Special Synergy between Electrolyte Additives and Positive Electrode Surface Coating to Enhance the Performance of Li[Ni_0.6Mn_0.2Co_0.2]O₂/Graphite Cells

Li[Ni 0.6 Mn 0.2 Co 0.2 ]O 2 (NMC622) is one of the most attractive positive electrode materials for Li-ion cells. Ultra high precision coulometry, impedance spectroscopy, gas evolution and long-term cycling were used to explore the combined impact of several electrolyte additives and an Al 2 O 3 surface coating on NMC622/graphite cells tested to 4.4 V at 40°C. An excellent correlation between the short-term coulombic efficiency measurements and the long-term cycling results was noted. An electrolyte containing 1 M LiPF 6 in ethylene carbonate: ethyl methyl carbonate (3:7 by weight) with additives 2 wt% prop-1-ene-1,3 sultone (PES) + 1 wt% methylene methane disulfonate (MMDS) + 1 wt% tris (trimethylsilyl) phosphite (TTSPi) (this electrolyte is called PES 211) was found to give the best performance in both coated and uncoated NMC622/graphite cells. The surface coating was found to improve coulombic efficiency, reduce impedance and improve capacity retention in all cases studied. However, uncoated NMC622/graphite cells with PES211 were found to perform better than coated NMC622/graphite cells which used any other electrolyte. The special synergy between Al 2 O 3 -coated NMC622 and PES211 needs to be understood so that even better coatings and electrolyte combinations can be developed.