Thermal behavior of LiFePO4 battery at faster C-rates and lower ambient temperatures

Abstract

Extensive focus is needed on the thermal safety of lithium-ion batteries at extreme operating conditions to prevent fire incidents. To accurately investigate the thermal behavior, this study employs a robust electrochemical-thermal model (P2D-T). The model incorporates temperature and concentration-dependent electrochemical parameters translating into an accurate prediction of temporal-spatial temperature variations, heat generation terms and shift in controlling mechanisms. The model has been validated with experimental data (18650 LiFePO4) and computed for a discharge rate of 1 C to 5 C and ambient temperature of 258.15 K to 328.15 K. Results reveal that at normal ambient temperature, the cell temperature gradient rises with the discharge rate (∆T5c2.3=∆T3c1.6=∆T1C). Since, the discharge is electronic conductivity controlled (σs <<σl), spatial non-uniform heat generation peaks form at the positive electrode. The dominant polarization heat (47% contribution, affected by overpotential and local current density) rises, complementing ohmic heat (27% contribution affected by transport resistance), while reversible reaction heat shifts to exothermic near the end of each discharge, contributing to the overall rise in cell temperature. At extreme conditions such as the discharge rate of 5 C and ambient temperature below 263.15 K (-10°C), the cell revealed a critical shift in heat generation mechanisms. Ohmic heat generation starts dominating (31% contribution) where ion transfer limitations (electrolyte diffusivity controlled) lead to non-uniform heat generation at the negative electrode rendering unsafe heat distribution with underutilization of energy (drops by 1.9 Wh/kg).