Impact of Pyrocarbonate Additives and Salt Chemistries (LiFSI and LiPF6) on Gassing and Performance in Silicon-Containing Pouch Cells

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Abstract
Silicon (Si)-based negative electrodes are increasingly being integrated into commercial electric vehicle (EV) batteries due to their significantly higher theoretical capacity compared to graphite, enabling improved energy density. However, Si-based electrode materials show substantial volume expansion during lithiation (150 to 300%), which can lead to mechanical degradation, continuous solid electrolyte interphase reformation, and capacity loss over extended cycling. To mitigate these challenges and enhance cell performance, a novel class of electrolyte additives, alkyl pyrocarbonates, have been investigated for their ability to thermally decompose within the electrolyte, generating CO2 in situ. In this study, micron-scale silicon (μSi) and a mechanical silicon-carbon composite (Si-C) both blended with natural graphite were employed as negative electrodes in pouch cells with LiN i 0.83 M n 0.06 C o 0.11 O 2 (Ni83) as the positive electrode. The impact of pyrocarbonate additives on cycling stability, gas evolution, and electrochemical performance was systematically evaluated using long-term cycling, in situ volume measurements, EIS, GC-MS, and NMR spectroscopy. Among the tested pyrocarbonates, cells with diallyl pyrocarbonate demonstrated superior electrochemical stability, enhancing cycle life and storage characteristics while minimizing gas evolution and reducing charge transfer resistance (Rct). Furthermore, the role of electrolyte salts, specifically LiPF6 and LiFSI, were examined, revealing that LiPF6 more effectively suppressed gas generation compared to LiFSI.

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