A Study of Vinylene Carbonate and Prop-1-ene-1,3 Sultone Electrolyte Additives Using Polycrystalline Li[Ni0.6Mn0.2Co0.2]O2in Positive/Positive Symmetric Cells

Vinylene carbonate (VC) and prop-1-ene-1,3 sultone (PES) have been reported to have beneficial effects on both negative and positive electrodes of Li-ion cells. To simplify evaluation of these two additives with LiNi0.6Mn0.2Co0.2O2(NMC622), positive/positive symmetric cells were used to exclude the influence from any negative electrode. The compatibility of electrolytes containing 2%VC or 2%PES with NMC622 in 1M LiPF6EC:DEC (v/v 1:2) electrolyte was evaluated by multiple voltage hold periods at 55 degrees C to accelerate the capacity fade. EIS spectra showed 2%PES is superior for impedance control compared to 2%VC. dV/dQ vs V fitting results showed that active mass loss is worse at high voltage than low voltage regardless of electrolyte used. Cross-sectional SEM images showed more microcracking of NMC622 particles at high voltage than at low voltage, which was more severe at the end of testing than at the beginning of testing. Symmetric cell storage at 0 V (ca. 3.8 V vs Li/Li+) and 0 degrees C showed a significant increase in cell impedance for cells stored after the end of testing and a moderate impedance increase for cells stored at the beginning of testing suggesting the accumulation of deleterious reaction products in the cells during testing.