Preparation And Modification Of Lithium-Rich Manganese-Based Cathode Materials Based On Ion Exchange/Diffusion Mechanism

Lithium-rich manganese-based cathode materials are considered as one of the most promising cathode material candidates for next-generation high-energy lithium-ion batteries due to their high specific capacity and low cost.However,problems such as capacity/voltage fading and insufficient rate capability hinder its practical application.In this paper,Li-rich manganese-based cathode materials were prepared based on ion exchange/diffusion mechanism,and their electrochemical performance was improved through synthesis process optimization,bulk doping and surface nickel enrichment.The main research contents are as follows:（1）Li_1.40Mn_0.60Ni_0.20Co_0.20O2+δ（LMNC）was prepared based on ion exchange/diffusion mechanism under acidic conditions using Li₂MnO₃ as the matrix material.The effects of calcination temperature and acid treatment time on the electrochemical properties of the cathode material were studied,and it was concluded that when the calcination temperature was 750℃ and the acid treatment time was 5 h,the overall performance of the prepared cathode material was the best:under the charge-discharge conditions of 0.1 C(20 mA g^-1)and 2.0-4.6 V,the specific capacity of initial discharge was 239.3 mAh g^-1.The capacity retention rate could reach 85.7%after 200cycles at 0.5 C rate;even if at a high rate of 10 C,the material still had a specific discharge capacity of 140.7 mAh g^-1.（2）In order to improve the electrochemical performance of the material,also based on the ion exchange/diffusion mechanism,the Cu²⁺bulk doped lithium-rich manganese-based cathode material Li_1.40(Mn_0.60Ni_0.20Co_0.20)_1-xCu_xO2+δwas prepared with Li₂MnO₃ as the matrix material.The large ionic radius of Cu²⁺（0.73 A）can enlarge the spacing between lithium layers,which is beneficial to the diffusion of Li⁺and thus improves the rate performance of the material.At the same time,Cu²⁺can replace a small amount of Ni²⁺in the lattice of the lithium layer,reducing the mixing of cations and improving the cycle performance of the material.Experiments showed that when the doping amount x was 0.02,the electrochemical performance of the material was the best,and its capacity retention rate was 92.1%after 200 cycles at 1 C current density,while the undoped sample was only 77.8%.In addition,the Cu²⁺-doped Li-rich material still had a discharge specific capacity of 153.1 mAh g^-1 at a rate of 10 C,while the undoped sample was only 140.7 mAh g^-1.（3）In order to improve the working voltage of the material and slow down the voltage decay,so as to further improve the energy density of the material.In this chapter,based on the ion exchange/diffusion mechanism,the lithium-rich manganese-based cathode material prepared in Chapter 1 was used as the matrix,and it was treated with secondary acid to introduce a small amount of nickel ions on the surface of the matrix to obtain the surface Ni-enriched cathode material Li_1.40Mn_0.60Ni_0.20Co_0.20O_2+δ@Ni.We all know that Ni can not only improve the working voltage and alleviate the voltage decay,but also inhibit the reduction of Mn⁴⁺to avoid the formation of spinel structure,thus improving the rate performance and cycling stability of the material.When the surface Ni enrichment was 3 wt%,the obtained sample showed the best electrochemical performance,the capacity retention rate was 92.3%after 200 cycles at 1 C,and the discharge specific capacity at 10 C high rate could reach 158.7 mAh g^-1,the median voltage of the first discharge was increased from 3.6808 V（the raw material）to 3.7766V,and the voltage decay was only 0.3632 V after 200 cycles,while the raw material up to 0.7301 V.