Study On Surface Modification Of Lithium-rich Manganese-based Cathode Materials

Lithium-rich manganese-based oxide（LMRO）is considered to be one of the most promising cathode materials for lithium-ion batteries with high energy density requirements,but its practical application is still restricted by some problems,such as low coulombic efficiency for the first time,insufficient rate performance and obvious capacity/voltage attenuation.It is generally believed that the electrochemical performance degradation of LMRO cathode material is caused by irreversible oxygen loss,metal ion migration and structural evolution on the surface.Therefore,in this paper,the electrochemical properties of materials were improved by uniform doping and coating modification on the surface of materials through precision surface engineering.The main research contents are as follows:（1）Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ Li-rich manganese-based cathode material with uniform deposition of tannic acid（TA）molybdenum（Mo）complex coating on the surface was obtained by hydrothermal solid-state reaction and reaction kinetics regulation,and then the material with uniform doping modification on the Mo surface was obtained by heat treatment.XRD and TEM characterization showed that most Mo ions were doped into the lattice structure of the electrode material,and a small amount of Mo ions did not enter the lattice structure formed spinel phase Li₂Mo O₄ on the surface,which could effectively improve the structural stability of the material and the Li⁺diffusion rate.Among them,LMNCO-6.5-Mo sample has the best electrochemical performance.The initial discharge capacity of LMNCO-6.5-Mo sample at 2-4.6 V and 0.1 C is 280.0 m Ah g^-1.After 100 cycles at 0.5 C,the capacity retention is still 80.2%,and the corresponding voltage decay is only 0.3369 V.At 2 C density,the capacity remained 72.1%after 150 cycles,and the rate performance was also significantly improved.（2）TA-Al complex coating layers with different thicknesses were uniformly deposited on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material by chemical kinetics control.The comprehensive modification of Al³⁺doping and Al₂O₃ coating on the surface of electrode material was realized by further heat treatment.Al³⁺doping is conducive to stabilizing the lattice structure and improving the Li⁺transmission rate.Al₂O₃ alleviates the occurrence of interfacial side reactions and improves the rate performance and cycle stability of the material.The initial discharge capacity and coulombic efficiency of LMNCO-12-Al at 2-4.6 V and 0.1 C are 283.8m Ah g^-1 and 82.9%,respectively.After 100 cycles at 0.5 C,the capacity retention rate was still 80.7%,and the corresponding voltage attenuation was only 0.3512 V.Even after 150 cycles at 2 C high current density,the capacity retention rate was still 70.4%,and the rate performance was good.At 5 C high current density,the discharge specific capacity was still 149.0 m Ah g^-1.（3）Uniform Zn O nano-coating was constructed on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material by wet chemical method and heat treatment.An appropriate amount of electrochemical inert Zn O coating can alleviate the occurrence of interfacial side reactions such as electrolyte corrosion,transition metal dissolution and oxygen release during cycling,and improve the cycle performance and rate performance without affecting the Li⁺transmission rate.The Li-rich Mn-based cathode material with Zn O coating thickness of 3 nm exhibits the best electrochemical performance.At 2.0-4.6 V,0.1 C,the initial discharge capacity is278.9 m Ah g^-1.After 100 cycles at 0.5 C,the capacity retention is 79.0%,and the rate capability is improved.Even at 5 C,the discharge capacity is 151.9 mAh g^-1.