Deterioration mechanism of the wettability of a lithium-ion battery separator induced by low-temperature discharge

Under low-temperature and discharge coupling conditions, serious performance degradation remains a severe challenge for lithium-ion batteries. The low-temperature discharge-induced evolution and deterioration of the wettability of the separator, which strongly affects the cycling stability and safety of lithium-ion batteries, need to be considered. The present work indicates that the separator wettability exhibits significant differences when subjected to various discharge ambient temperatures and discharge currents. The deterioration of separator wettability is accelerated at a relatively lower discharge ambient temperature and larger discharge current. Through the characterization of the microstructure, composition, chemical species and ionic impedance of the surface of the invalid separator coating, it is observed that the electrochemical reaction products of the positive active material and electrolyte deposit appear on the surfaces of the separator coating, which severely affects the wettability. A gradually decreasing ambient temperature and increasing discharge current increase the contact angle, which further induces the agglomeration of particles, the formation of sediments, an increase in the separator ion impedance and a decrease in the compatibility between the separator and electrolyte.