Controllable Preparation And Performance Improvement By Carbon Coating And Cation Doping Of Nanosized Limnpo4cathode Materials

Exploring novel cathode materials is crucial to meet the demand of high-performance lithium ion battery. The increasing research interests have been focused onthe new active materials to replace the typical LiCoO2regarding to the costs and thetoxicity of cobalt. The potential alternatives are lithium transition metal phosphates,such as LiFePO4or LiMnPO4, thanks to their high thermal and electrochemical stability,low toxicity, low costs and their environmental friendly. The low operating potential ofLiFePO4indicates the low energy density which limits its application for electricvehicles. The successful commercialization of LiFePO4has stimulated the search forother olivines such as LiMnPO4. LiMnPO4exhibits an operating voltage of about4.1Vvs. Li+/Li, making the theoretical energy density (171mAh·g-1×4.1V=701Wh·kg-1)1.2times larger than that of LiFePO4(170mAh·g-1×3.45V=586.5Wh·kg-1). However, theelectrochemical performance of LiMnPO4is poor due to the slow lithium diffusionkinetics within the grains and the low intrinsic electronic conductivity, even much worsethan those of LiFePO4. Excellent electrochemical performance, especially the ratecapability of the material can be obtained by particle-size minimization, carbon coating,cation substitution and morphology control.LiMnPO4/C materials were prepared by a facile two-step solid state methodcombining with the ball-milling process. For comparison, a sample was also preparedby a one-step solid state method. The sample prepared by the two-step ball-millingmethod shows not only a uniform and reduced particle size, but also an enhancedelectrochemical performance. Moreover, solid-state method is suitable for industrializedproduction, which implies that the material prepared by this method can be a goodcandidate cathode material for high performance LIBs. The sample prepared by two-step ball-milling method exhibits initial discharge capacities of148mAh·g-1,141mAh·g-1,134mAh·g-1and134mAh·g-1at0.05C,0.1C,0.5C and1C, respectively.The capacity retentions are over90%after40cycles.Flakelike LiMn0.9Fe0.1PO4nanorods with Mn-site substitution with Fe2+wassynthesized via a simple solvothermal process in water-ethanol solvent mixtures. First,the effects of pH on the morphology and structure of LiMnPO4were studied, and the optimized pH of5.5was obtained. Base on this, the effects of ascorbic acid on physicaland electrochemical properties of flakelike LiMn0.9Fe0.1PO4/C nanorods were conductedin details. The uniform and monodispersed sample obtained in the presence of0.8gascorbic acid delivers a high capacity of145mAh·g-1,134mAh·g-1,119mAh·g-1,97mAh·g-1and68mAh·g-1at0.1C,0.5C,1C,5C and10C, respectively, with anexcellent cycling stability.In order to further improve the rate capability of the material, LiMnPO4nanorodswith Mn-site Co-substitution with Fe and Mg was prepared. The effects of Mn-site co-substitution with Fe and Mg on the structure, morphology and electrochemicalperformance of the material were investigated. Although cation substitution has littleeffect on the morphology of the material, it reduces lattice parameters and cell volume.What’s more, the electrochemical performance was improved significantly. The Fe andMg co-substitution LiMn0.8Fe0.19Mg0.01PO4/C leads to a significantly increasedreversible capacity of165mAh·g-1,150mAh·g-1,143mAh·g-1and110mAh·g-1at0.1C,0.5C,1C and5C with an excellent cycling stability, respectively.