Evolutionary mechanism and frequency response of graphite electrode at extreme temperatures

The battery management system is employed to monitor the external temperature of the lithium-ion battery in order to detect any potential overheating. However, this outside–in detection method often suffers from a lag and is therefore unable to accurately predict the battery's real-time state. Herein, an inside–out frequency response approach is used to accurately monitor the battery's state at various temperatures in real-time and correlate it with the solid electrolyte interphase (SEI) evolution of the graphite electrode. The SEI evolution at temperatures of −15, 25, 60, and 90 °C exhibits certain regular characteristics with temperature change. At a temperature of −15 °C, the Li+-solvent interaction of lithium-ion slowed down, resulting in a significant reduction in performance. At 25 °C, a LiF-rich inorganic SEI was identified as forming, which facilitated lithium-ion transportation. However, high temperatures would induce decomposition of lithium hexafluorophosphate (LiPF6) and lithium-ion electrolyte. At the extreme temperature of 90 °C, the SEI would be organic-rich, and LixPyFz, a decomposition product of lithium salts, was further oxidized to LixPOyFz, which led to a surge in the charge-transfer resistance at SEI (Rsei) and a reduction in Coulombic efficiency (CE). This changing relationship can be recorded in real time from the inside out by electrochemical impedance spectroscopy (EIS) testing. This provides a new theoretical basis for the structural evolution of lithium-ion batteries and the regular characterization of EIS.