Insights on the degradation mechanism for large format prismatic graphite/LiFePO4 battery cycled under elevated temperature

Increasingly, batteries are being exposed to elevated temperatures to promote charging capability and satisfy various requirements. A comprehension of the degradation mechanism under elevated temperature is essential for advanced battery management. Here, commercial 100 Ah prismatic graphite/LiFePO4 batteries cycled under 45 °C are investigated. Less than 500 cycles are reached when capacity fades to 90 % of the nominal value, while more than 1300 cycles are reached under room temperature. Comprehensive post-mortem analysis is conducted to explore the mechanism of accelerated aging for 45 °C cycled batteries. The concentration of electrolyte salt dropping from 13.19 wt% to 12.98 wt% indicates the decomposition of electrolyte which brings about the gas release and battery volume expansion. Loss of electrolyte associated with enhanced evolution of solid electrolyte interface (SEI) film also leads to loss of lithium inventory. Larger LiFePO4 particles tend to fracture and Fe dissolves from them. It is proposed that Fe deposition accelerates the SEI film generation and the deposition blocks the graphite layers and limits lithium intercalation. From time-of-flight secondary ion mass spectroscopy depth profiling results, it is found that cathode electrolyte interphase film thickens moderately while SEI film thickens sharply, causing anode porosity to drop from 46.22 % to 40.11 %.