Mechanistic Insights Into the Electrochemical and Thermal Safety Degradation of Lithium Titanate Batteries Under Constant Voltage Overcharge Conditions

Lithium titanate oxide (Li4Ti5O12, LTO) anode-based batteries are widely recognized for their excellent safety characteristics and long cycle life. However, constant-voltage overcharging (CVOC), arising from cell-to-cell variations and delays in the response of a battery management system (BMS), can accelerate capacity degradation and increase the risk of thermal runaway. In this work, we systematically investigated the effects of CVOC at different voltages on the capacity retention and thermal safety of LTO-based batteries. By subjecting cells to overcharging cycling at elevated voltages, we elucidate the aging mechanisms and degradation pathways. Noninvasive diagnostics combined with postmortem analyses are employed to correlate the electrochemical behavior with the electrode morphology and composition under CVOC conditions. A critical voltage of 3.5 V is identified, beyond which severe degradation occurs. At 4.0 V, continuous CVOC induces the growth of a thick organic-rich solid-electrolyte interphase (SEI) and byproducts, leading to only 60% capacity retention compared with nearly 100% retention under 3.0 V CVOC or conventional constant-current/constant-voltage charging. Furthermore, the self-heating onset temperature decreases by 44.8 °C, indicating a significant reduction in thermal stability associated with high-voltage overcharge. These findings are corroborated by detailed postmortem characterization. Overall, this study demonstrates that the CVOC critically impacts both the electrochemical performance and thermal safety of LTO-based batteries, offering important insights for the design of high-stability, highly safe power sources for electric transportation systems.