Rate-Dependent Anisotropic Lattice Strain in LiFePO₄ Verified by Simultaneous Operando X-ray Diffraction and...

LiFePO₄ is particularly popular as a positive electrode material for lithium-ion batteries owing to the natural abundance of Fe. However, the phase transformations of LiFePO₄ and the resulting increase in its rate capability are insufficiently understood. Herein, we employed almost simultaneous operando X-ray diffraction and X-ray absorption measurements combined with multivariate-curve resolution to re-explore the phase transformation between Li-rich (LFP) and Li-poor (FP) phases by focusing on the differences in the orthorhombic lattice parameters during charging and discharging at the same capacities (δa_o, δb_o, and δc_o). At the low rate of 1/14 C (1 C is equal to one full charge for 1 h), all δ parameters remained approximately 0%, indicating a symmetric and isotropic phase transformation. By contrast, at a moderate rate of 5/14 C, δa_o and δb_o for the LFP phase had opposite signs with values of -0.05% and +0.1%, respectively, whereas those for the FP phase were almost 0%. These results indicate that the anisotropic and asymmetric strains generated in the LFP phase solely contribute to the high rate capability. The δ parameter thus provides a quantitative and comparable framework for evaluating the phase transformation behavior in LiFePO₄ and can be extended to other solid-state ionic materials.