Electrolyte‐Dependent Aging Analysis in SiO _x ‐Based Li‐Ion Batteries via Composite Electrode Model

The performance of lithium‐ion batteries with silicon oxide (SiO _x ) is influenced by solid electrolyte interphase (SEI) formation and associated lithium consumption. This study investigates the role of electrolyte composition in governing SEI evolution, lithium consumption, and aging behaviors in LiCoO ₂ |SiOx cells using complementary experimental and modeling approaches. Electrolytes containing fluoroethylene carbonate (FEC) exhibit significantly improved cycle retention compared with conventional ethylene carbonate (EC)‐based electrolytes. Nuclear magnetic resonance spectroscopy reveals two distinct SEI formation pathways: EC decomposition yields an organic‐rich SEI, whereas FEC and PF ₆ ⁻ decomposition produce an inorganic, LiF‐rich SEI. The total SEI mass remains nearly constant after aging, indicating dynamic breathing behavior rather than continuous growth. Based on these insights, we developed a pseudo‐two‐dimensional model that couples electrolyte consumption and SEI growth by distinguishing organic and inorganic SEI components with different mechanical responses to SiO _x volume changes. The model successfully predicts electrolyte‐dependent aging, reproducing the influence of FEC concentration on overpotential, loss of lithium inventory, and capacity fade. The rate performance of the cell is primarily limited by lithium transport within SiO _x particles and by electrolyte transport across the electrode, particularly under fast‐charging conditions. Our combined experimental‐modeling approach provides a foundation for electrolyte‐specific aging assessment and electrolyte design for SiO _x ‐based batteries.