Experimental and theoretical investigation of fluorine substituted LiFe0.4Mn0.6PO4 as cathode material for lithium rechargeable batteries

As a cathode material for lithium rechargeable batteries, fluorine-substituted lithium iron manganese phosphate (LiFe0.4Mn0.6PO4-δFδ, δ = 0 to 0.09) without additional carbon sources was synthesized by solid state reaction by using planetary mill grinding and characterized by XRD, SEM, TEM with EDX mapping, XPS and galvanostatic charge–discharge testing. LiFe0.4Mn0.6PO4-δFδ samples were shown to have single-phase crystalline nature with X-ray diffraction analysis and enhanced discharge capacity at various C-rates as compared to bare LiFe0.4Mn0.6PO4. Among them, the LiFe0.4Mn0.6PO3.970F0.03 with the best cycleability exhibited an initial discharge capacity of 153 mAh g− 1 at 0.1 C and 113 mAh g− 1 at 1 C. It is proven that the enhanced electrochemical properties of LiFe0.4Mn0.6PO4-δFδ by fluorine substitution are incorporated with not only electrical conductivity but also stability of crystal structure. First principle computations were also performed to determine the optimum molar ratio of Fe and Mn and evaluate the improvement in electrical conductance through fluorine substitution to oxygen.