Impact of VC on high temperature cycling of LFP/Gr cells

With the growing adoption of LiFePO₄ (LFP)/Graphite (Gr) cell chemistry in electric vehicles and grid energy storage, understanding and enhancing its performance under high-temperature conditions has become increasingly critical. In this study, various concentrations of vinylene carbonate (VC) (1% to 5%) were introduced to LFP/Gr pouch cells cycled at 70°C to evaluate their impact on cell lifetime. Additionally, two different lithium salts, LiFSI and LiPF₆, were investigated. Upon reaching the end-of-life (80% capacity retention), detailed post-mortem analyses were performed, including qNMR and GC-MS to determine changes in electrolyte composition, micro X-ray fluorescence (μXRF) to quantify Fe deposition on the negative electrode, and electrochemical impedance spectroscopy (EIS) to assess charge-transfer resistance.

Various LFP/Gr pouch cells were evaluated, encompassing four distinct graphite types, two LFP surface area variations, and two cell form factors. The results demonstrate that higher VC concentrations significantly improve cell lifetime, reduce Fe dissolution and suppress electrolyte degradation pathways, including the formation of ethyl methyl carbonate (EMC) and dimethyl 2,5-dioxahexane carboxylate (DMOHC). Furthermore, while LiFSI-based LFP/Gr cells exhibit enhanced performance in certain metrics, they suffer the production of gas at 70°C, which can be mitigated by incorporating LiPF₆ salt.

Saad Azam is a battery scientist whose research focuses on improving the lifetime, safety and performance of lithium-ion batteries for electric vehicles and grid energy storage. He completed his PhD at Dalhousie University in the Jeff Dahn research group, where his work centered on electrolyte additives, high-temperature degradation, transition-metal dissolution, gas evolution and long-term cycling of LFP/graphite and NMC/graphite pouch cells. His research combines electrochemical testing with advanced post-mortem methods, including qNMR, GC-MS, EIS, and micro-X-ray fluorescence, to connect cell performance with chemical degradation mechanisms. His recent work in the Journal of The Electrochemical Society examines how higher concentrations of vinylene carbonate improve the lifetime of LFP/graphite pouch cells cycled at 70°C.