Role of AlF3 Coating on LiCoO2 Particles during Cycling to Cutoff Voltage above 4.5 V

Abstract

Cell tests demonstrated that the use of an AlF3 coating enabled a LiCoO2 electrode to deliver a higher discharge capacity (208 mAh g(-1)) 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 LiCoO2 was only 60% at 4.5 V. The improved electrochemical performance with the AlF3 coating was attributed to the delay of structural degradation of LiCoO2 during cycling. A structural analysis of the cycled LiCoO2 electrode revealed that the pristine LiCoO2 transforms to a cubic spinel phase via an intermediate phase triggered by progressive chemical leaching of Co during cycling. The AlF3 coating protected LiCoO2 from the chemical attack by HF and thus helped to delay the eventual phase transformation to a spinel phase. By delaying this transformation, the AlF3 coating was able to reduce the charge-transfer resistance and maintain the structural stability when cycled above 4.5 V.