Role of AlF[sub 3] Coating on LiCoO[sub 2] Particles during Cycling to Cutoff Voltage above 4.5 V

Cell tests demonstrated that the use of an coating enabled a electrode to deliver a higher discharge capacity at an upper cutoff voltage of 4.54 V. About 94% of the initial capacity was retained after 50 cycles, while the capacity retention of pristine was only 60% at 4.5 V. The improved electrochemical performance with the coating was attributed to the delay of structural degradation of during cycling. A structural analysis of the cycled electrode revealed that the pristine transforms to a cubic spinel phase via an intermediate phase triggered by progressive chemical leaching of Co during cycling. The coating protected from the chemical attack by HF and thus helped to delay the eventual phase transformation to a spinel phase. By delaying this transformation, the coating was able to reduce the charge-transfer resistance and maintain the structural stability when cycled above 4.5 V.