Orientation-Dependent Lithium Miscibility Gap in LiFePO₄

[100] Pnma is believed to be a tough diffusion direction for Li + in LiFePO 4, leading to the belief that the rate performance of [100]-oriented LiFePO 4 is poor. Recent work revealed that reducing the dimension of the LiFePO 4 phase to 12 nm in the [100] direction increased the extent of Li solid solution between LiFePO 4 and FePO 4, which produced an increase in the active population and excellent rate performance, but the lithiation/delithiation mechanism for this interesting phenomenon remains to be unraveled. Here we report, by using operando X-ray diffraction, the phase transition path involved in the lithiation/delithiation process is single-phase featured. This work presents one of the first experimental demonstrations that decreasing the dimension of LiFePO 4 in the [100] direction to the LiFePO 4 /FePO 4 equilibrium phase boundary width can improve the solid solubility of both the end solid solutions (Li α FePO 4 and Li 1−β FePO 4 ) and decrease the miscibility gap of LiFePO 4, demonstrating a highly orientation-dependent lithium miscibility gap.