Improving the electrochemical kinetics of lithium manganese phosphate via co-substitution with iron and cobalt

High Mn content multicomponent olivine phosphate LiMn0.9Fe0.1−xCoxPO4/C (x = 0, 0.05 and 0.1) are synthesized by two-step solid-state method and characterized by X-ray powder diffraction, FTIR spectra, XPS, electron microscopy and electrochemical measurements. Upon substitution, the cell volume decrease as a result of the smaller ionic radii of Fe2+ and Co2+ compared to Mn2+. Structural analysis demonstrates that Fe and/or Co substitution decrease the Mn-O and P-O bond length and increase the Li-O bond length, leading to enhanced physical stability and expanded Li diffusion channels. The Fe2+ and/or Co2+ substituted LiMnPO4 is found to have improved kinetics, lithium storage performance, and electronic conductivity during electrochemical reaction. In particular, LiMn0.9Fe0.05Co0.05PO4/C delivers a capacity of 145 mA h g−1 at a current of 0.05 C and exhibits enhanced capacity retention compared to that of unmodified LiMnPO4/C. The enhanced electrochemical performance of co-substituted LiMnPO4 is attributed to the enhanced physical stability, the favorable delithiation/lithiation induced by previous redox reaction of Fe2+/Fe3+and Co2+/Co3+ and the improved electric/ionic transport properties.