And Modification Of Li-Mn-O As Cathode Materials For Li-ion Batteries

Rechargable Lithium ion batteries（LIBs）have been widely applied in various fields,from small consumer electronics,such as smart phone,laptop computer,digital cameras etc,to transportation tools including hybrid electric vehicles（HEVs）,plug in hybrid electric vehicles（PHEVs）and electric vehicles（EVs）,and stationary energu storage systems,such as off-grid,micro-grid solar/wind power station and distributed generation system,due to their high energy density,wide operating temperature range and long cyclic life.In this paper,spinel Li-（Ni）-Mn-O as cathode material for lithium ion battery was studied and a serious of issues was discussed,which mainly include synthetic process of spherical porous LiMn₂O₄,improvement on electrochemical performance of spinel LiMn₂O₄ by surface coating and surface modification of LiNi_0.5Mn_1.5O4.Therefore,the research work will be stated by three parts:1,The porous LiMn₂O₄ micro-spheres can be successfully and rapidly synthesize via the micro-emulsion demulsified method and high temperature solid state method.The crystal structure and morphology of precursor MnCO3,caybyne Mn2O3 and end product LiMn₂O₄ are studied by X-ray diffraction（XRD）and scanning electron microscope（SEM）,which shows the influence of different concentration of aqueous phase on the morphology of product.The electrochemical performance of end-products is studied by cyclic voltammetry（CV）,electrochemical impedance spectroscopy（EIS）and cell performance testing,with which the influence of different concentration of aqueous phase for electrochemical performance of end-products.In summary,There are three conclusion can be drew: Firstly,the manganese ion concentration of aqueous phase in emulsion have a certain influence on the microstructure and cell performance of end-products.The higher Mn2+ in the water phase,the samller nanoparticles that reunited into the spherical porous end-products,and the bigger the specific surface area,the better the rate performance of material.Secondly,the larger specific surface area make manganese dissolution worse,which lead to a bad cyclic performance.Thirdly,althought the products with low Mn2+ concentration delievers a bad rate performance,the capacity and cyclic performance show well.2,Through precipitation and heat-treatment,the spherical porous LiMn₂O₄ is produced,followed by one-pot hydrothermal method,a favorable electrochemical performance of spherical LiMn₂O₄ coated with LiMnPO4 is get.The Mn2+-rich cladding layer effectively prevents the Mn2+ dissolving into electrolyte,suppressing the disproportionation of Mn³⁺ into Mn2+ and Mn4+ so that can dramatically enhance the cycle performance.Both LiMn₂O₄@3wt%LiMnPO4 and LiMn₂O₄@10wt%LiMn PO4 have shown improved electrochemical properties under several cycling testing in different charge/discharge current density.In addition,with the existence of lower manganese valence state in electrode material,the conductivity and lithium diffusion ability of LMO have been improved,which can result in an enhanced rate performance.Therefore,modification with stable Mn2+-rich phase on the surface of LiMn₂O₄ have a positive effect to improve electrochemical performance for high-performance LIBs.3,Spherical porous LiNi_0.5Mn_1.5O4 particles were produced by co-precipitation and heat-treatment,LiCoPO4（LCP）as shell has been grown on the surface of LiNi_0.5Mn_1.5O4（LNM）particles under hydrothermal condition.The morphologies of composite and the coating structure are confired by SEM and HRTEM,the cladding layer LiCoPO4 is proved by XRD and XPS.The data from XPS,CV and cell performance testing shows the growth of LiCoPO4 has simultaneously induced Mn³⁺ on the interface of LCP and LNM,and the amount of Mn³⁺ increases with the growth of LCP shell.The appearance of Mn³⁺ has dramatically improved the conductivity and lithium diffusion ability of LNM（EIS and GITT）.From analysis of cell cycling performance,it can be concluded that an appropriate amount of Mn³⁺ is necessary for improving the capacity and rate performance,while the stable shell of LCP preventing the dissolution of Mn³⁺ ensures the excellent cyclic performance of LNM@5%LCP.It is worth noting that LiCoPO4 as a stable shell effectively reduces the decomposition of the electrolyte LiPF6.The sample LNM@5%LCP with an appropriate amount of Mn³⁺ has shown the best electrochemical properties,128 mAh g-1 at 0.5 C rate and 96 % capacity remained after 100 cycles.At 20 C rate,LNM@5%LCP has demonstrated the best performance of 115 mAh g-1,much higher than 57 mAh g-1 of pure LNM.