Surface Decoration And Lithium Storage Mechanism Of Lithium-rich Manganese Oxide As Cathode In Lithium-Ion Batteries

Lithium-rich manganese oxides[x Li₂MnO₃（1-x）Li MO₂（M=Ni、Co、Mn,0<x<1）]have attracted more and more attentions due to high specific capacity of>250 m Ah g^-1,low cost and environmental friendliness.However,these materials experience low initial coulombic efficiency,poor rate capability,and unsatisfactory cycling stability,limiting the commercial application.In this thesis,the means of coating and surface treatment have been applied on Lithium-rich manganese oxides,and the effects on the electrochemical properties have been analyzed.1.NH₄F has been used to react with Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ on the surface layer,under high temperature,NH₄F can be decomposed to produce HF,which will activate Li₂MnO₃ by a proton exchange reaction.Therefore,the irreversible release of lattice oxygen in the first charge process can be effectively inhibited,increasing the initial columbic efficiency.When treated with NH₄F,the initial coulombic efficiency increase from 82.6%to 94.5%.Meanwhile,the structural stability is improved due to the reduction of lattice oxygen extraction and the formation of spinel layers,hence the cycling stability of the treated Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ has been improved with a capacity retention of 82.8%after 50 cycles at 0.1 C.2.NH₄H₂PO₄ has been used to treat Li_1.2Mn_0.54Co_0.13Ni_0.13O₂,here,H₃PO₄in-situ generated by NH₄H₂PO₄ decomposition has the function to pre-activate Li_1.2Mn_0.54Co_0.13Ni_0.13O₂,forming Li₃PO₄（lithium-ion conductor）and spinel lithium manganate in the surface layer.It is found that the formed interface can effectively inhibit the irreversible release of lattice oxygen in the first charge process,increasing the initial columbic efficiency.Meanwhile,the hybrid modification layer can effectively increase the lithium diffusion coefficient due to spinel lithium manganate with 3D channel for fast Li⁺diffusion and lithium-ion conductor Li₃PO₄,which helps to the rate capability.When treated with NH₄H₂PO₄,the initial coulombic efficiency can be elevated from 82.6%to 90.2%,while the discharge capacity at 5 C is increased from 60.2 m Ah g^-1 to 99.2 m Ah g^-1.3.Polymerized ionic liquids（PILs）with high lithium-ion conductivity and excellent electrochemical stability is used to coat on the surface of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ to cut off the direct contact of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ and liquid electrolyte.LMRO@PILs2%material exhibits a capacity retention rate of 79.6%after 50 cycles at the current density of 0.1 C while still maintaining a discharge specific capacity of 106.2m Ah g^-1 at the current density of 5 C.The improved electrochemical performance should be mainly contributed by PILs with the function to allow the fast diffusion of Li⁺the molecular chains and isolate liquid electrolyte and Li_1.2Mn_0.54Co_0.13Ni_0.13O₂,benefiting for cycling stability and rate capability.