Special Synergy between Electrolyte Additives and Positive Electrode Surface Coating to Enhance the Performance of Li[Ni0.6Mn0.2Co0.2]O2/Graphite Cells

Li[Ni0.6Mn0.2Co0.2]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 Al2O3 surface coating on NMC622/graphite cells tested to 4.4 V at 40 degrees C. An excellent correlation between the short-term coulombic efficiency measurements and the long-term cycling results was noted. An electrolyte containing 1 M LiPF6 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 Al2O3-coated NMC622 and PES211 needs to be understood so that even better coatings and electrolyte combinations can be developed. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.