X-ray absorption spectroscopic study on the electronic structure of Li1-xCOPO4 electrodes as 4.8 V positive electrodes for rechargeable lithium ion batteries

Changes in the electronic structure of olivine Li1-xCoPO4, 4.8 V positive electrode material for lithium ion batteries, were investigated using the X-ray absorption spectroscopy (XAS) technique. The threshold energy in the Co K-edge increased with electrochemical Li removal, indicating the oxidation of cobalt ions due to charge compensation. Moreover, P and 0 K-edge XAS showed a slight shift in threshold energy with Li removal. Although it is generally believed that the electrons of PO4 polyanion do not contribute to the oxidation process, present experimental results indicate changes in the electronic structure around PO4 units. Such results would be interpreted by the idea of the hybridization effect between the Co 3d and O 2p orbitals and of the polarization effect introduced by Li ions.