Rate-Dependent Anisotropic Lattice Strain in LiFePO ₄ Verified by Simultaneous Operando X-ray Diffraction and Absorption Measurements

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 (δao, δbo, and δco). 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, δao and δbo 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.