Effect of Fe2+ substitution on the structure and electrochemistry of LiCoPO4 prepared by mechanochemically assisted carbothermal reduction

Abstract

LiCo_1−yFe_yPO₄ solid solutions (0 ≤ y ≤ 1) were prepared by the mechanochemically assisted carbothermal reduction of Co₃O₄ and Fe₂O₃. Mechanical activation was performed using a high-energy planetary mill AGO-2. The samples were characterized in detail by X-ray powder diffraction (XRD) using a Rietveld refinement, Fourier transform infrared spectroscopy (FTIR), Mössbauer spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), galvanostatic cycling, and galvanostatic intermittent titration technique (GITT). According to XRD, all the samples are single-phase solid solutions, crystallized in an orthorhombic structure (S.G. Pnma). The cell volume of LiCo_1−yFe_yPO₄ linearly increases vs. the Fe content. All the Fe ions are in the 2+ oxidation state and are octahedrally coordinated. The LiCo_1−yFe_yPO₄ solid solutions show improved electrochemical performance, compared with LiCoPO₄. Based on the data from XRD and GITT, the improvement is attributed to the enhanced Li⁺ diffusion, due to the enlargement of the 1D diffusion channels in the polyanion structure of LiCoPO₄ and the reduced cell volume change in the material during the Li extraction/insertion process. Moreover, a systematic decrease in the average potential of the Co²⁺/Co³⁺ redox pair is observed with the increased Fe content, leading to the reaction termination in the electrochemical window of conventionally available electrolytes. In situ synchrotron diffraction shows that upon charging LiCo_0.5Fe_0.5PO₄, the two-phase mechanism of Li (de)intercalation at the Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ redox stages changes to a solid solution-like mechanism, contrary to the pristine LiFePO₄ and LiCoPO₄ materials.