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

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 LiMn0.5Fe0.5PO4 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 LiMn0.5Fe0.5PO4 reaches 625 Wh kg–1, which is 16% higher than that of LiFePO4. Moreover, the high-performance LiMn0.5Fe0.5PO4 nanocrystals are prepared by a microwave-assisted hydrothermal synthesis in pure water.