Improving and Understanding Lifetime of LFP/Graphite Pouch Cells with Higher Concentrations of Vinylene Carbonate in the Electrolyte
Saad Azam, W. B. Black, Holden MacLennan, Anu Adamson, Eniko Zsoldos, E. R. Logan, J. R. Dahn
Abstract
With the growing adoption of LiFePO 4 (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 LiPF 6 , 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 LiPF 6 salt.