Revealing Insights into Li x FePO 4 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 (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 (Delta E) 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 Li0+delta FePO4, which have a direct relationship with electrochemical performance during (de)lithiation procedures.