Research On The Structure Regulation And Electrochemical Performance Of Cobalt-free Lithium-rich Manganese-based Cathode Material

Lithium-rich manganese-based materials have the advantages of high discharge specific capacity,wide operating voltage window,low cost,and excellent safety,making them the main candidates for the next generation of lithium-ion battery cathode materials.However,there are still many issues with this type of material,such as low initial efficiency,poor cycling performance,and severe voltage attenuation.In this thesis,rich lithium manganese-based cathode materials were prepared using the sol-gel method,and various methods were used to modify the bulk and surface of the materials.The structural characteristics of the materials and their relationship with electrochemical performance were investigated through physical characterization and electrochemical performance testing.Firstly,the influence of process parameters on the morphology and structure of the material was investigated,and high crystallinity and excellent electrochemical performance cathode materials were prepared.Subsequently,Li_1.2Ni_0.2Mn_0.6O₂ was modified using Al ions.The Al doping,achieved by forming stronger Al-O bonds,inhibited the transformation from a layered structure to a spinel structure and the release of oxygen,resulting in a material with improved cycling performance.The capacity retention of Li_1.2Ni_0.2Mn_0.56Al_0.04O₂ after 200 cycles was 83.8%at 1 C,with an average voltage decay of 1.11 m V per cycle.Secondly,hydrochloric acid dopamine was used as a precursor for carbon coating,and the above-mentioned Li_1.2Ni_0.2Mn_0.56Al_0.04O₂ material was surface-modified.After high-temperature treatment,a carbon layer doped with both N and Cl was formed on the material surface.Simultaneously,hydrochloric acid dopamine decomposed,generating ammonia gas that partially reduced Mn ions and created numerous oxygen vacancies on the material surface.The N and Cl co-doped carbon layer reduced the direct contact between the active material and electrolyte,thereby reducing excessive metal dissolution.The presence of the carbon layer also increased the resistance to oxygen release.The reduction of Mn ions resulted in a higher capacity of the material.The presence of oxygen vacancies disrupted the Li-O-Li structure,forming an inert layer for anionic redox reactions on the material surface,thereby reducing oxygen release during the initial charge-discharge process and enhancing the material’s high capacity and stability.After modification,the material exhibited a capacity of 276.4 m Ah·g^-1 at 0.1 C,with a capacity retention of 93.4%after 200 cycles and an average voltage decay of 0.55 m V per cycle at 1 C.Finally,deionized water was used to modify the surface structure of Li_1.2Ni_0.2Mn_0.56Al_0.04O₂.The results showed that water treatment pre-activated the Li₂Mn O₃ on the surface of the lithium-rich manganese-based material and created lithium vacancies and oxygen vacancies on the material surface.The presence of lithium vacancies and oxygen vacancies further disrupted the Li-O-Li structure,resulting in a loss of the material’s surface anionic redox capacity.As the cycling progressed,the lithium vacancies and oxygen vacancies were gradually filled,leading to an increase in capacity.The sample treated with deionized water exhibited a capacity retention of 109.45%after 200 cycles and 96.0%after 500 cycles,with an average voltage decay of 0.18 m V per cycle.Additionally,anhydrous ethanol was also used for pretreating Li_1.2Ni_0.2Mn_0.6O₂.The research results indicated that ethanol dispersed the particles between the materials,increased the specific surface area,and increased the reaction sites for lithium extraction and insertion.This improved the utilization of Li₂Mn O₃ and greatly enhanced the material’s capacity.Furthermore,ethanol treatment reduced the residual lithium on the particle surfaces,which suppressed the formation of the space charge region and increased the lithium extraction ability,thereby improving the rate performance.After ethanol treatment,the material exhibited a capacity of 310.2 m Ah·g^-1 at 0.1 C,with an initial coulombic efficiency of 81.9%and a capacity of 179.2 m Ah·g^-1 at 5 C.