Directionality of thermal gradients in lithium-ion batteries dictates diverging degradation modes

Intentionally applied interelectrode thermal gradients (ITGs) accelerate capacity loss in 35 degrees C cells, and the directionality of the thermal gradient dictates the responsible degradation mode. By simulating cell self-heating at various temperatures and C-rates, we identify 35 degrees C and C/5 as a condition that does not typically exhibit lithium (Li) plating under isothermal conditions but is sensitive to thermal gradients. When subjected to an ITG, we observe 77% capacity fade over 20 cycles when the negative electrode (NE) is warmer than the positive electrode (PE) (Delta T-int = +2 degrees C) and 100% capacity fade when the PE is warmer than the NE (Delta T-int = 2 degrees C). Incremental capacity analysis diagnoses PE degradation for Delta T-int = +2 degrees C and NE degradation for Delta T-int = 2 degrees C. Electrochemical impedance spectroscopy and postmortem optical investigation corroborate these findings. We identify ITGs as a means to achieve accelerated aging of Li-ion cells with the capability to dictate a limiting electrode and/or decouple degradation of each electrode.