X-ray absorption fine structure analysis for crystal structure change of vanadium-substituted lithium silicate, Li2.2Fe0.4Mn0.4Si0.8V0.2O4, during charge-discharge cycles

Abstract Li2.2Fe0.4Mn0.4Si0.8V0.2O4 as a cathode material for Li-ion batteries is synthesized by solid-state reaction, and the effect of V substitution for Si on an Mn–O bond is studied by charge-discharge tests, X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis. Charge-discharge tests suggest that the nature of the covalent bond in Mn–O is intensified by the inductive effect due to the decreased electronegativity by V substitution for Si. However, XRD reveals that the crystal structure of Li2.2Fe0.4Mn0.4Si0.8V0.2O4 changes into an amorphous state during the first charge process similarly to that of Li2MnSiO4. The extended X-ray absorption fine structure (EXAFS) analysis reveals that the Mn–O distance increases and decreases drastically during the charge-discharge process due to Jahn-Teller distortion, showing an elongated bond along the c-axis of MnO4 tetrahedra. We consider the elongated bond length of Mn–O in the tetrahedra further affects the coordination structure of Fe, Si, and V, which leads to change to the crystal structure of Li2.2Fe0.4Mn0.4Si0.8V0.2O4 into an amorphous state. The V substitution for Si in Li2Fe0.5Mn0.5SiO4 enhances the covalent bond nature of Mn–O to a certain extent. However, V substitution is still insufficient to suppress the amorphization.