Elucidating Failure Mechanisms in Li-ion Batteries Operating at 100 °C

Rechargeable batteries that function at temperatures as high as 100 °C are desired for drilling instruments, autoclavable medical electronics, and space exploration. However, at 100 °C and beyond, lithium-ion batteries (LIBs) exhibit rapid capacity fade that prevent their use in many applications. Here, an in-depth study of the failure mechanisms was undertaken for LIBs operating at 100 °C containing graphite anodes, LiNi0.33Mn0.33Co0.33O2 (NMC111) cathodes, and organic electrolytes containing lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4) salts. Electrochemical methods including differential analyses (dV/dQ) of galvanostatic cycling, electrochemical impedance spectroscopy (EIS), and linear polarization (LP) revealed that capacity loss was caused by a loss of lithium inventory in the cell due to film-forming reactions that siphon capacity. We hypothesized that capacity fade results from continual degradation and reformation of the anode solid electrolyte interphase (SEI), causing low coulombic efficiencies and poor cycle life. Crucially, electrode replacement of either the graphite or NMC111 with lithiated electrodes enabled full capacity recovery after high-temperature cycling, revealing that the electrode materials themselves were not inherently unsuitable for use at 100 °C. Overall, this study identifies the failure mechanisms of LIBs cycling at 100 °C, which are expected to guide the development of electrolyte formulations that improve electrode interphase stability and hence the performance of LIBs operating as high as 100 °C.