The negative and positive structural effects of Ga oping in the electrochemical performance of LiCoO2

The electrochemical performance of LiGayCo(1-y)O(2) electrodes, y = 0.005 and 0.1, was studied. Charge/discharge curves in galvanostatic condition were obtained for a series of voltages ranging from 4.35 to 4.7 V. Samples were previously characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), X-ray absorption spectroscopy (XAS) and conductivity measurements. Comparison of data obtained for LiGayCo1-yO2 and the pristine oxide LiCoO2 is provided. Ga doping at 0.5 mol% drastically reduces the electrode capacity fading when charges at 4.35 or 4.5 V are applied, while Ga doping at 10 mol% spoils the Ga-doped oxides electrochemical response, even at a cut-off limit of 4.35 V. These positive and negative effects were attributed to changes in both the electronic conductivity and local structure of LiCoO2, induced by substitution of Co for Ga and cationic disorder. For y = 0.005, a slight distortion in the LiCoO2 local structure occurs, and formation of states in the band gap that the raise oxide electronic conductivity is observed, explaining the electrochemical improvement in LiGayCo1-yO2. In contrast, hybridization with oxygen occurs when y = 0.1, opening LiCoO2 band gap and making LiGayCo1-yO2 an insulator and a cationic disorder material that leads to poor electrochemical performance. Finally, XAS results show that Ga doping does not change the LiCoO2 charge transfer mechanisms, where oxygen is reduced during lithium extraction. (C) 2005 Elsevier Ltd. All rights reserved.