Thermodynamic and kinetic degradation of LTO batteries: Impact of different SOC intervals and discharge voltages in electric train applications

Lithium-titanate-oxide (LTO) based lithium-ion batteries show promise for longer lifespan, higher power capability, and lower life cycle cost for energy storage and electric transportation applications than graphite-based counterparts. However, the degradation mechanisms of LTO-based cells in the high and low state-of-charge (SOC) intervals and different discharge cut-off voltages are not clearly investigated. In this study, the application-related lifetime performance of high-power Li₄Ti₅O₁₂/LiCoO₂ batteries is investigated at five independent SOC intervals with 20 % depth-of-discharge (DOD) and three discharge cut-off voltages. Our results show that degradation increases significantly when the batteries are cycled within lower SOC intervals or with lower cut-off voltages. Additionally, thermodynamic degradation is more significant when cycled at 20 % DOD, while kinetic degradation dominates at 100 % DOD. For thermodynamic degradation, the determining degradation mode is shown to be the loss of active material in the negative electrode, while the active material loss at the cathode has a greater impact on the equilibrium voltage curve. The kinetic degradation is mainly due to the slower charge transfer process and diffusion process at the cathode, which increases polarization impedance.