Synthesis And Performances Of LiMnPO₄ As Cathode Materials For Li-ion Batteries

Li-ion batteries（LIBs）are attracting more and more attentions in terms of high energy/power density,environmental benignity and good safety.Comparied with LiFePO₄,Li MnPO₄ possesses a higher voltage plateau（4.1 V）and theoretical energy density,and hence is considered as a promising alternative.However,the low electronic conductivity(<10^-1010 S cm^-1)and Li⁺ion conductivity(10^-7 cm² s^-1)seriously hamper its practical application.To overcome the shortcomings,great efforts focus on three issues:（i）adjusting particle size and morphology;（ii）doping with cations and/or anions;（iii）surface modification with high conductive materials.In this work,well-dispersed LiMnPO₄ nanomaterial was synthesized using solvothermal method.Surface coating and dopping were further adopted to improve the electrochemical performances.X-ray diffraction（XRD）,scanning electron microscrope（SEM）,transmission electron microscope（TEM）,and energy dispersive spectroscope（EDS）were performed to characterize the composition,structure,and morphology.Galvanostatic charge-discharge,cyclic voltammetry（CV）,and electrochemical impedance spectroscope（EIS）were carried out to investigate the electrochemical performance.（1）Three strategies（solid-state method,sol-gel route,and solvothermal treatment）were used to fabricate LiMnPO₄ nanomaterials.It showeded that obvious impurity existed in the product by solid-state method and was difficult to remove.The product from sol-gel method consisted of large microscale particles.By constrast,the sample derived from the solvothermal treatment was comprosed of uniform nanoparticles（50¹00 nm）.On this basis,the effects of Li:Mn:P ratios on the morphology and electrochemical performances were investigated.When the ratio was 2.5:1:1,the obtained LiMnPO₄ nanoparticles exhibited good electrochemical performancewith an initial discharge capacity of 132.2mAh g^-1。（2）LiMnPO₄ nanoparticles were further encapsulated by carbon nanolayers by a EG-assisted method to obtain ultrathin carbon coating LiMnPO₄/C composites（ULMP/C）.The ULMP/C demonstrated much better cycling performances than pure LiMnPO₄（LMP）and LiMnPO₄/C（LMP/C）composites using the traditional hydrothermal coating.The initial discharge capacity of ULMP/C was 130.8 mAh g^-1 and remained83%after 50 cycles.（3）LiMn_xFe_1-xPO₄/C compound was synthesized using solvothermal method.The effect of different FeSO₄ additionswas studied.It suggested that the product gain from Mn:Fe of 8︰2.The initial discharge capacity was 123 mAh g^-1 and kept 88%after 50 cycles.To sum up,the nanotechology,carbon coating,and Fe²⁺doping can effectively improve the lithium storage performance of LiMnPO₄materials.