Investigation of the Li-Co antisite exchange in Fe-substitued LiCoPO4 cathode for high-voltage lithium ion batteries

Carbon coated olivine Pnma LiCoPO4 (LCP/C) and Fe-substituted LiCo0.8Fe0.2PO4 (LCFP/C) were synthesized by a solvothermal method and their structural features and electrochemical properties were investigated. The electrochemical performance of LCFP/C is better than that of LCP/C, owing to the partial substitution of Co by Fe which effectively suppresses the increasement of antisite exchange between Li+ and Co2+ ions within the structure during cycling, despite a similar amount of Li–Co antisite exchange in pristine LCP/C and LCFP/C samples. Furthermore, direct visualization of Co in Li sites in the pristine samples and after 50 cycles was achieved through high-resolution scanning transmission electron microscopy for both LCP/C and LCFP/C. It was found that LCP/C locally formed a new cation-ordered structure after cycling due to the Li–Co antisite exchange, while the structure of LCFP/C remains almost the same. This study provides direct evidence that Fe substitution reduced the Li–Co exchange and improved the electrochemical cycling life of the LiCoPO4 cathode for high-voltage lithium ion batteries