(Invited) Microcalorimetry of Silicon Anodes for Lithium-Ion Batteries

Abstract

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.