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Small pouch cells containing Si/graphite electrodes were systematically charged to 100, 120, 140, 160, 180 and 250% state of charge. Characterization of the Si/graphite electrode after the overcharge showed some features similar to those found in a pure graphite electrode. Since the amount of silicon in the electrode was ∼15 wt%, this was not unexpected. The effect of silicon was seen in the composition of the SEI layer and the trends in two components of the SEI layer.

Cells based on NMC/graphite, containing poly(vinylidene difluoride) (PVDF) binders in the positive and negative electrodes, were systematically overcharged to 100, 120, 140, 160, 180, and 250% state-of-charge (SOC). At 250% SOC the cell vented. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of the anodes showed several state-of-overcharge-dependent trends. Starting at 120% SOC, dendrites appeared and increased in concentration as the SOC increased. Dendrite morphology appeared to be dependent on whether the active material was on the “dull” or “shiny” side of the copper collector. Significantly more delamination of the active material from the collector was seen on the “shiny” side of the collector particularly at 180 and 250% SOC. Transition metals were detected at 120% SOC and increased in concentration as the SOC increased. There was considerable spatial heterogeneity in the microstructures across each laminate with several regions displaying complex layered structures.

Cells based on nickel manganese cobalt oxide (NMC)/graphite electrodes, which contained polyvinylidene difluoride (PVDF) binders in the electrodes, were systematically charged to 100, 120, 140, 160, 180, and 250% state of charge (SOC). Characterization of the anodes by inductively-coupled-plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), and high-performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC-ESI-MS) showed several extent-of-overcharge-dependent trends. The concentrations (by wt) of nickel, manganese, and cobalt in the negative electrode increased with SOC, but the metals remained in the same ratio as that of the positive. Electrolyte reaction products, such as LiF:LiPO3, increased with overcharge, as expected. Three organic products were found by HPLC-ESI-MS. From an analysis of the mass spectra, two of these compounds seem to be organophosphates, which were formed by the reaction of polymerized electrolyte decomposition products and PF3 or O=PF3. Their concentration tended to reach a constant ratio. The third was seen at 250% SOC only.

A stable solid electrolyte interphase (SEI) layer is key to high performing lithium ion and lithium metal batteries for metrics such as calendar and cycle life. The SEI must be mechanically robust to withstand large volumetric changes in anode materials such as lithium and silicon, so understanding the mechanical properties and behavior of the SEI is essential for the rational design of artificial SEI and anode form factors. The mechanical properties and mechanical failure of the SEI are challenging to study, because the SEI is thin at only ~10–200 nm thick and is air sensitive. Furthermore, the SEI changes as a function of electrode material, electrolyte and additives, temperature, potential, and formation protocols. A variety of in situ and ex situ techniques have been used to study the mechanics of the SEI on a variety of lithium ion battery anode candidates; however, there has not been a succinct review of the findings thus far. Because of the difficulty of isolating the true SEI and its mechanical properties, there have been a limited number of studies that can fully de-convolute the SEI from the anode it forms on. A review of past research will be helpful for culminating current knowledge and helping to inspire new innovations to better quantify and understand the mechanical behavior of the SEI. This review will summarize the different experimental and theoretical techniques used to study the mechanics of SEI on common lithium battery anodes and their strengths and weaknesses.

Silicon anodes are a promising choice to replace graphite anodes in lithium ion batteries for electric vehicle applications because they have a near 10-fold increase in theoretical energy density. Unfortunately, the solid electrolyte interphase (SEI) on silicon has been found to degrade both mechanically and chemically. The strain due to SEI formation versus the strain in the electrode due to lithiation and delithiation is not well understood. This work used in situ moiré microscopy to make in-plane strain measurements with an emphasis on the observed strain during SEI formation. A novel method was used to create smooth, 50 nm silicon thin films on 2 µm thick copper foil electrodes. A sample grating was patterned on the backside of the electrodes for use with an in-house moiré microscope (see figure). The reference grating was incorporated into the microscope and imaged onto the sample to form moiré patterns which were then used to calculate in-plane strain. Strain measurements were obtained in situ with constant current cycling and cyclic voltammetry. Initial cycles show a greater strain response during the formation of the SEI. Special care was taken to try and deconvolute the different sources of strain. The changes in strain due to variations in temperature, electrolyte additives, and the introduction of electrolyte were explored. Figure 1

Two types of small pouch cells based on LiFePO4/graphite, one containing a N-methyl pyrrolidinone (NMP)-soluble binder, poly(vinylidene difluoride), and the other an aqueous-soluble binder, styrene-butadiene rubber/carboxymethylcellulose were systematically charged to 100, 120, 140, 160, 180, and 250% state of charge (SOC). The cells were then discharged to 3.0 V at room temperature before being disassembled for postmortem analysis. Microstructural changes in the anode associated with increasing SOC were more pronounced in the aqueous processed cells in comparison to the NMP-processed cells. Dendrite formation was observed on the aqueous-processed anode at 120% SOC, while the NMP-processed anode surface does not show dendrites until 250% SOC. Overall, the aqueous-processed anode surfaces displayed more evidence of microstructural degradation as a function of increasing SOC. In the NMP-processed cells, four organic compounds on the anode surface were found to show a dependence on SOC, while only two displayed a similar dependency in the aqueous-processed cells. The nature of the binder changed the number and composition of the species found at the anode.

Cells based on NMC/graphite, containing styrene-butadiene rubber/carboxymethylcellulose binder in the anodes and pVdF latex/carboxymethylcellulose in the cathodes, were systematically overcharged to 100, 120, 140, 160, 180, 250% and 270% state-of-charge. The impact of the binder was characterized by elemental analysis, SEM, and HPLC. These results were compared to similar cells just using the poly (vinylidene difluoride) binder. Not only did the binder impact the rate of transition metal transport from the cathode to the anode, it also had a marked effect on the microstructure and composition of the materials on the anode surface.

Author(s): Bartelt, NC; Li, Y; Sugar, JD; Fenton, K; Kilcoyne, ALD; Shapiro, DA; Tyliszczak, T; Chueh, WC; El Gabaly, F | Abstract: Incorporation of ions into battery electrodes can lead to phase transformations. When multiparticle phase-transforming electrodes charge or discharge, two processes must occur in each particle: the new phase must nucleate, and then grow until the particle is fully charged or discharged. A fundamental question is which of these two processes is rate limiting. Here we construct a simple stochastic model that shows how the relative rate of nucleation compared with growth determines the particle state-of-charge distributions in the electrode. We find that the number of particles that are partially charged at any time increases as the relative nucleation rate increases. The maximum number of particles that are actively charging occurs just before the time when the first particles are becoming completely charged. By comparing measured state-of-charge distributions with the model, we determine the relative rate of nucleation. We apply this procedure to measurements of the evolution of particles in LiFePO4 cathodes and show we can account for the particle state-of-charge distribution as a function of the electrode state of charge.

The objectives of this report are as follows: elucidate degradation mechanisms, decomposition products, and abuse response for next generation silicon based anodes; and Understand the contribution of various materials properties and cell build parameters towards thermal runaway enthalpies. Quantify the contributions from particle size, composition, state of charge (SOC), electrolyte to active materials ratio, etc.

Anion receptors that bind strongly to fluoride anions in organic solvents can help dissolve the lithium fluoride discharge products of primary carbon monofluoride (CFx) batteries, thereby preventing the clogging of cathode surfaces and improving ion conductivity. The receptors are also potentially beneficial to rechargeable lithium ion and lithium air batteries. We apply Density Functional Theory (DFT) to show that an oxalate-based pentafluorophenyl-boron anion receptor binds as strongly, or more strongly, to fluoride anions than many phenyl-boron anion receptors proposed in the literature. Experimental data shows marked improvement in electrolyte conductivity when this oxalate anion receptor is present. The receptor is sufficiently electrophilic that organic solvent molecules compete with F– for boron-site binding, and specific solvent effects must be considered when predicting its F– affinity. To further illustrate the last point, we also perform computational studies on a geometrically constrained boron ester that exhibits much stronger gas-phase affinity for both F– and organic solvent molecules. After accounting for specific solvent effects, however, its net F– affinity is about the same as the simple oxalate-based anion receptor. Lastly, we propose that LiF dissolution in cyclic carbonate organic solvents, in the absence of anion receptors, is due mostly to the formation of ionicmore » aggregates, not isolated F– ions.« less

Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode lithium iron phosphate (LiFePO4; LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes.

We demonstrate an ex situ method for analyzing the chemistry of battery electrode particles after electrochemical cycling using the transmission electron microscope (TEM). The arrangement of particles during our analysis is the same as when the particles are being cycled. We start by sectioning LiFePO 4 battery electrodes using an ultramicrotome. We then show that mapping of the Fe 2+ and Fe 3+ oxidation state using energy-filtered TEM (EFTEM) and multivariate statistical analysis techniques can be used to determine the spatial distribution of Li in the particles. This approach is validated by comparison with scanning transmission X-ray microscopy (STXM) analysis of the same samples [Chueh et al. Nanoletters, 13 (3) (2013) 866–72]. EFTEM uses a parallel electron beam and reduces the electron-beam dose (and potential beam-induced damage) to the sample when compared to alternate techniques that use a focused probe (e.g. STEM–EELS). Our analysis confirms that under the charging conditions of the analyzed battery, mixed phase particles are rare and thus Li intercalation is limited by the nucleation of new phases.

Isothermal microcalorimetry is a promising characterization tool to both enhance understanding of degradation processes of silicon anodes in lithium-ion batteries, as well as to measure the impacts of efforts to improve the performance of these electrode materials. Isothermal microcalorimetry is a thermal characterization technique in which a sample is held in isothermal conditions while heat flow into / out of the sample to maintain a constant temperature is measured. This heat flow is determined by endothermic or exothermic reactions within the prepared sample. Isothermal titration calorimetry is a similar technique in which controlled amounts of a reactant are introduced to the sample during measurement, allowing capture of the reaction energies between materials. Both techniques are employed in our study of silicon anodes. Silicon has long been pursued as a next generation anode material for lithium-ion batteries due to its very high theoretical capacity relative to the currently employed graphite anode. So far, however, the practical realization of silicon anodes has been limited by both mechanical degradation caused by volume change during lithiation / delithiation as well as electrode surface instability that combine to shorten battery life and reduce capacity. Mitigation of these issues is being pursued through advanced binder materials, electrolyte additives, nano-scale architectures, and composite active materials.1 Previous microcalorimetry work on graphite anodes has demonstrated the ability of the technique to discern different lithiation reactions, measure the severity of parasitic losses, and identify the points of electrolyte degradation.2 Sandia National Laboratories is employing isothermal microcalorimetry to study new composite anodes of 15 wt.% silicon combined with standard graphite active material. Our microcalorimetry characterization approach uses a TA Instruments TAM IV with resolution to ± 200 nW to analyze samples of silicon-composite electrodes and compare their thermal signatures to baseline graphite samples to offer an understanding of the impact of silicon during electrode processing and fabrication, initial active material passivation upon electrolyte exposure, and electrochemical cycling of the battery. Our initial work focuses on the impacts of processing various silicon materials using a commonly employed LiPAA binder in an aqueous solvent. While silicon has a naturally passivating surface oxide layer, heat and gas generation can be observed during slurry mixing with the sub-micron sized silicon materials used in lithium-ion batteries. This reactivity can serve to rob capacity and increase material impedance before the electrode is even built into a battery. Later work will focus on characterizing the composite silicon electrodes during electrochemical cycling. When cycled in situ, the microcalorimeter can capture heat flow related to both surface electrolyte reactions and lithiation reactions of the silicon electrodes as well as the impacts of new binders, additives, or surface passivation approaches to mitigate these effects. References Yang J., Wang B. F., Wang K., Liu Y., Xie J. Y., Wen Z. S., Solid-State Lett., 6, A154 (2003). Krause L. J., Jensen L. D., Dahn J. R., Electrochem. Soc., 159, A937 (2012). Acknowledgment: Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Lithium-ion batteries have become the leading energy storage technology for automotive applications. During operation, a passivating or solid electrolyte interphase (SEI) layer forms on both electrodes. The structure and composition of the SEI layers at the electrode surfaces are complex under normal operating conditions [1-4]. However, there are very few reports in the literature that describe how the SEI changes under abusive conditions, such as overcharge. Understanding these changes is potentially very useful from a safety viewpoint since it might help us create robust cells that are tolerant of overcharge. Argonne, Oak Ridge and Sandia National Laboratories collaborated to understand the physical and chemical changes that occur in small pouch cells that were charged to 100, 120, 140, 160, 180 and ~250% SOC. The SEI layer on the graphite anode surface was characterized using scanning-electron microscopy, X-ray photoelectron spectroscopy, chemical analysis and high-performance liquid chromatography. We found that the structure and chemistry of the SEI layer was sensitive to the extent of overcharge, the binder in the cathode, and the nature of the cathode. These and other findings will be discussed. References Peled, J. Electrochem. Soc., 126 (1979) 2047. G. Thevenin and R.H. Muller, J. Electrochem. Soc., 134 (1987) 273. J. An, J. Li, C. Daniel, D. Mohanty, S. Nagpure, and D.L. Wood, Carbon, 105 (2016) 52–76. Aurbach, I. Weissman, A. Zaban, and O. Chuzid (Youngman), Electrochim Acta, 39 (1994) 51. We gratefully acknowledge support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. The work at Oak Ridge National Laboratory was sponsored by U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Applied Battery Research. Oak Ridge National Laboratory is managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration (NNSA) under contract DE-NA0003525. The U.S. government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the government.

This article will briefly discuss the genesis of the Li-ion chemistry and its meteoric rise to prominence, supplanting aqueous rechargeable batteries such as NiCd and NiMH. The principal intent of this article is to discuss the issues with thermal instability of common Li-ion electrolytes, which detract from the positive attributes of this chemistry. The development of an innovative low-cost non-flammable electrolyte will greatly improve the safety and reliability of lithium batteries, a key technological hurdle that must be overcome for the wider application of this chemistry. This article will also include the advancements made in combating/mitigating solvent flammability through the addition of fire retardants, fluoro-solvents, ionic liquids etc. The scope of the article will be limited to the flammability of non-aqueous solvents and will not include the thermal instability issues of anodes and cathodes. We will elaborate using examples from our in-house research aimed at mitigating solvent flammability by using hydrofluoro ethers (HFEs) as a cosolvent. Additionally, we will describe in-house capabilities for prototyping 18,650 cells and in-house thermal abuse test capabilities that allow us to evaluate materials and their thermal responses in actual cell configurations.

This report describes the preparation and performance of electro-spun polyester-based separators for lithium-ion batteries. Polyester fibers (200–500 nm) are electro-spun into nonwoven mats and pressed into separator sheets ∼55 μm thick. The resulting polyester separators are 75% porous, highly permeable (Gurley number (s/100 mL) = 6), and have good wettability with conventional carbonate-based electrolyte. In NMC/graphite lithium-ion cells, results show comparable performance to commercially available polyolefin separators (rate, capacity fade, and reactivity) but with improved thermal stability to >200 °C. The use of this higher melting temperature polymer separator is one approach to close the gap between potential thermal instabilities (softening, shrinking, melting, etc.) of separators and the onset of thermal runaway reactions of commonly used cathode materials.

By utilizing an equilibrium processing strategy that enables co-firing of oxides and base metals, a means to integrate the lithium-stable fast lithium-ion conductor lanthanum lithium tantalate directly with a thin copper foil current collector appropriate for a solid-state battery is presented. This resulting thin-film electrolyte possesses a room temperature lithium-ion conductivity of 1.5 × 10(-5) S cm(-1) , which has the potential to increase the power of a solid-state battery over current state of the art.

We have demonstrated a novel microfluidic technique for aqueous media, which uses super-hydrophobic materials to create microfluidic channels that are open to the atmosphere. We have demonstrated the ability to perform traditional electrokinetic operations such as ionic separations and electrophoresis using these devices. The rate of evaporation was studied and found to increase with decreasing channel size, which places a limitation on the minimum size of channel that could be used for such a device.

As lithium-ion battery technologies mature, the size and energy of these systems continues to increase (> 50 kWh for EVs); making safety and reliability of these high energy systems increasingly important. While most material advances for lithium-ion chemistries are directed toward improving cell performance (capacity, energy, cycle life, etc.), there are a variety of materials advancements that can be made to improve lithium-ion battery safety. Issues including energetic thermal runaway, electrolyte decomposition and flammability, anode SEI stability, and cell-level abuse tolerance continue to be critical safety concerns. This report highlights work with our collaborators to develop advanced materials to improve lithium-ion battery safety and abuse tolerance and to perform cell-level characterization of new materials.

Organosilicon electrolytes exhibit several important properties for use in lithium carbon monofluoride batteries, including high conductivity/low viscosity and thermal/electrochemical stability. Conjugation of an anion binding agent to the siloxane backbone of an organosilicon electrolyte creates a bi-functional electrolyte. The bi-functionality of the electrolyte is due to the ability of the conjugated polyethylene oxide moieties of the siloxane backbone to solvate lithium and thus control the ionic conductivity within the electrolyte, and the anion binding agent to bind the fluoride anion and thus facilitate lithium fluoride dissolution and preserve the porous structure of the carbon monofluoride cathode. The ability to control both the electrolyte conductivity and the electrode morphology/properties simultaneously can improve lithium electrolyte operation.

The application space for lithium ion cells is rapidly expanding to include transportation, grid storage, space system, and military power needs. Increasingly demanding mission requirements in these applications has necessitated higher energy and higher power energy storage systems. As the size and capacity of these power systems increases, so do the consequences of an off normal battery event (thermal runaway, short circuit, mechanical crush, etc.). Significant scientific efforts have been made to understand the consequences and requirements for runaway, understand policy decisions pertaining to battery safety, understand safety response in packs, and model thermal abuse of cells. Despite great progress, the overall technology still poses safety concerns for the public which, along with other considerations, have hindered the widespread adoption of many lithium based storage technologies. Several high-profile safety occurrences in recent history have brought the subject of lithium ion battery safety into the forefront of discussion within the community. While many solutions have been proposed, there has yet to be a robust and cost effective solution for the issue of battery safety, particularly in high capacity storage systems. Additionally, next generation materials such as silicon graphite/graphene anodes and high voltage cathodes are posed to increase the electrochemical performance of future systems, but little is known with regards to the abuse response of these materials. Our research efforts have focused on developing understanding and strategies to develop intrinsically safe lithium ion based systems. This material science based strategy to ensure battery safety offers benefits over engineered solutions to battery safety. As next generation materials mature, it becomes increasingly important to apply similar methodologies to understand and optimize the abuse response and safety envelope for these batteries. Intrinsically safe battery research topics have focused on materials development and coating efforts for anode and cathode safety, electrolyte flammability and reactivity reduction efforts, separator instability considerations, and solutions targeted at the mitigating the energy released in the case of a cell abuse. The understanding of next generation silicon carbon composites will be discussed to compare with current graphite based systems. Industrially relevant 18650 cell were built using silicon carbon composite anodes versus NMC 523 cathodes to understand both electrochemical and abuse performance, see Figure 1. Initial experiments show similar performance for specific capacity and rate capability for these cells. However, these next generation anode materials appear to have some potential differences in the degradation mechanisms and overall response during abuse conditions. These results may help to inform future development efforts, particularly as industry pushes towards higher silicon content in anode materials or towards significantly higher energy dense materials. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

Silicon has long been sought as a replacement to graphite anodes in next generation lithium-ion batteries due largely to it’s extremely high theoretical capacity of ~3500 mAh g-1 (for the formation of Li15Si4). This full value has yet to be realized in a practical electrode system due to both mechanical degradation caused by volume change during lithiation / delithiation and electrode surface instability that combine to cause rapid loss of capacity. However, advances in binder materials, materials synthesis of nano-silicon, and electrolyte additives have enabled the development of graphite and nano-silicon (Graphite + nSi) composite anodes that possess greater specific capacity that typical graphite electrodes and can be practically incorporated into improved lithium-ion cells. Sandia National Labs is conducting both materials scale and battery scale safety tests to determine the effects of nano-silicon incorporation on the safety performance of lithium-ion batteries. This study will focus on the effects of varying the silicon content in Graphite + nSi electrodes, using different binder materials, and using different electrolytes or additives on the failure mechanisms and characteristics of lithium-ion battery materials. Performance of Graphite + 15 % nSi Electrodes Electrodes of both graphite and Graphite + 15 % nSi composite were prepared using an aqueous LiPAA binder and 1.2 M LiPF6 in EC:EMC (3:7) electrolyte1. The electrodes possessed areal capacities of approximately 1.6 mAh cm-2 and were cycled in both half-cell configuration vs. lithium foil and in full cell configuration vs. an NCM 523 cathode. The voltage vs. capacity curve for the two electrodes, see Figure 1a, show several plateaus below 0.2 V arising from graphite activity with lithium while the 15 % nSi electrode also has significant capacity at higher voltages arising from lithiation of silicon which gives Graphite + 15 % nSi a specific capacity of 610 mAh g-1 compared to 319 mAh g-1 for graphite only. The differential capacity plot in Figure 1b shows more clearly the capacity contribution from the silicon peaks which occur at roughly 0.45 V and 0.25 V during charge and discharge. Figure 1d shows the cathode capacity retention and coulombic efficiency for the composite anode tested in full cell configuration, demonstrating the continued need for performance improvement for greater capacity retention. Figure 2 shows post cycling DSC analysis carried out on Graphite + 15 % nSi electrodes extracted from cycled full cells. After several formation cycles the cells were discharged to a prescribed voltage corresponding to a given state-of-charge (SOC). As expected the higher SOC electrodes generated more heat during testing. The various voltages were also selected for the given lithiation state of the anode: at 3.1 V the anode is fully delithiated, showing minimal thermal response; at 3.4 V the graphite is delithiated and the silicon is lithiated, making it the primary reactant and contributor to the thermal response at this state; at 4.1 V both graphite and silicon are lithiated so the resulting DSC will be a combined response of the two lithiated materials. As such, by looking at the 3.4 V DSC curve it can be seen that silicon has two strong exothermic peaks at ~ 200 °C and 260 °C, while the 4.1 V curve shows additional exothermic peaks at both 150 °C and 300 °C corresponding to lithiated graphite. So far this data agrees with previous results comparing graphite to silicon electrodes. References Klett M., Gilbert J. A., Trask S. E., Polzin B. J., Jansen A. N., Dees D. W., Abraham D. P., J. Electrochem. Soc., 163, A875 (2016). Acknowledgment: Sandia Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation for the U. S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL 85000. Figure 1

Battery electrodes are complex multiphase composites which must provide efficient bicontinuous networks for transport of electrons (through the particle phase) and positive lithium ions (through the electrolyte filled pores of the electrode). A crucial but often neglected element of battery electrodes is the binder, typically a mixture of polyvinylidene fluoride (PVDF) and carbon black. The binder has two primary roles – to provide mechanical integrity and to improve electrical conduction of the electrodes. Migration of the binder has also been implicated as a potential mechanism of capacity fade in rechargeable lithium ion batteries. We will present experimental characterization of the polymer binder for battery applications. Mechanical properties of the composite binder will be shown for both dry films and also binder swollen with carbonate electrolytes used in rechargeable lithium batteries. The electrical properties are strongly dependent on the applied stress and more modestly on the strain rate. The evolution of mechanical and electrical properties of the binder after repeated cycling will be shown. Mesoscale simulations will be presented using experimentally determined three dimensional structures of battery cathodes. Using these realistic microstructures, the role of polymer binder properties on the effective modulus of the electrode will be examined. The complex particle scale geometry results in a heterogeneous stress distribution with the binder between particle contacts experiencing high localized stresses. Implications for battery internal resistance and cycling stability will be discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2015-10738 A

Anion receptors that bind strongly to fluoride anions in organic solvents can help dissolve the lithium fluoride discharge products of primary carbon monouoride (CFx) batteries, thereby preventing the clogging of cathode surfaces and improving ion conductivity. The receptors are also potentially beneficial to rechargeable lithium ion and lithium air batteries. We apply Density Functional Theory (DFT) to show that an oxalate-based pentauorophenyl-boron anion receptor binds as strongly, or more strongly, to uoride anions than many phenyl-boron anion receptors proposed in the literature. Experimental data shows marked improvement in electrolyte conductivity when this oxalate anion receptor is present. The receptor is sufficiently electrophilic that organic solvent molecules compete with F - for boron-site binding, and specific solvent effects must be considered when predicting its F - affinity. To further illustrate the last point, we also perform computational studies on a geometrically constrained boron ester that exhibits much stronger gas-phase affinity for both F - and organic solvent molecules. After accounting for speci_c solvent effects, however, its net F - affinity is about the same as the simple oxalate-based anion receptor. Finally, we propose that LiF dissolution in cyclic carbonate organic solvents, in the absence of anion receptors, is due mostly to the formation of ionic aggregates, not isolated F - ions. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corpo ration, for the U.S. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

Introduction: Thermal instability of LiPF6 at elevated temperatures is well known. Thus there is an imminent need to develop thermally stable salt to replace LiPF6. A variety of new salts have been studied in Li-ion batteries without much success. One of the salts we are currently evaluating is LiF. This is a low molecular weight salt that has high voltage and thermal stability. However, it doesn’t dissolve in common battery solvents. This limitation can be solved with the addition of anion binding agents (ABAs). For example, Mehta and Fujinami1 proposed boroxine (tri coordinate boron ring) which is a Lewis Acid, to solubilize salt in common battery electrolyte. This compound traps the anion of the salt and frees up the Li+ for conduction. A similar idea was developed by McBreen etal 2-4 where they synthesized boron containing additives to dissolve LiF in organic solvents. However, most of the ABAs investigated are heavy with molecular weight exceeding 500 g/mole. Bulky ABAs impede access of the anion of the salt to the binding site (boron center) thus leading to poor dissolution of the salt5, 6. Inspired by McBreen’s approach we launched a program and developed lower molecular weight (~250 g/mole) ABAs. We quickly learned that the as prepared ABAs do have DMSO solvent bound to the boron (this is designated as impure ABA). We purified the impure ABA by reacting with LiF in acetone which yielded a pure ABA+LiF salt. The purified salt gave a clear solution whereas the impure ABA+LiF salt gave a cloudy solution in EC:EMC (Photo-1). We have prepared a variety of ABAs and tested them for electrochemical performance. Most of them could solubilize LiF only poorly and showed very low conductivity. However, two of the ABAs are promising and the results on one of them will be described here. Electrochemical Performance: Figure 2 compares conductivity for the pure and impure Oxalic-ABA+LiF in EC:EMC We have prepared a variety of ABAs and tested them for electrochemical performance. Most of them could solubilize LiF only poorly and showed very low conductivity. However, two of the ABAs are promising and the results on one of them will be described here. Electrochemical Performance: Figure 2 compares conductivity for the pure and impure Oxalic-ABA+LiF in EC:EMC solvent blend at different temperatures. The pure ABA shows higher conductivity than the impure. Electrochemical measurements and cell building was performed with the purified salt. This showed stability up to 4.5V vs. Li+/Li but unstable close to Li voltage, which was eliminated by adding low concentration of LiPF6 to the solution. Figure 3 shows formation of an 18650 cell containing carbon anode, NMC (523) cathode in EC:EMC(3:7 w%)-ABA+LiF_10mM LiPF6. The reversible capacity is ~ 1.03Ahrs.

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