Additive Screening of LFP/graphite Pouch Cells for High Temperature Cycling at 70 °C

In recent years, LiFePO₄ (LFP)/graphite cell chemistry has gained renewed interest for commercial use in electric vehicles and grid energy storage due to its low cost, utilization of an abundantly available transition metal, and better intrinsic safety characteristics. However, LFP/graphite cells exhibit inferior cycling performance at high temperatures compared to some Li[Ni_xMn_yCo_z]O₂ (NMC) type materials. This study evaluated three classes of electrolyte additives - vinylene carbonate (VC), fluoroethylene carbonate, and prop-1-ene-1,3-sultone to improve the lifetime of LFP/graphite pouch cells stored and cycled at 60 °C and 70 °C, respectively. Co-additives such as methylene methanedisulfonate, 1,3,2-dioxathiolane-2,2-dioxide, and tris(trimethylsilyl) phosphite were also screened. Once the cells completed their respective testing, cell components were characterized via EIS, μXRF, and NMR. After screening 25 electrolyte combinations, it was found that VC at 4−5% (w/w) is the most effective additive for improving the lifetime of LFP/graphite pouch cells during cycling and storage at high temperatures. This improvement is due to the formation of an efficient negative electrode SEI and the limitation of Fe dissolution from the positive electrode, which maybe responsible for catalyzing further lithium inventory loss. The efficacy of 4−5% VC is largely due to having residual VC remaining in the electrolyte after the formation cycle.