Phase Transformation of Doped LiCoPO₄ during Galvanostatic Cycling

In situ X-ray diffraction was employed to investigate the crystal structure changes in Cr/Si co-doped Li(Co,Fe)PO₄ cathode material during a galvanostatic charge/discharge process at a slow rate of C/30. The evolution of the X-ray patterns revealed that the phase transformation between the Cr/Si-Li(Co,Fe)PO₄ and Cr/Si-(Co,Fe)PO₄ is a two-step process, which involves the formation of an intermediate compound of Cr/Si-Li_0.62(Co,Fe)PO₄ upon the extraction of Li ions from the pristine phase. Different from the previously reported two biphasic transition steps, the phase transformation of the Cr/Si-Li(Co,Fe)PO₄ followed a solid solution and a biphasic reaction pathway at different stages of the delithiation/lithiation process, respectively.