Effects of Antisite Defects on Li Diffusion in LiFePO₄ Revealed by Li Isotope Exchange

Li–Fe antisite defects are commonly found in LiFePO 4 particles and can impede or block Li diffusion in the single-file Li diffusion channels. However, due to their low concentration (∼1%), the effect of antisite defects on Li diffusion has only been systematically investigated by theoretical approaches. In this work, the exchange between Li in solid LiFePO 4 (92.5% enriched with 6 Li) and Li in the liquid Li electrolyte solution (containing natural abundance Li, 7.6% 6 Li and 92.4% 7 Li) was measured as a function of time by both ex situ and in situ solid-state nuclear magnetic resonance experiments. The experimental data reveal that the time dependence of the isotope exchange cannot be modeled by a simple single-file diffusion process and that defects must play a role in the mobility of ions in the LiFePO 4 particles. By performing kinetic Monte Carlo simulations that explicitly consider antisite defects, which allow Li to cross over between adjacent channels, we show that the observed tracer exchange behavior can be explained by the presence of channels with paired Li–Fe antisite defects. The simulations suggest that Li diffusion across the antisite is slow (10 –16 cm 2 s –1 ) and that the presence of antisite defects is widespread in the LiFePO 4 particles we examined, where ∼80% channels are affected by such defects.