Highlights

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

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.

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.

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

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

Accomplishment

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.

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

Supporting Material

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 C₂H₄O₂^2- reaction products, consistent with experiments1 but previously unpredicted by computation, as well as expected CO₃^2- 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

Supporting Information

Electroosmotic Flow Rectification in Membranes with Asymmetric Nanopores – Demonstrating a ‘Flow Diode’

Charles Martin (UFL) - Thrust A

Accomplishment

• 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

• 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

Reference

Controlled Growth of Functional Groups Preserves Carbon Nanotube Properties

Y.H. Yang (UMD) - Thrust D

Accomplishment

• 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

Reference

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

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

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

First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on Li_xMn₂O₄(100) Surfaces

Thrust C

Accomplishment

Quantum mechanical modeling of ethylene carbonate (EC) on the surface of Li_0.6Mn₂O₄ (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