Synthesis And Modification Of LiMnPO₄as Cathode Material For Lithium-ion Batteries

Olivine structured lithium manganese phosphate (LiMnPO4) is considered as one ofthe most potential and applicable cathode materials for lithium ion batteries due to themerits of abundant supply, low cost, structural stability, high energy density and cyclingstability. With a theoretical capacity of170mAh g-1and a voltage platform of4.1V vs.Li+/Li, the energy density of LiMnPO4is1.2times as much as that of LiFePO4. However,the intrinsic low electric conductivity and ionic conductivity largely limit theelectrochemical performance and thus the practical application of LiMnPO4.Traditional modification strategies, such as carbon coating, particle size minimizingand guest ion doping, are mostly often used to improve the electrochemical behavior ofLiMnPO4. Based on the structural design, carbon coating and particle size minimizing areachieved simultaneously, combined with cationic doping to overcome the intrinsic lowelectrical conductivity, thus improving the electrochemical performance of LiMnPO4.X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), differential thermoanalysis thermogravimetry (TG/DTA), cyclicvoltammetry (CV) and charge/discharge test were used to study the crystal structure, themorphology and electrochemical properties of the obtained materials. The main contents ofthis paper are as follows:1. Two-level hierarchical structured LiMnPO4nanocomposites (THS LMP/C-NC) wereprepared via solid-state method using oleic acid as the surfactant and sucrose as thesupplementary carbon source. The size of primary particles is in a range of8-10nm,which greatly shortens the diffusion distance of Li ions. These nanoparticles form intosecondary particles of50-80nm with large amounts of3D conductive carbon network existing between them, which is beneficial to the electron transfer in the material. Theeffect of surfactant concentration, carbon content and carbon precursors on thestructure and electrochemical performance of LiMnPO4are systematically discussed.The optimized synthesis conditions are as follows: the volume ratio of oleic acid andacetone is1:10, the carbon content is7.5wt%and the carbon source is sucrose. THSLMP/C-NC obtained from the optimized condition exhibits superior electrochemicalperformance. The initial discharge capacity of THS LMP/C-NC at0.1C is126mAh g-1,and at high current rates of5C and10C, the specific capacities of95mAh g-1and87mAh g-1are obtained, respectively. The capacity retention of THS LMP/C-NC at0.1C for50cycles is97.5%.2. LiFePO4/C with different carbon contents were synthesized using the above mentionedmethod. It is found that supplementary carbon source is unnecessary for the synthesis.When the carbon content increases, too thick carbon layer blocks the transfer of Li ions.Then a series of Fe-doped LiMn1-xFexPO4/C was synthesized. When the doping contentis0.2, the material exhibits the best electrochemical performance, slightly higher thanthat of THS LMP/C-NC. For further study, Mn1-xFexC2O4·2H2O (x=0.1,0.2) were firstprepared through co-precipitation method and then used as the precursor to synthesizeLiMn1-xFexPO4/C via solid-state method. The products have high crystallinity,homogeneous particle size distribution with no obvious changes found in themorphology. Compared with THS LMP/C-NC, the electrochemical performance ofFe-doped LiMnPO4is greatly improved, it delivers157mAh g-1at0.1C,139mAh g-1at1C and101mAh g-1at10C.3. Li3V2(PO4)3was prepared through sol-gel reaction and the presintering product afterthe heat treatment at350℃was used as the vanadium source to synthesizeLiMnPO4-Li3V2(PO4)3/C composite. CV and charge/discharge test show that twophases coexist in the material. By deducting the capacity delivered by Li3V2(PO4)3, thedischarge capacity of LiMnPO4is calculated to be159mAh g-1, demonstrating that theelectrochemical performance of the composite is improved. In comparison, NH4VO3 was also used as vanadium source to prepare V-doped LiMnPO4. Experimental resultsshow that the stoichiometric ratio and the temperature have great influence on thecomposition and performance of the material. CV and charge/discharge test confirmthat only LiMn0.85V0.1PO4/C prepared at750℃is the composite of LiMnPO4andLi3V2(PO4)3. And the material exhibits a longer and more stable charge/dischargeplateau with much lower polarization. After capacity conversion, it is found that thedischarge capacity and rate capability of LiMn0.85V0.1PO4/C are greatly improved,demonstrating that the electrochemical kinetics of the material is greatly improved afterV-doping.