Study On The Synthesis And Electrochemical Performance Of LiFe_0.6Mn_0.4PO₄/C

Olivine type LiFe_1-yMn_yPO₄with a theoretical capacity of170mAh·g^-1,providing higher energy density compared to those of industrial LiFePO4cathodematerial （Mn3+/Mn2+，4.1V vs Li⁺/Li）, are considered to be an ideal lithium ioncathode materials for EV and HEV due to its low cost, nontoxicity, high thermalstability and superior cycling performance.In this study, carbon-coated LiFe_1-yMn_yPO₄（y=0,0.2,0.4,0.6,0.8,1.0） cathodematerials were prepared via one-step solid-state method using citric acid as carbonsource. In order to examine the effect from different ratios of Fe to Mn on theelectrochemical properties of these cathode materials, SEM（scanning electronmicroscopy）、XRD（X-ray diffraction） and charge/discharge test were performed. Theresults indicated that LiFe_0.6Mn_0.4PO₄/C possessed the best electrochemical propertiesand exhibited the highest energy density of557Wh·kg^-1at0.1C rate, and LiFePO4/Ccathode material exhibited a energy density of534.9Wh·kg^-1under the sameconditions. Furthermore, we also investigated the influence of different amounts ofcarbon contents on the electrochemical properties of LiFe_0.6Mn_0.4PO₄, using cirtricacid as carbon source. The results of XRD、SEM、electrochemical properties andEIS（electrochemical impedance spectroscopy） test showed that the kinetic lithiumintercalation and de-intercalation of LiFe_0.6Mn_0.4PO₄/C was effected by the differentcarbon amount. The sample with20%（wt） of cirtric acid had the best electrochemicalproperty.A serial of Ti⁴⁺doping on Li(Fe_0.6Mn_0.4)_1-2xTi_xPO₄/C （x=0,0.01,0.02,0.03） weresynthesized via one-step solid-state method. The influence of different amounts ofTi⁴⁺doping on properties of the LiFe_0.6Mn_0.4PO₄/C electrode were characterized byXRD、SEM、EDS、CV、EIS and charge/discharge test. The result suggested thatLi(Fe_0.6Mn_0.4)0.96Ti0.02PO4/C show the best excellent rate performance and thesmallest charge transfer resistance. It showed discharge capacity of160.03mAh·g^-1and124.4mAh·g^-1at0.1C and20C rate, respectively, while LiFe_0.6Mn_0.4PO₄/Cwithout Ti⁴⁺doping possessed discharge capacity of149.2mAh·g^-1and68.6mAh·g^-1at0.1C and20C rate respectively.Different particle size of LiFe_0.6Mn_0.4PO₄/C were successfully prepared bysolid-state with second ball grinding method and solid-state method combined with spray drying technology. Through the two methods, LiFe_0.6Mn_0.4PO₄/C showsnanoparticles and micro-spherical morphology, respectively. Though the optimizingof thermal treatment at different temperature, the best electrochemical performancewas obtained at650℃with the initial discharge capacities of160.2mAh·g^-1and118.8mAh·g^-1at0.1C and30C rate, respectively. Compared to the nanoparticleLiFe_0.6Mn_0.4PO₄/C, the micro-LiFe_0.6Mn_0.4PO₄/C had the higher tap density of1.4g·cm^-3obtained by Spray drying technology, and owned the volumetric energydensity of772.5Wh·L^-1at0.1C rate.