Role of AlF₃ coating on LiCoO₂ particles during cycling to cutoff voltage above 4.5 v

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 AlF ₃ 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.