Extending the high-voltage operation of Graphite/NCM811 cells by constructing a robust electrode/electrolyte interphase layer

The cycling life of layered Ni-rich LiNi$_{1-x-y}$CoxMn$_y$O$_2$ (NCM, 1-x-y ≥ 0.8) is typically extended by restricting the upper cut-off voltage during cycling to below 4.2 V, sacrificing, however, the untapped additional capacity above the cut-off voltage. To make this additional capacity available, we investigate graphite/LiNi$_{0·8}$Co$_{0·1}$Mn$_{0·1}$O$_2$ cells cycled to high upper cut-off voltages up to 4.5 V at high electrode areal capacities of 4.8 mAh/cm$^2$ in a standard electrolyte consisting of 1 M lithium hexafluorophosphate (LiPF$_6$) in ethylene carbonate and ethylene methyl carbonate (ethylene carbonate:ethylene methyl carbonate = 3:7 vol% + 2% vinylene carbonate). Although the initial capacity reaches 190 mAh/g, the capacity retention after 300 cycles to 4.5 V is only 66%. Employing a combination of tris(trimethylsilyl)phosphite and lithium difluoro(oxalato)borate as electrolyte additives, we demonstrate excellent capacity retention of 85% after 300 cycles to 4.5 V. Moreover, graphite/LiNi$_{0·8}$Co$_{0·}$1Mn$_{0·1}$O$_2$ cells with additives show improved capacity retention also at elevated temperatures of 60 °C. ... mehrA detailed post-mortem analysis reveals the formation of a compact and LiF-rich and B-containing cathode/electrolyte interphase layer on the LiNi$_{0·8}$Co$_{0·1}$Mn$_{0·1}$O$_2$ particles cycled with tris(trimethylsilyl)phosphite and lithium difluoro(oxalato)borate additives, substantially suppressing the transition metal dissolution and the cation-disordered layer formation on the exposed particles' surface.