Improving the High Voltage Cycling of Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/Graphite Pouch Cells Using Electrolyte Additives

Abstract

Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells with 1 M LiPF6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by weight) electrolyte containing 2 wt% vinylene carbonate (VC), 2 wt% prop-1-ene-1,3 sultone (PES), or 2 wt% PES + 1 wt% tris(trimethylsilyl) phosphite (TTSPi) + 1 wt% methylene methanedisulfonate (MMDS) (“PES-211”) were subjected to cycling tests at various upper cut-off voltages from 4.2 to 4.7 V. After every 10 cycles, the pouch cells were charged or discharged to 3.8 V and evaluated using electrochemical impedance spectroscopy (EIS). This approach allows the impact of electrolyte additives on the impedance of cells cycled to different cut-off voltages to be studied systematically. The addition of 2% PES or “PES-211” proved to be beneficial in suppressing impedance growth when cells are cycled up to and above 4.4 V. Long-term cycling tests between 3.0 V and 4.4 V at 45°C showed that “PES-211” had the best capacity retention (∼85%) and the least impedance growth after 500 cycles. The results suggest the use of “PES-211” could lead to longer lived and higher power NMC/graphite Li-ion cells with better tolerance to high voltage (4.4 V) which will improve energy density.