Synthesis And Surface Modification Of Single Crystal Lithium-rich Manganese-based Cathode Materials

Lithium-rich manganese-based cathode materials are considered to have great potential as the next generation of high-energy lithium-ion battery positive electrode materials due to their high specific capacity,low cost,and high thermal stability.However,problems such as low compaction density and poor cycling performance seriously restrict their commercialization.In view of the above problems,this paper focuses on the exploration and research of single crystal lithium-rich manganese-based materials and further surface modification.The main research contents are as follows:(1)Research on the preparation of single crystal lithium-rich manganese-based materials using a single additive.Introducing additives into the mixture of precursor and lithium carbonate,and then conducting high-temperature solid-state sintering to synthesize lithium-rich manganese-based materials.Various lithium-rich materials were prepared using chloride,boric acid,and lithium tungstate as additives.The effects of key parameters such as additive type,sintering temperature,and additive ratio on the physical properties,crystal structure,microstructure,and electrochemical performance of lithium-rich manganese-based materials were systematically studied,and the mechanism of action of different additives was explored.The research shows that the boric acid as the assistant promotes the particle growth,the maximum single crystal particle size of more than 10μm can be obtained,but the particle size distribution is wide,the uniformity is not easy to control,and the electrochemical performance of the material is poor;Lithium tungstate as an additive has the effect of refining grains,with relatively uniform particle size,with a primary particle size of only 130 nm.But no single crystal particles are formed.Based on this,we combine the characteristics of different additives and consider utilizing the synergistic effect of different additives to carry out research on the synthesis of single crystal lithium-rich manganese-based materials using composite additives.(2)Research on the preparation of single crystal lithium-rich manganese-based materials using composite additives.By combining the different mechanisms of boric acid and lithium tungstate in the crystal growth process,boric acid and lithium tungstate were used as composite additives.With extremely low addition amounts,single crystal lithium-rich manganese-based cathodes were successfully obtained,and the compaction density and cycling stability were significantly improved compared to polycrystalline materials.The sintering process and mechanism of composite additives for preparing single crystal lithium-rich manganese-based materials were studied using semi-in-situ SEM,in-situ XRD,and TG/DSC characterization techniques.The effects of sintering temperature and proportion of additives on the preparation of single crystal lithium-rich manganese-based materials with composite additives were systematically studied.The results show that using boric acid and lithium tungstate as composite additives can obtain single crystal lithium-rich manganese-based materials with regular morphology and uniform particle size distribution;Composite additives help to form layered structures of materials at lower temperatures(650℃)during the synthesis process;The morphology and particle size of the obtained material can be controlled by adjusting the sintering temperature and addition ratio.Compared with polycrystalline lithium-rich materials,the compaction density increases from 2.94 g cm-3 to 3.38 g cm-3,an increase of 14.97%,and there is no significant bending of the plate after rolling.The specific surface area decreases from 2.896 m2 g-1 to 1.206 m2 g-1.However,the first cycle discharge specific capacity and rate performance of single crystal lithium-rich manganese-based materials have slightly decreased.Surface modification will be carried out in the next chapter to optimize electrochemical performance.The traditional process of synthesizing single crystal lithium rich materials using molten salt requires a large amount of potassium chloride/sodium and the addition of washing and filtration processes.The method in this paper has the characteristics of low additive dosage(4-6 wt%),simple process(one-time sintering),and significant effects(uniform and adjustable particle size).It has important reference significance for developing high-performance and low-cost single crystal lithium-rich manganese-based materials.(3)Research on surface modification of single crystal lithium-rich manganese-based cathode materials.In order to further improve the first cycle coulomb efficiency and rate performance of the single crystal materials,the surface modification was studied.The single crystal lithium-rich manganese-based material was treated with ammonium sulfate solution,and the residual lithium on the material surface was reacted with sulfate ions to form a Li2SO4 conductor coating layer,greatly improving the first cycle Coulomb efficiency and rate performance of the material.The changes in material structure,surface characteristics,and lithium-ion diffusion coefficient before and after surface modification were analyzed by XRD,TEM,XPS,and GITT.The results showed that the Li2SO4 layer was formed after surface modification,which improved the lithium-ion diffusion coefficient and material rate performance.The first cycle discharge specific capacity of material 0.1 C increased from 235.8 mAh g-1 to 259.9 mAh g-1,and the discharge specific capacity at 5 C magnification increased from 125.7 mAh g-1 to 162.0 mAh g-1.