-by-layer fashion.
  • Pure axial lithiation on Ge/Si core/shell NWs showed Li+ ions do not penetrate the ultrathin surface Si shell.
  • Core-shell lithiation was observed on pure Ge nanowires (NWs) indicating inward diffusion from surface to core.

References

Yang Liu, Xiao Hua Liu, Binh-Minh Nguyen, Jinkyoung Yoo, John P. Sullivan, S. Tom Picraux, Jian Yu Huang, and Shadi A. Dayeh, "Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale", Nano Letters, 13, 4876-4883 (2013) DOI: 10.1021/nl4027549

Summary

This work presents the first direct observation of the dramatic interfacial effect on ionic transport at the nanoscale. This is also the first demonstration that the lithiation behavior of a nanostructure can be controlled by interface and bandgap engineering, a potential tool to minimize mechanical degradation and to improve battery performance.

Deposition of a conformal, epitaxial and ultrathin (down to ~1 nm) silicon (Si) shell on germanium (Ge) nanowires can completely suppress the commonly observed surface insertion of Li ions in all reported nanoscale systems to date. The Ge/Si core/shell nanostructure induces a purely axial lithiation along the <111> direction in a layer-by-layer fashion. The presence of the Si shell slows down the lithiation reaction at the surface and forms a chemical potential barrier that blocks Li ion diffusion through the shell.

Diagnosing Nanoelectronics with Electron Holography (EH)

Diagnosing Nanoelectronics with Electron Holography (EH)

Scientific Achievement

Using off-axis electron holography (EH), we made a direct observation of the electric potential distribution in the vicinity of a single carbon nanotube (CNT). The uneven resistivity of different contacts causes an asymmetric EH phase shift.

Significance and Impact

EH, combined with finite element calculations, offers a unique and precise method to identify & quantify the electrostatic potential drops across the metal contacts at the interface and along the length of the nanotube, with a spatial resolution of 10 nm or less.

Research Details

  • Prototype device consists of a CNT connecting with two closely spaced contacts.
  • Multiwalled nanotubes (MWNTs) were used, synthesized by chemical vapor deposition, with diameters ~50 nm and lengths up to a few µms.
  • COMSOL Multiphysics calculations using a potential difference model suggest one bad contact at the ground electrode, confirmed by the experimentally observed image profile.

References

Kai He and John Cumings, "Diagnosing Nanoelectronic Components Using Coherent Electrons" Nano Letters (2013) 13, 4815-4819 DOI: 10.1021/nl402509c

Work was performed at University of Maryland.

Summary

For detecting defective contacts in nanoelectronic devices, external transport measurements often yield unsatisfactory results. Microscopic characterization, such as electron beam-induced current imaging can provide locations in more detail, but without a quantifiable electrostatic potential profile. To solve these problems, investigators from EFRC-NEES demonstrated that off-axis electron holograph (EH), which utilizes coherent electrons in a transmission electron microscope (TEM}, in combination with finite element calculations can be a powerful technique to provide direct spatial details of the potential at the nanometer scale. The system studied was a single carbon nanotube (CNT) with two closely-space electrical contacts, as a prototype structure for nanoelectric devices. Two different resistive contacts give rise to asymmetric features of the phase shift. When combined with finite element calculations (COMSOL Multi physics software with a potential difference model), the results demonstrated the ability to separately observe the electrostatic potential drops across the metal contacts and the interface, and along the length of CNT itself. The technique is extensible to a wide array of important nanoelectronic systems.

Energy Textile from Weavable High-Capacity Electrodes

Energy Textile from Weavable High-Capacity Electrodes

YuHuang Wang Group (UMD)

Scientific Achievement

A highly scalable process was developed to produce a mechanically robust and electrically conductive Si-carbon nanotube (CNT) composite yarn, with discharge capacity five times that of graphite anodes.

Significance and Impact

NEES researchers have taken a further step to translate their scientific advance in hybrid Si@CNT nanostructures into fabric-like form factors amenable to evaluation as prototype battery technology.

Research Details

  • Vertically aligned CNT arrays (50 nm in diameter,750 ?m in length) were spun into a porous yarn, layered with a-Si via low pressure CVD at Si:CNT mass ratio of 0.63.
  • Si-CNT composite yarn showed a discharge capacity of 2200 mA h/g based on the Si mass, in the first 10 cycles with a cutoff of 0.1-1.0 V.
  • Mechanically durable, flexible, scalable, with high electrical conductivity
  • Fabrication control on porosity (twisting force) & Si mass

Summary

Adopting from textile technology, scientist at NEES EFRC designed and synthesized a Si-carbon nanotube (CNT) composite yarn that demonstrates a multitude of materials qualities desirable for electrical energy storage. It is mechanically strong, flexible, and highly scalable and possesses high electrical conductivity. A Li-storage capacity as high as 2200 mA h/g based on the Si mass in the first 10 cycles with a cutoff of 0.1-1.0 V was achieved, five times higher than that of the typical graphite electrodes. This high capacitance also exceeds previous reports from fabrication of supercapacitor yarns by electrodeposition using MnOx as the active materials.

Vertically aligned CTN arrays (each CNT approximately 750 ?m in length and 50 nm in diameter) grown by chemical vapor deposition (CVD) were spun into a yarn, followed by low pressure CVD of amorphous Si at a Si:CNT mass ratio of 0.63. The loading of active material Si per area for this yarn is ?2.5 mg/ and can be controlled by the growth time for Si. The porosity of the yarns can be controlled by cm2 adjusting the twisting force during the spinning process. The resulting yarn is highly porous, with ability to accommodate the volume expansion of Si upon electrochemical charging.

Reference

Chuan-Fu Sun, Hongli Zhu, Edward B. Baker III, Morihiro Okada, Jiayu Wan, Adrian Ghemes, Yoku Inoue, Liangbing Hu, YuHuang Wang Nano Energy (2013) 2, 987. http://dx.doi.org/10.1016/j.nanoen.2013.03.020

Real Time Observation of Breathable Silicon Beads on a Robust Carbon Nanotube String

Real Time Observation of Breathable Silicon Beads on a Robust Carbon Nanotube String

YuHuang Wang's research group - Thrust B

Scientific Achievement

Researchers have built upon the heterogeneous carbon nanotube (CNT) as the current collector and Si as the ion storage material of ultrahigh capacity through selective chemical functionalization of the Si-CNT interface layer. A stable silicon nanobeads on a CNT structure was formed that radially swells and shrinks without cracking while maintaining electrical connectivity during lithiation/delithiation cycling. Chemical functional groups on the Si/C interface localize, separate, and anchor the beads, making Si's high energy storage capacity attainable.

Significance and Impact

Interfacial structural instability is a long-standing challenge in lithium ion battery anodes where silicon expands ~300% in volume during lithiation/delithiatin cycling. Studies here demonstrated that chemically controlled Si-Carbon nanotube interface can accommodate the huge volume change during lithiation/delithiation cycling, with stable structural integrity (no cracking) and intact electrical conductivity. This is a scientific breakthrough to harness silicon's potential for ultrahigh capacity as an anode material.

Summary

The Nanostructures for Electrical Energy Storage (NEES) EFRC investigates how the structure of materials at the nanoscale affects electrical energy storage. By growing silicon into the geometry of anchored beads on a carbon nanotube string, researchers created a nanostructure that can withstand significant radial expansion during lithiation/delithiation cycles without cracking.

Earlier NEES work reported that silicon nanoparticles under the critical diameter of 150 nm did not fracture when lithiated (DOI: 10.1021/nn204476h). Building on work that confined the propagation of functional groups on carbon nanotubes (DOI: 10.1038/ncomms1384), researchers grew silicon beads 200 nm in diameter that were anchored on carboxylic acid functional groups attached to the CNT. As observed by In situ TEM, cracks formed during charging and discharging in the tubular core-shell nanostructures but not in the functionalized beaded-string nanostructures. DFT and FEM computations revealed that in both cases tensile stresses developed in the unlithiated silicon due to the large volume expansion in the lithiated silicon phase, but strong covalent bond between the carboxylic acid function group and silicon prevented the silicon beads from cracking. These findings provide important new insights in the synthesis of high performance Si electrodes, laying a foundation for the next generation lithium ion batteries.

Research Details

  • Each silicon bead of 200 nm in diameter was anchored on carboxylic acid functional groups attached to the CNT surface.
  • During lithiation, the lithiated-silicon phase grows starting both from the outer bead surface and from the CNT-silicon interface.
  • In situ TEM observation showed that cracks formed in the tubular core-shell nanostructures but not in the functionalized beaded-string nanostructures during charging and discharging cycling.
  • DFT and FEM computations revealed that in both cases tensile stresses developed in the unlithiated silicon due to the large volume expansion in the lithiated silicon phase, but strong covalent bond between the carboxylic acid function group and silicon prevented the silicon beads from cracking.

References

Chuan-Fu Sun, Khim Karki, Zheng Jia, Hongwei Liao, Yin Zhang, Teng Li, Yue Qi, John Cumings, Gary W. Rubloff, and YuHuang Wang, "A Beaded-String Silicon Anode" ACS Nano (2013), 7, pp 2717-24. http://dx.doi.org/10.1021/nn4001512

Shunliu Deng, Yin Zhang, Alexandra H. Brozena, Maricris Lodriguito Mayes, Parag Banerjee, Wen-An Chiou, Gary W. Rubloff, George C. Schatz & YuHuang Wang, "Confined Propagation of Covalent Chemical reactions on Single-Walled Carbon Nanotubes" Nat Commun (2011) 2, p382. http//dx.doi.org/10.1038/ncomms1384

Lithium-Assisted Electrochemical Welding in Silicon Nanowire Battery Electrodes

Lithium-Assisted Electrochemical Welding in Silicon Nanowire Battery Electrodes

Thrust B

Accomplishment

  • Inside a TEM, the researchers crossed two silicon nanowires connected to a common substrate and touched the free end of one wire to lithium metal with a LiO2 oxide coating. With an applied bias to the substrate, both nanowires lithiated and fused, creating a lithium-ion-conducting region between them.
  • Applying force to the cantilevered nanowires with the probe tip, the researchers estimated the shear strength of the node. Their analytical models gave the minimum shear strength to be 200 MPa, which compares to the strength of stainless steel (~205 MPa) and ceramic silicon carbide (~200 MPa), and a finite-element model found 308 MPa.
  • The researchers proposed a kinetic driving force for the fusion. During lithiation, lithium ions bind with the surface silicon atoms on both wires simultaneously, creating metastable Li-Si bonds. During delithiation, the Si-Li-Si bonds break, and neighboring surface Si atoms bond to each other and fuse the nanowires together.

Significance

  • The formation of fused regions between silicon nanowires is a critical first step toward self-healing networks of silicon and energy storage materials that may otherwise crack and lose contact with a current collector during charging and discharging.
  • The room temperature method for creating the nodes is a facile method for controlling the number and location of fused regions between wires. Tailoring architectures of nanowire networks could provide a platform for further probing energy storage properties of nanostructures.

Reference

K. Karki et al., Lithium-Assisted Electrochemical Welding in Silicon Nanowire Battery Electrodes. Nano Letters 12, 1392 (2012/03/14, 2012). (coauthors: Karki, Khim; Epstein, Eric; Cho, Jeong-Hyun; Jia, Zheng; Li, Teng; Picraux, S. Tom; Wang, Chunsheng; Cumings, John) DOI: 10.1021/nl204063u

MnO2-Gold Nanowires with Ultra-high Charge Storage

MnO2-Gold Nanowires with Ultra-high Charge Storage

R Penner Group (UCI) - Thrust A

Scientific Achievement

MnO2 nanowire electrodes with gold cores show ultra-high charge storage for 1000 charging and discharging cycles. Controlling the structure down to 2 nm scale increased the specific capacity, and employing a dry electrolyte greatly increased cycle stability. The nanowires can precisely measure MnO2 electrochemical properties without ohmic drops, which would occur in real battery electrodes. The mesoporous-MnO2 nanowires synthesized here showed a hybrid ultra-high specific capacity of 1020 F/g at 5 mV/s.

Significance and Impact

Controlling structure at the nanoscale unlocks nearly the full theoretical specific capacity from two of three storage mechanisms in MnO2. The present design of core/shell nanowires allows researchers to partially decouple the mechanisms that contribute to the observed ultra-high capacity, making it feasible to further increase the capacity, rate, and cycle stability in MnO2 and other charge storage materials.

Research Details

  • The LPNE (lithographically patterned nanowire electrodeposition) technique deposited gold current collector nanowires 40 nm tall and 150-250 nm wide with MnO2 shells 50-250 nm thick. The porous nanowires were 0.1 - 1 cm long and were made of 2 nm-thick fibrils.
  • Electrochemical measurements were taken with 1.0 M LiClO4 in dry acetonitrile. The dry electrolyte prevents Mn2+ from dissolving out of the material and degrading it.
  • Because fibrils are so small, all Mn ions are close to surfaces and therefore can contribute to multiple charge storage mechanisms.

Summery

The Nanostructures for Electrical Energy Storage (NEES) EFRC investigates precision nanoparticles, nanowires, nanotubes, and nanometer-thick films to understand how the structure of materials affects charge storage mechanisms. Most reported nano-MnO2 electrodes show specific capacity values of 200-600 F/g, with a few reports up to 1380 F/g. The mesoporous-MnO2 nanowires synthesized here show hybrid ultra-high specific capacity of 1020 F/g at 5 mV/s. Cycle stability of this capacity up to 1000 cycles is achieved using dry acetonitrile electrolytes to eliminate dissolution of MnO2 during cycling.

The charge storage in MnO2 electrodes occurred via two faradaic and one nonfaradaic mechanisms. Electron transfers between ions in the electrolyte and Mn ions on the surface of MnO2 occur in faradaic pseudocapacitance mechanism. For the faradaic insertion mechanism, the electrolyte ions diffuse into the bulk MnO2 materials for the oxidation-reduction reactions to occur, which is the dominant charge storage in batteries. The one nonfaradaic charge storage in MnO2 occurs from double-layer capacitance at interface, which is the dominant charge storage in supercapacitors.

Using variable dependency of voltammetric current on potential scan rates, researchers were able to deconvolute insertion vs. noninsertion capacitances. They found that the high specific capacity in thin-shell MnO2 nanowires is largely due to changes in the faradaic insertion capacity, as it is rate-limited by cation diffusion in MnO2.

Reference

W. Yan, J. Y. Kim, W. Xing, K. C. Donavan, T. Ayvazian, R. M. Penner "Lithographically Patterned Gold/Manganese Dioxide Core/Shell Nanowires for High Capacity, High Rate, and High Cyclability Hybrid Electrical Energy Storage" Chemistry of Materials, 2012, 24, 2382-2390. DOI: 10.1021/cm3011474

First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces

First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces

Thrust E

Accomplishment

Quantum mechanical modeling of ethylene carbonate (EC) on the surface of Li0.6Mn2O4 (manganese spinel positive electrode) reveals an unexpected mechanism for EC breakdown. EC is a component of standard lithium-ion battery electrolytes. The local interaction between EC and the surface ions, not electron transfer (oxidation), initiates the EC decomposition, which eventually also weakens the electrode surface structure.

Significance

No previous atomistic modeling study has examined an organic solvent molecule on a lithium ion battery positive electrode oxide surface. Finding that the reactive manganese spinel surface is enough to destabilize EC contributes to understanding how electrolytes and positive electrodes degrade after many battery cycles.

Research Details

  • The breakdown of EC on negative electrode surfaces has been well studied. There the breakdown products lead to the considerable growth of a solid-electrolyte interphase, a composite that both inhibits and protects battery operation. On positive electrode surfaces, previous computational studies have modeled electrodes under ultra high vacuum conditions or water on oxide surfaces.
  • Since the intrinsic potential of EC is higher than manganese spinel, electrons are not expected to transfer between EC and the oxide, and if surface effects were not considered, no EC degradation therefore would be expected to occur despite their proximity to each other.
  • The molecular dynamics calculations provided a five-stage process for the decomposition of EC due to surface interactions. Additional ultrahigh vacuum condition calculations quantified the energy profile of the system across the stages and quantified the activation energy for reaction steps.

Reference

Kevin Leung, "First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces." J. Phys. Chem. C, 2012, 116 (18), pp 9852-9861 DOI: 10.1021/jp212415x

Highly flexible pseudocapacitor based on freestanding heterogeneous MnO2/conductive polymer nanowire arrays

Highly flexible pseudocapacitor based on freestanding heterogeneous MnO2/conductive polymer nanowire arrays

Thrust A

Scientific Achievement

Duay, et. al. synthesized asymmetric pseudocapacitors made of PEDOT nanowire anodes opposite coaxial MnO2/PEDOT nanowire cathodes. Outperforming symmetric pseudocapacitors of MnO2/PEDOT nanowires alone, the asymmetric cells had a larger voltage window leading to higher energy density, had high power density and cycle life, and maintained 86 % of the energy density in a highly flexed state.

Significance & Impact

Flexible, high capacity, high power energy storage is critical for widespread use of wearable device fabrics and flexible biomedical devices. By discerning the limitations of coaxial MnO2/PEDOT nanowires as anodes yet capitalizing on their properties as cathodes, the researchers improved the supercapacitor performance with a new architecture, providing for higher performance than existing supercapacitors reported in literature.

Research Details

  • MnO2/PEDOT coaxial nanowires show superior performance as electrodes in half-cell measurements. PEDOT acts as a flexible mechanical scaffold with high ion permeability and high electrical conductivity. MnO2 provides high energy density. This study examined their performance in full-cell pseudocapacitor configuration.
  • Experiments on the symmetric full cells showed that coaxial MnO2/PEDOT nanowires were limiting as anode materials because the wide voltage window of PEDOT overlapped the irreversibility window of MnO2. As the coaxial nanowire anode cycled, electrochemically inactive Mn2+ started to form, decreasing the amount of energy storage material available.
  • With reconfigured electrodes, the asymmetric pseudocapacitors of PEDOT anodes and MnO2/PEDOT cathodes reached 0.26 F total capacitance and a maximum voltage at 1.7 V. The energy density was 9.8 Wh/kg at a power density of 850 W/kg.

Reference

Jonathon Duay, Eleanor Gillette, Ran Liu, and Sang Bok Lee
Phys. Chem. Chem. Phys., 2012, 14, 3329-3337
DOI: 10.1039/C2CP00019A

Mapping of near field light and fabrication of complex nanopatterns by diffraction lithography

Mapping of near field light and fabrication of complex nanopatterns by diffraction lithography

Mark Reed Group (Yale) - Thrust B

Accomplishment

  • In a lithography experiment, the researchers quantified to very high accuracy the intensity of light diffracting from a mask onto photoresist by using a simple, single-step process. The diffraction followed classical Fresnel diffraction theory.
  • Taking advantage of the Poisson spot from the diffraction, they synthesized nanostructures with subwavelength features using UV light (? = 405 nm).
    • Starting with a photomask with opaque, circular discs, they synthesized a regular array of uniform single-crystal silicon nanotubes (~500 nm diameter, ~10 nm wall thickness, ~2 ?m tall). See Figure (a). The wall thickness of the silicon tubes is <~100 nm, less than ~1/4 of the used wavelength, which conventional optical lithographies can hardly achieve.
    • With masks containing opaque triangles and squares, they finely tuned the diffraction patterns to create nanostructures of photoresist with multifold symmetries. See Figures (b) and (c).

Significance

  • The diffraction technique and the ability to transfer patterns onto substrates open the opportunity for researchers to synthesize uniform nanostructures on a large scale with novel geometries. The method can fabricate sub-wavelength features on a large-scale. By tuning geometries, researchers may study the effect of structure on lithium-ion charge storage.
  • This sub-wavelength feature fabrication is simple synthesis process; it does not require any extra fabrication steps or extra experimental apparatus over conventional lithographies.
  • This process is less expensive than other non-optical (e.g. e-beam lithography) or alternative optical (e.g. interference) lithographies that are capable of sub-wavelength feature fabrication but are generally very costly and require a series of fabrication process and experimental apparatus.

Reference

Y. Jung, A. Vacic, Y. Sun, E. Hadjimichael, M. A. Reed, Mapping of near field light and fabrication of complex nanopatterns by diffraction lithography. Nanotechnology 23, 045301 (2012).
DOI: 10.1088/0957-4484/23/4/045301

Controlled Growth of Functional Groups Preserves Carbon Nanotube Properties

Controlled Growth of Functional Groups Preserves Carbon Nanotube Properties

Y.H. Yang (UMD) - Thrust D

Accomplishment

A new electrokinetic phenomenon - electroosmotic flow rectification (EOF) - has been demonstrated.
  • Developed a process for the controlled growth of functional groups on single-walled carbon nanotubes (SWNTs).
  • At the step edges of implanted sp3 defects, functional groups covalently bond in sequential steps via a Billups-Birch alkylcarboxylation reaction and extend around the tube into a band. Continuing the reaction propagates the functionalized regions of -(CH2)nCOOH to controlled length along the tube.
  • Functional regions induce water solubility while intact regions maintain the SWNTs remarkable optical and electronic properties.

Significance

  • Covalent functionalization for the first time is separated from defect nucleation on a graphene lattice. This mechanism affords control of carbon surface chemistry much like the separation of crystal growth from seed nucleation.
  • Covalent modification is required for many applications of CNTs, but adding the necessary functional groups usually destroys the electronic and optical properties that make CNTs an attractive material. By localizing the functional groups in bands, the properties are largely preserved.
  • This discovery affords new control in making use of the excellent electrical conductivity of CNTs in lithium-ion battery studies.

Collaborators

S. Deng, Y. Zhang, A. H. Brozena, M. L. Mayes, P. Banerjee, W. A. Chiou, G. W. Rubloff, G. C. Schatz, Y. H. Wang

Reference

Deng, S.; Zhang, Y.; Brozena, A. H.; Mayes, M. L.; Banerjee, P.; Chiou, W. A.; Rubloff, G. W.; Schatz, G.C.; Wang, Y. H.*, Nature Communications 2:382 (2011) | DOI: 10.1038/ncomms1384
Nature Website Article
Maryland NanoCenter News Story

Real-time Observation of Charging a Single SnO2 Nanowire Anode with Lithium

Real-time Observation of Charging a Single SnO2 Nanowire Anode with Lithium

Jianyu Huang, John P. Sullivan (Sandia) - Thrust D

Accomplishment

  • Creation of the first nano battery inside a transmission electron microscope (TEM) - consisting of a single SnO2 nanowire anode, an ionic liquid electrolyte (ILE), and bulk LiCoO2 cathode - and the in-situ observation of the lithiation of the SnO2 nanowire during electrochemical charging.
  • The low vapor pressure of the ILE allowed direct observation of the process at the nanoscale in the vacuum environment of a TEM.
    • The SnO2 nanowire was in contact with an ILE drop that contained Li salt and that touched the LiCoO2 cathode.
    • Real-time TEM observations show the reaction front moving through the nanowire.
  • Li moves primarily through bulk diffusion into SnO2, reacting to form Li2O and initiating mechanical changes observed as nanowire bending and distortion.

Significance

  • These are the first definitive experiments to directly observe an electrochemically-induced reaction in Li-ion battery materials in real time with atomic-scale resolution inside a TEM.
  • Such real-time observations at the nanoscale promise a new level of understanding for the fundamental mechanisms of Li-ion battery reactions, e.g. the relative roles of bulk diffusion versus surface Li diffusion.
  • The approach is general and may be applied to most any Li-ion battery material of suitable thin cross-section or even to other electrochemical phenomena, such as electrodeposition.

Collaborators

Jianyu Huang, John P. Sullivan (Sandia)
Chongmin Wang (Pacific Northwest Lab)
Scott Mao (Univ. Pittsburg)
Ju Li (Univ. Pennsylvania)

Supporting material

"In Situ Observation of the Electrochemical Lithiation of a Single SnO2 Nanowire Electrode" Jian Yu Huang, Li Zhong, Chong Min Wang, John P. Sullivan, Wu Xu, Li Qiang Zhang, Scott X. Mao,Nicholas S. Hudak, Xiao Hua Liu, Arunkumar Subramanian, Hongyou Fan, Liang Qi,Akihiro Kushima, and Ju Li Science 10 December 2010: 1515-1520. DOI: 10.1126/science.1198591

Reprinted with permission from AAAS.

Lithographic Patterning of α-MnO2 Nanowires On Glass

Lithographic Patterning of α-MnO2 Nanowires On Glass

R Penner Group (UCI) - Thrust A

Accomplishment

  • Lithographically patterned nanowire electrodeposition (LPNE) created high density, uniform, and long (mm-scale) nanowires in well-defined patterns, with adjustable width and height
  • LPNE couples relaxed top-down lithography with self-alignment of nanowires at patterned edges
  • Phase-pure α-MnO2 nanowires synthesized on glass by LPNE and verified by x-ray diffraction

Significance

  • LPNE method is broadly applicable to electrochemically deposited materials. It provides excellent control of nanowire geometry and surface area using simple lithography
  • LPNE enables high areal density of nanowire surfaces in well-defined patterns, with benefit to quality of characterization and to investigation of size-dependent behavior of electrochemical nanowires

Collaborators

Wenbo Yan, Yongan Yang, Reg Penner, UC Irvine Chemistry

Virus-Templated Silicon Anode for Li Ion Batteries

Virus-Templated Silicon Anode for Li Ion Batteries

CS Wang group (UMD) - Thrust B

Accomplishment

  • Assembly of a novel Si nanowire anode from Tobacco Mosaic Virus (TMV1cys) template
    • TMV's are identical nanotubes 300nm long, 4 nm ID, 18 nm OD
    • Self-assemble TMV on stainless steel through TMV 3' thiol group
    • Electroless deposition Ni current collector, then Si sputter deposition, onto TMV
    • Room temperature, neutral pH process
  • High capacities (3300mAh/g), nearly 10x capacity of graphite
  • Excellent charge-discharge cycling stability (0.20% loss per cycle at 1C), and consistent rate capabilities (46.4% at 4C) between 0 and 1.5 V

Significance

  • TMV provides precisely reproducible template for a nanostructured electrode, ideal for assessing the benefits of highly regular nanostructures
  • High capacity, comparable to other silicon nanostructured electrodes, demonstrates viability for TMV, biologically based nanoassembly strategy
  • TMV offers technology advantages: very lost cost, easily self-assembled on surfaces, highly reproducible nanostructures, in room temperature, neutral pH processes

Collaborators

Xilin Chen, Konstantinos Gerasopoulos, Juchen Guo, Adam Brown, Chunsheng Wang, Reza Ghodssi, James N. Culver

Supporting material

Xilin Chen et al, "Virus-Enabled Silicon Anode for Lithium-Ion Batteries", ACS Nano, Article ASAP (Aug. 13, 2010); DOI:10.1021/nn100963.

Redox Exchange Induced MnO2-Nanoparticle Enrichment in PEDOT Nanowires

Redox Exchange Induced MnO2-Nanoparticle Enrichment in PEDOT Nanowires

SB Lee Group (UMD) - Thrust A

Accomplishment

  • Synthesis of MnO2 nanoparticles (likely alpha) in PEDOT conductive polymer matrix, with control over nanoparticle size and ability to achieve uniform distribution in the PEDOT.
  • High electrochemical performance: very high specific capacitance (410 F/g) as the supercapacitor electrode materials as well as high Li ion storage capacity (300 mAh/g) as cathode materials of Li ion battery with good cyclability.
  • Revealed the mechanism of MnO2 nanoparticle formation in the PEDOT: triggered by the reduction of KMnO4 via the redox exchange of permanganate ions with the functional group 'S' on PEDOT.

Significance

  • Identified a new reaction pathway model to synthesize metal oxide nanoparticles in conductive polymer and graphitic carbon matrices.
  • Determined that the reaction primarily involves the S group on PEDOT, rather than the oxidized polymer backbone as previously believed
  • This synthesis route offers design flexibility to control and optimize MnO2 nanoparticle size for Li storage (insertion/desertion)

Collaborators

Ran Liu, Jonathon Duay, Zhe Gui, Stefanie Sherrill, Sung Kyoung Kim

Supporting Material

R. Liu, J. Duay, S.B. Lee. ACS Nano, 2010, 4 (7), pp 4299-4307. DOI: 10.1021/nn1010182

Predictions of Ethylene Carbonate Breakdown & Solid Electrolyte Interphase (SEI) Onset

Predictions of Ethylene Carbonate Breakdown & Solid Electrolyte Interphase (SEI) Onset

Kevin Leung (SNL) - Thrust C

Accomplishment

  • Computational methods predict the electrolyte decomposition products at an electrode-electrolyte interface during charging.
  • In the ab initio molecular dynamics model, two electrons transferring from LiC6 anode to ethylene carbonate-based electrolyte instigates breakdown.
  • Results indicate formation of CO and C2H4O22- reaction products, consistent with experiments1 but previously unpredicted by computation, as well as expected CO32- and C2H4 compounds.

Significance

  • The stability and safety of many electrochemical systems for electrical energy storage depend on a critical solid-electrolyte interphase (SEI), which forms from the reaction products and passivates the electrode layer.
  • The formation and character of the SEI layer is largely unclear.
  • This work is the first theoretical to address the initial chemical mechanisms of electrolyte breakdown at explicit electrode-liquid electrolyte interfaces and therefore SEI formation.

Collaborators

Kevin Leung (Sandia National Lab)
Joanne Budzien (Frostburg State)

Supporting Information

Leung & Budzien, Phys. Chem. Chem. Phys. 12, 6583 (2010)
1Onuki et al., JECS 155 A794 (2008).
2Lithium ion batteries: solid electrolyte interface, ed. Wang & Balbuena (Imperial College, London, 2004)

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Outer Wall Selectively Oxidized, Water-Soluble Double-Walled Carbon Nanotubes

Outer Wall Selectively Oxidized, Water-Soluble Double-Walled Carbon Nanotubes

YH Wang group (UMD) - Thrust B

Accomplishment

  • Selective oxidation of the outer wall of double-wall carbon nanotubes (DWCNTs) by oleum and nitric acid made the CNTs water soluble.
  • Inner wall remains intact, preserving CNT electrical conductivity properties. Outer wall is mostly functionalized, but intact regions enable contacts to inner walls.
  • Thin film conductivity of functionalized DWCNTs is up to 65% better than for SWCNTS

Significance

  • CNT benefits in conductivity are normally compromised by the functionalization often needed for nanoassembly and use of the CNTs
  • Two walls of DW-CNTs allows outer wall to be functionalized, providing for flexible design, assembly and use of CNT's in nanostructures, while retaining unique conductivity properties of CNTs

Collaborators

Brozena, A. H.; Moskowitz, J.; Shao, B.; Deng, S.-L.; Liao, H. W.; Gaskell, K. J.; Wang, Y. H.

Supporting material

Brozena, A. H.; Moskowitz, J.; Shao, B.; Deng, S.-L.; Liao, H. W.; Gaskell, K. J.; Wang, Y. H.* J. Am. Chem. Soc. 2010, 132, pp 3932-3938, DOI: 10.1021/ja910626u.

Electroosmotic Flow Rectification in Membranes with Asymmetric Nanopores - Demonstrating a 'Flow Diode'

Electroosmotic Flow Rectification in Membranes with Asymmetric Nanopores - Demonstrating a 'Flow Diode'

Charles Martin (UFL) - Thrust A

Accomplishment

A new electrokinetic phenomenon - electroosmotic flow rectification (EOF) - has been demonstrated.
  • EOF is a well-known electrokinetic phenomenon used to pump solutions through electrochemical devices.
  • We are studying EOF in nanopore membranes.
  • EOF rectification was demonstrated, for the first time, in mica membranes with pyramidally shaped nanopores (Figure 1 A).
  • One face of the membrane contains small openings (pore tips) and the other face has large pore openings (bases) (Figure 1 B).
  • This asymmetric pore shape, and the negative charge on the pore wall, are key to observing EOF rectification
When current passes through the nanopores:
  • EOF causes solution to be pumped through the membrane
  • With these asymmetric-pore membranes, the flow rate in one direction through the membrane is different than the flow rate in the opposite direction - EOF rectification (Figure 2)
  • Hence, the membrane is to flow what a diode is to current
  • he flow rate in the direction from base to tip is high and the flow rate from tip to base is low

Significance

  • A new electrokinetic phenomenon has been demonstrated and a first-order theoretical model was proposed.
  • The studies show the importance of surface charge and pore shape on the transport properties of nanopores, leading to a greater understanding of how confining electrolytes to nanopores affect transport properties in nanopore systems.
  • EOF measurements are also being used to quantitatively determine the surface charge density and a related parameter, the zeta potential.
  • Currently exploring electrochemical modulation of surface charge density.

Collaborators

Jin, P.; Mukaibo, H.; Horne, L. P.; Bishop, G. W.; Martin, C. R.

Reference

Leung & Budzien, Phys. Chem. Chem. Phys. 12, 6583 (2010)
J. Am. Chem. Soc. 2010, 132 (7), 2118-2119.