Study On Modification And Properties Of Layered Li-rich Mn-based Cathode Material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂

The layered lithium-rich manganese-based cathode materials have distinct advantages such as high specific capacity,high energy density,and environmental friendliness,which have received wide attention fromresearchers upon their discovery and are considered to be the next generation of commercial cathode materials.However,due to the low coulombic efficiency of the first time,the poor structural stability,and the continuous decay of mediumvoltage during the long cycle,the commercialization of layered lithium-rich manganese-based materials has been restricted.In this thesis,we have used Co₃O₄,a derivative of ZIF-67,glacial acetic acid,a mono-organic weak acid,and lithiumphosphate to modify the layered lithium-rich manganese-based cathode materials with various methods including material compounding,surface acid treatment,and combined surface treatment.It is expected to obtain excellent performance of layered lithium-rich manganese cathode materials and provide new technical methods for the development of their modification treatment and commercialization process.The main research contents are as follows:（1）Metal-organic backbones（MOFs）are ideal for designing micron/nanocomposites.When MOFs-derived materials are used as electrode materials for secondary batteries,their good spatial structure can improve the contact efficiency between electrodes and electrolytes and provide better channels for charge transfer.Lithium-rich manganese-based cathode materials prepared by co-precipitation method using acetate as the systemwere materially compounded with Co₃O₄,a derivative of metal-organic skeleton ZIF-67,by a simple solid-phase sintering method.The results show that the introduction of the appropriate amount of derivatives can reduce the particle size of the lithium-rich material and expand the specific surface area of the material,which are conducive to improve the contact efficiency of the cathode material with the electrolyte.Thus,the initial coulombic efficiency,cycling stability,and structural stability are improved.Among them,the specific discharge capacity of the 10:1LMR@67 sample with a composite ratio of 10:1 is 258.9 mAh g^-1at 0.1 C,and the first coulombic efficiency is 74.93%.In particular,it has specific capacity of 234.1 mAh·g^-1at low current density of 0.5 C and capacity retention rate of 86.28%after 100 cycles.（2）A mono organic weak acid,glacial acetic acid,is taken to activate the surface of lithium-rich manganese-based cathode material.The results show that glacial acetic acid is a mild and convenient treatment,which can effectively activate the Li₂Mn O₃phase without seriously breaking the ring of the crystal structure of the material.Moreover,the acid treatment can improve the particle distribution of the material and make it more loosely distributed,which is beneficial to improve the contact efficiency between the cathode material and the electrolyte and thus increase the discharge specific capacity and the first coulomb efficiency.The electrochemical test results showed that the GAA 2h sample treated with glacial acetic acid for 2 h had a specific capacity of 273.0 mAh·g^-1at a current density of 0.1C and a first coulomb efficiency of 79.87%.In addition,the capacity decay and corresponding d Q/d V plots of all samples at 0.5 C long cycle were also investigated,and it was found that the small amount of spinel-like phase produced by the glacial acetic acid treatment helped to retard the phase transition fromthe layered to spinel-like structure of the Li-rich material during the long cycle,thus retarding the voltage decay.（3）Glacial acetic acid and lithiumphosphate are taken to co-modify the surface of lithium-rich manganese-based cathode materials.The treatment with glacial acetic acid,a mono-organic weak acid,is mild and convenient,and can improve the particle distribution of the material to make it more loosely distributed,which can effectively activate the Li₂Mn O₃phase without seriously breaking the crystal structure of the material.In addition,the addition of lithiumphosphate helps to reduce the side reaction with electrolytes and replenish the loss of lithiumions during charging and discharging,thus increasing the diffusion efficiency and improving the stability of lithiumions.The results show that the combined surface treatment method is superior to any single treatment method.The electrochemical test results showed that the first discharge capacity and coulomb efficiency of GP-LMR treated with combined glacial acetic acid and lithiumphosphate surface treatment increased fromthe initial 229.7mAh·g^-1,69.25%,to 289.7 mAh·g^-1,83.53%,at 0.1 C current density.In addition,the discharge specific capacities at higher current densities of 0.5 C and 2 C were 243.6 mAh·g^-1and 189.3 mAh·g^-1,respectively.