Surface Coating Modification And Electrochemical Properties Of Li-Rich Manganese-Based Cathode Materials

Recently,with the rapid development of new energy vehicles,smart grids and large-scale energy storage power stations,higher requirements have been put forward for the specific capacity,energy density,cycle life and safety of secondary energy storage devices,represented by lithium ion batteries.Layered Li-rich manganese-based cathode materials,xLi2MnO3·(1-x)LiMO2(0≤x≤1,M=Ni,Co,Mn),have been considered as one of the most promising cathode materials of next-generation highperformance lithium-ion batteries due to their large specific capacity(≥250 mA h g-1),high energy density(about 1000 W h kg-1),relatively high operating voltage and low cost.However,Li-rich manganese-based cathode materials also suffer from problems such as low initial coulombic efficiency,poor rate performance,and severe voltage/capacity degradation,which seriously hinder their commercial application.The main reasons include the decomposition of the electrolyte and side reactions with electrode materials,the low intrinsic diffusion coefficient of lithium ions in the Li-rich phase Li2MnO3,and the phase transition that occurs during cycling.In this context,this article aims to improve the electrochemical performance of Lirich manganese-based cathode materials via surface modification.Lithium phosphate(Li3PO4),as a solid electrolyte of lithium-ion batteries,has high ionic conductivity.The strategy surface coating modification by Li3PO4 has been proven to be one of the effective strategies to improve the electrochemical performance of Li-rich manganesebased cathode materials.The Li3PO4 coating layer can not only effectively prevent the direct contact between the electrode material and the electrolyte so as to reduce interfacial side reactions,increase the transmission rate of lithium-ions and electrons at the interface between the electrode material and the electrolyte,but also inhibit the oxygen release on the surface of the Li-rich manganese-based cathode material,thereby delaying the phase transition from layered structure to spinel structure and then to rock salt structure.As a result,the initial Coulombic efficiency,rate performance and cycling stability of lithium-rich manganese-based cathode materials are enhanced to a certain extent.In addition,the surface coating modification by lithium trivanadate(LiV3O8)is also an effective method to improve the lithium ion diffusion rate of lithium-rich manganese-based cathode materials,so its rate performance has also been improved to a certain extent.The main work and experimental conclusions of this article are summarized as follows:(1)Firstly,a layered Li-rich manganese-based cathode material with a microspherical morphology was prepared by the co-precipitation and high-temperature calcination strategy.A uniform Li3PO4 coating layer was successfully induced on the surface of the lithium-rich manganese-based cathode material by means of the hightemperature decomposition reaction of ammonium dihydrogen phosphate(NH4H2PO4).The experimental results show that the modified Li-rich manganese-based cathode material has good initial discharge specific capacity of 286 mA h g-1 at a current density of 0.1 C(1 C=250 mA g-1)and the initial Coulombic efficiency is 79.5%.The discharge specific capacities remain at 240 mA h g-1 after 100 cycles at 0.2 C with a capacity retention rate of 86.4%,at 219 mA h·g-1 after 200 cycles at 0.5 C with a capacity retention rate of 82.9%,at 196 mA h·g-1 after 500 cycles at 1 C with a capacity retention rate of 77.2%,showing excellent cycling stability.When matched with lithium titanate(Li4Ti5O12)anode material,the first cycle discharge specific capacity of the constructed full cell is 210 mAh·g-1 at a current density of 0.1 C.After 100 cycles at 1 C,the discharge specific capacity is 159 mA h·g-1 with a capacity retention rate of 82.5%.(2)The layered Li-rich manganese-based cathode material was prepared by coprecipitation and high-temperature calcination method,and then a layer of LiV3O8 was coated on its surface.The experimental results show that the modified material has higher specific discharge capacity and better cycling stability than the pristine material under 3 C and 5 C current densities.The discharge specific capacities under 3 C current density are 209 mA h g-1 at first cycle and 183 mA h g-1 after 100 cycles with a capacity retention rate of 87.5%.The discharge specific capacities at 5 C are 173 mA h g-1 at first cycle and 137 mA h g-1 after 200 cycles with a capacity retention rate of 75.1%.Furthermore,when cycled at a high current density of 10 C,its average discharge specific capacity is still 100 mA h g-1,showing excellent rate performance.