Revealing Insights into LixFePO4 Nanocrystals with Magnetic Order at Room Temperature Resulting in Trapping of Li Ions

A systematic study on the structures, magnetic properties, and electrochemical performance of LixFePO4 was carried out. Interestingly, it was found that the magnetic phase transition temperature is around 295 K in LixFePO4 (x ≤ 0.12) nanocrystals, which is higher than that of LiFePO4 (TN = 51(1) K) and FePO4 (TN = 114(1) K) phases. The origin of magnetic order around room temperature (RT) in LixFePO4 (x ≤ 0.12) is proposed to be due to both Fe2+-Fe3+ superexchange and the broken local symmetries induced by Fe2+-Li+ antisite defects. Ab initio calculations were adopted to calculate the total energy change of the ferromagnetic state and antiferromagnetic state (ΔE) to qualitatively estimate TN and proved the existence of the magnetic order around room temperature. Because of the strong Lorentz force in RT magnetic domains, Li ions will be trapped and form sub-10 nm solid solution domains of Li0+δFePO4, which have a direct relationship with electrochemical performance during (de)lithiation procedures.