Determination of Interphasial (Electro-)Chemical Mechanisms Governing Stability and Reactivity in Silicon Anodes Using Gas Characterization

Silicon electrodes present a strategic opportunity to strengthen domestic lithium-ion battery manufacturing and increase energy density in next-generation batteries. However, capacity fade during storage ("calendar aging") driven by instability at the solid-electrolyte interphase (SEI) limits state-of-charge retention at long timescales. The underlying processes inhibiting stability are difficult to resolve because they arise from coupled electrochemical, chemical, and transport phenomena occurring simultaneously across solid, liquid, and gaseous phases within the evolving interphase. Specifically, electrolyte decomposition produces soluble and volatile products; the soluble intermediates diffuse to and react with the electrode, while gases alter local composition and transport. These pathways (electrolyte decomposition, SEI dissolution, and gas evolution) interact through shared intermediates and mass transport and often coexist. This study targets that coupling and crosstalk through nanomolar-scale quantitative gas phase analysis at the evolving SEI. Results indicate that the solvent alone contributes little to gassing, but in the presence of silicon and salt, decomposition of cyclic carbonates occurs, intensified at elevated temperatures. These quantitative gas species data offer experimental validation of predicted reaction mechanisms based on continuum-level molecular-scale reaction network-based models. This study produces fundamental insights into SEI evolution and chemical degradation in Si-based systems. Future work will enable crosstalk between SEI modeling efforts and experimental characterization to enable more robust models.