Zero Lithium Miscibility Gap Enables High-Rate Equimolar Li(Mn_,Fe)PO₄ Solid Solution

Abstract

Forming olivine-structured Li(Mn,Fe)PO₄ solid solution is theoretically a feasible way to improve the energy density of the solid solutions for lithium ion batteries. However, the Jahn-Teller active Mn³⁺ in the solid solution restricts their energy density and rate performance. Here, as demonstrated by operando X-ray diffraction, we show that equimolar LiMn_0.5Fe_0.5PO₄ solid solution nanocrystals undergo a single-phase transition during the whole (de)lithiation process, with a feature of zero lithium miscibility gap, which endows the nanocrystals with excellent electrochemical properties. Specifically, the energy density of LiMn_0.5Fe_0.5PO₄ reaches 625 Wh kg^-1, which is 16% higher than that of LiFePO₄. Moreover, the high-performance LiMn_0.5Fe_0.5PO₄ nanocrystals are prepared by a microwave-assisted hydrothermal synthesis in pure water.