Study On Surface Modification And Electrochemical Performance Of Lithium-rich Manganese-based Cathode Material

Lithium-rich manganese-based cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂（LMNC）is expected to be a new generation of cathode material for lithium-ion batteries because of its high specific capacity and low cost.However,the high irreversible capacity,poor cycling stability and multiplicative performance for the first time limit its commercial application.LMNC consists of layered hexagonal phase Li TMO₂ and layered monoclinic phase Li₂Mn O₃,in which the performance of Li₂Mn O₃ phase directly affects the electrochemical performance of lithium-rich manganese-based cathode materials.Based on this,this thesis starts from the Li₂Mn O₃ phase and treats the Li-rich manganese-based cathode material with NH₄VO₃ and Na BH₄ respectively to construct heterostructures on its surface and study the influence law of heterostructures and treatment methods on its electrochemical performance and charging/discharging mechanism as follows:（1）Li₂Mn O₃ cathode materials were prepared by high-temperature solid-phase method,and Li₂Mn O₃-Na cathode materials were synthesized by Na doping.The results showed that after Na doping,the discharge capacity was 35.8 m Ah g^-1 at 5 C discharge multiplicity and 11.2 m Ah g^-1 for Li₂Mn O₃,and the discharge capacity was improved by24.6 m Ah g^-1;the discharge capacity and retention rate were still 85.7 m Ah g^-1 and 94.0%after 100 cycles at 1 C,while the undoped cathode was only 57.2 m Ah g^-1（53.7%）,indicating that Na doping is effectiveness in improving the cycling stability and rate capability of Li₂Mn O₃.（2）Lithium-rich manganese-based cathode materials with Li₃VO₄ outer layer were obtained by treating lithium-rich manganese-based cathode materials with NH₄VO₃.The results showed that when NH₄VO₃ treatment was 3 wt%,the initial coulomb efficiency was improved from 59.97%to 70.45%,and the discharge capacity and capacity retention were improved from 151.9 m Ah g^-1（85.2%）to 169.3 m Ah g^-1（100.7%）after 120 cycles;the capacity under 5 C was improved from 69.8 m Ah g^-1 to 97.7 m Ah g^-1,an improvement of 27.9 m Ah g^-1,indicating that NH₄VO₃ treatment can significantly improve the electrochemical properties of the material.（3）Based on the above study,the Na-doped lithium-rich manganese cathode material was modified with 3 wt%NH₄VO₃ to the effect of the dual modification on the electrochemical performance of the material.The results showed that when the NH₄VO₃treatment content was 3 wt%,the first coulomb efficiency was improved from 59.97%to 70.45%,and the discharge capacity and capacity retention after 120 cycles were improved from 151.9 m Ah g^-1（85.2%）to 169.3 m Ah g^-1（100.7%）;the discharge capacity at 5 C was improved from 69.8 m Ah g-¹ to 97.7 m Ah g^-1,an improvement of27.9 m Ah g^-1,indicating that NH₄VO₃ pretreatment can significantly improve the electrochemical properties of the material.（4）NH₄VO₃ treatment can effectively improve the cycling performance of materials,but the improvement of material rate performance is limited.To investigate the effect of the way for constructing spinel structures on LMNC surface,Na BH₄ was used to pretreat the precursor of lithium rich manganese based cathode materials,and spinel structures were constructed in situ on their surfaces.The same method was used to pre-treat the Li₂Mn O₃ precursor,further verifying the mechanism of improving electrochemical performance.The results show that the surface spinel structure can effectively inhibit the irreversible escape of oxygen and provide Li⁺ion fast channels to improve the multiplicative performance of the material,and when the Na BH₄ treatment concentration is 0.01 M,the first coulomb efficiency of the Li-rich manganese-based cathode material is improved from 64.2%to 72.8%,and the first discharge specific capacity at 1 C is as high as 199.04 m Ah g^-1,which is much higher than that of the original sample 167.93m Ah g^-1;after 200 cycles,the material still had a reversible discharge capacity of 164.62m Ah g^-1 with a capacity retention rate of 82.71%,higher than that of the original sample of 135.17 m Ah g^-1（80.49%）.the lower discharge capacity at 5 C was 127.4 m Ah g^-1,an improvement of 54.6 m Ah g^-1 compared to the untreated sample.using The Li₂Mn O₃material precursor was treated with 0.01 M Na BH₄,and after 200 cycles at 1 C,its discharge capacity and capacity retention were 90.2 m Ah g^-1（151.3%）,respectively,which were much higher than that of the untreated sample of 69.3 m Ah g^-1（97.1%）.In summary,Na-doped Li₂Mn O₃ can inhibit the activation of Li₂Mn O₃ and improve the cycling stability and multiplicity performance of the material.The cycle stability of the cathode material was improved by 2%-15%after the surface construction of heterostructure,and the capacity was increased by 27-54.5 m Ah g^-1 under 5 C.The NH₄VO₃ treatment can effectively improve the structural stability and cycle stability of the material;the dual modification of Na doping and NH₄VO₃ is greater than the single modification,and the discharge capacity and cycle stability of the material are significantly improved.Compared with NH₄VO₃ treatment,Na BH₄ treatment of oxide precursors is more beneficial to improve the discharge specific capacity and multiplicity performance of the materials.