Size-Dependent Memory Effect of the LiFePO₄ Electrode in Li-Ion Batteries

In Li-ion batteries, the phase transition usually determines the electrochemical kinetics of some two-phase electrode materials, and it can be adopted to excellently interpret the memory effect of Li-ion batteries, therefore the size dependence of phase transition was expected to affect the memory effect significantly. In this work, we investigated the memory effect and phase transition of olivine LiFePO₄ in Li-ion batteries. Through electrochemical measurements, we found that the memory effect of LiFePO₄ was dependent on the particle size, especially after a long-time relaxation. By using the in situ X-ray diffraction, we found that the phase transition of nano-LiFePO₄ was ahead of the charging and discharging processes, while it took place concurrently or later for micro-LiFePO₄, which might be attributed to the high-specific two-phase boundary of nano-LiFePO₄. Furthermore, the phase-transition diagram was adopted to interpret the size-dependent memory effect schematically. Notably, it is the first time to report the phase transition ahead of (dis)charging for nano-LiFePO₄, which is significant to understand the phase transition of two-phase electrode materials, as well as the relevant phenomena, such as the memory effect.