Comprehensive Study of the Degradation of LiFePO₄/Graphite Cells at Elevated Temperatures

Previous findings for LiFePO 4 (LFP)/Graphite lithium-ion cells showed that the rate of capacity loss during cycle testing at elevated temperatures is highly dependent on the initial amount of vinylene carbonate (VC) in the electrolyte. To understand the potential mechanism behind this, a series of nominally identical cells with different VC content was prepared and tested for various cycle numbers at 55, 70, 85, and 100 °C. Cells were stopped at different cycle numbers to measure gas evolution and charge transfer impedance (R ct ) and then disassembled to track VC consumption, Fe deposition on the negative electrode, and diethyl-2,5-dioxahexane carboxylate (DEOHC) generation inside the cell. Higher concentrations of VC improves capacity retention, suppresses the generation of DEOHC and the rapid deposition of Fe on the negative electrode but is accompanied by more rapid gas evolution and R ct increase. Once all of the VC is consumed VC containing cells, Fe deposition accelerates and DEOHC appears in the electrolyte. These findings verify previous suggestions that lithium alkoxide accelerates Fe dissolution and the subsequent Fe deposition on the negative electrode of LFP/Graphite cells.