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 Li_xFePO₄ was carried out. Interestingly, it was found that the magnetic phase transition temperature is around 295 K in Li_xFePO₄ (x ≤ 0.12) nanocrystals, which is higher than that of LiFePO₄ (T_N = 51(1) K) and FePO₄ (T_N = 114(1) K) phases. The origin of magnetic order around room temperature (RT) in Li_xFePO₄ (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 T_N 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 Li_0+δFePO₄, which have a direct relationship with electrochemical performance during (de)lithiation procedures.