Long-Term Operando Quantification of Li Plating on Graphite Anodes

Lithium (Li) plating on graphite poses safety concerns for energy storage batteries, and quantifying its evolution in a nondestructive way is important to understand battery degradation pathways. Here, operando variable-temperature electrochemical solid-state ⁷Li nuclear magnetic resonance (NMR) spectroscopy is used to quantify Li plating (onset, plated amount, reversibility) in LiFePO₄/graphite batteries cycled until 30% state of health (SoH) at both 25 and 45 °C with three distinct electrolytes. We find a three-stage Li plating trend across cycles and that Coulombic efficiency (CE) drops are highly correlated to the jump in Li plating and the accumulation of dead Li, as Li plating induces significant electrolyte decomposition and SEI formation. Vinylene carbonate (VC) and Li difluoro(oxalato)borate (LiDFOB) are chosen specifically to generate organic-rich and inorganic solid electrolyte interphase (SEI), respectively. VC suppresses plating and dead Li formation throughout cycling, while LiDFOB initially delays plating but leads to abrupt degradation after decomposition into LiBF₄, which is more severe at an elevated temperature. High temperature improves the Li plating reversibility, minimizing dead Li accumulation, yet intensifies SEI formation and results in fewer cycles. Based on our findings, further advice on additives and temperature management for mitigating Li plating is presented.