Thermodynamic equilibrium theory-guided design and synthesis of Mg-doped LiFe0.4Mn0.6PO4/C cathode for lithium-ion batteries

Mn-rich LiFe1−xMnxPO4 (x>0.5), which combines the high operation voltage of LiMnPO4 with excellent rate performance of LiFePO4, is hindered by its sluggish kinetic properties. Herein, thermodynamic equilibrium analysis of Mn2+-Fe2+-Mg2+-C2O42−-H2O system is used to guide the design and preparation of in-situ Mg-doped (Fe0.4Mn0.6)1−xMgxC2O4 intermediate, which is then employed as an innovative precursor to synthesize high-performance Mg-doped LiFe0.4Mn0.6PO4. It indicates that the metal ions with a high precipitation efficiency and the stoichiometric precursors with uniform element distribution can be achieved under the optimized thermodynamic conditions. Meanwhile, accelerated Li+ diffusivity and reduced charge transfer resistance originating from Mg doping are verified by various kinetic characterizations. Benefiting from the contributions of inherited homogeneous element distribution, small particle size, uniform carbon layer coating, enhanced Li+ migration ability and structural stability induced by Mg doping, the Li(Fe0.4Mn0.6)0.97Mg0.03PO4/C exhibits splendid electrochemical performance.