Composition Control,Preparation Optimization And Electrochemical Performance Of LiFe_0.5Mn_0.5Po₄-based Cathode Materials

In the recent years,olivine-based LiFe_0.5Mn_0.5PO₄ has been highlighted as potential cathode materials and has already been commercialized in large scale due to its outstanding properties.However,the intrinsic disadvantages of the olivine-structure phosphate-based compounds such as low electronic conductivity,sluggish Li⁺diffusion rate,and the presence of anti-site defects also have a serious impact on the rate and cycle performance of LiFe_0.5Mn_0.5PO₄ samples.Thus it is still necessary to further improve the electrochemical performance of LiFe_0.5Mn_0.5PO₄ material by novel strategies.In order to overcome the problems existed in commercial application of LiFe_0.5Mn_0.5PO₄/C cathode material,such as complicated procedures in the synthesis process and/or low volumetric energy density of nanometer-sized sample,herein,two LiFe_0.5Mn_0.5PO₄/C samples were synthesized by the solid-state and Sol-gel routes,respectively,and then the morphology and electrochemical performance of these two samples were compared and analyzed.It is found that the LiFe_0.5Mn_0.5PO₄/C sample prepared from the solid-state route shows superior electrochemical performance than that of the other sample obtained from the Sol-gel route.Based on this fact,the effects of different experimental conditions,such as ball milling method,carbon sources,calcination temperature,carbon content and pre-calcination time,on the morphology and electrochemical performance of LiFe_0.5Mn_0.5PO₄/C composites were further investigated in detail,to achieve the optimum conditions for the preparation of LiFe_0.5Mn_0.5PO₄/C composites.The composition and structure of the final materials were characterized by XRD,N₂-adsorption and desorption,FTIR,Raman,SEM,TEM and XPS.The electrochemical performances of final LiFe_0.5Mn_0.5PO₄/C materials were tested under different conditions and were discussed by means of the analysis of the charge and discharging curves,CV curves and electrochemical impedance spectroscopies.The main research work of this paper is as follows:Carbon-coated olivine LiFe_0.5Mn_0.5PO₄/C cathode materials by were in-situ synthesized by two-step solid-state calcination route in combination of a surfactant-assisted ball milling process.The effects of different ball milling methods,calcination temperature,carbon content,carbon source and pre-calcination time on the electrochemical performance of Final LiFe_0.5Mn_0.5PO₄/C cathode materials were investigated.Based on the experimental results,some conclusions are summarized as follows:The dry ball milling in combination of wet ball milling processes are efficient for the improvement of electrochemical performances of LiFe_0.5Mn_0.5PO₄/C cathod material prepared from the solid-stae route.The appropriate calcining temperature and carbon content are contribute to the formation of LiFe_0.5Mn_0.5PO₄/C with high crystallinity and uniform carbon coating.The properties of carbon sources have great influence on the morphology,structure and electrochemical performance of the as-obtained LiFe_0.5Mn_0.5PO₄/C materials.The LiFe_0.5Mn_0.5PO₄/C sample with uniform carbon coating,reasonable pore structure and small particle size can be prepared successfully,using the mixture of polyvinyl alcohol and sucrose as carbon sources.Also,this LiFe_0.5Mn_0.5PO₄/C sample shows good electrochemical performance.The pre-calcining time has a great influence on the morphology of final LiFe_0.5Mn_0.5PO₄/C materials.The high performance LiFe_0.5Mn_0.5PO₄/C can be achived using suitable pre-calcining time during the preparation processes.Three olivine cathode materials with non-stoichiometric ratio(Li_1.05Fe_0.5Mn_0.5PO₄,Li_1.05Fe_0.5Mn_0.45PO₄ and Li_1.05Fe_0.45Mn_0.5PO₄)were prepared,and the physical properties and electrochemical performance were discussed.The electrochemical perfomances of LiFe_1-xMn_xPO₄（0<x<1）cathode materias can be further improved by design of non-stoichiometric samples.The energy density of Li_1.05Fe_0.5Mn_0.5PO₄,Li_1.05Fe_0.5Mn_0.45PO₄and Li_1.05Fe_0.45Mn_0.5PO₄ were 574.86 Wh·kg^-1,580.18 Wh·kg^-1and 582.52 Wh·kg^-1at 0.1 C,respectively.