Modification And Electrochemical Performance Of Li-rich Mn-based Cathode Material

As one of the convenient energy storage devices,lithium-ion batteries are widely used in the field of 3C electronic products and power systems.Nowadays,the energy density of commercial Li-ion batteries cannot meet the requirements of our daily life,which is limited by cathode material.Li-rich Mn based cathode x Li₂Mn O₃·（1-x）Li MO₂（M=Ni,Co,Mn）is considered as one of the most promising cathode materials for the next generation of Li-ion batteries with high energy density due to its high discharge capacity and low cost.In this thesis,Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material was modified by boron doping combined with carbon surface modification and replacing different metal elements with Al.（1）Study on boron doping and carbon surface modification for Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ material.The best doping amount of B was selected from the materials of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]_1-xB_xO₂（x=0.01,0.02,0.05）.Electrochemical performance results show that the[Li_0.2Mn_0.54Ni_0.13Co_0.13]_1-0.02B_0.02O₂（LRM-B-2%）owns the best cycling performance with capacity retention rate of 80.98%after 100cycles.Then,based on the preparation of LRM-B-2%,the boron doped layered@spinel@carbon heterostructured Li-rich Mn based cathode material（LRM-B/C）was obtained by carbon surface modification.The test results indicate that LRM-B/C displays a specific discharge capacity of 135.8 m Ah g^-1 at 5 C.Especially,it not only shows excellent thermal stability at 45℃with a capacity retention of 83.3%after100 cycles,but also displays a specific discharge capacity of 108.9 m Ah g^-1 at-20℃.The excellent electrochemical performance of LRM-B/C are attributed to that B-doping can increase the lattice spacing,which is conducive to Li⁺insertion/extraction and restrains oxygen loss.The spinel and carbon layer can reduce the corrosion of electrolyte decomposition on material surface and lessen the occurrence of phase transformation.The three-dimensional conductive structure can accelerate the transportation of Li⁺and electrons.（2）Al-substitution for different metal elements in Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was studied.The electrochemical performance of Al doped materials were investigated.Results indicate that the initial irreversible capacity,cycle performance and rate performance of Li_1.2Mn_0.54-xNi_0.13Co_0.13O₂（x=0.1,0.2,0.5）materials are improved.The Li_1.2Mn_0.52Al_0.02Ni_0.13Co_0.13O₂（LRM-Al-2%）material shows the best electrochemical performance with a capacity retention of 85.50%after 100 cycles.Therefore,Al（2%）was used to replace Li,Ni,Co and Mn in Li_1.2Mn_0.54Ni_0.13Co_0.13O₂,respectively,to study the distinctions of electrochemical performance including the initial coulomb efficiency,voltage and capacity fading,rate performance as well as high voltage performance of the materials.Results indicate that when Li replacing with Al,its first coulomb efficiency is improved.Al instead of Co promotes the rate performance of the cells.The substitution of Mn not only shows the highest capacity and voltage retention in the potential range of 2.0-4.8 V,but also delivers excellent capacity retention at a higher cut-off voltage of 5.0 V.Unfortunately,the replacement of Ni cannot perfect the electrochemical performance effectively.It is found that the electrochemical performance of Al doping materials for Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ are closely related to the migration,dissolution and valence change of transition metal ions as well as structural evolution.In conclusion,B-doping combined with carbon surface modification effectively improves the electrochemical performance of Li-rich Mn-based cathode materials.Replacing different metal elements with Al shows different effects on the electrochemical performance of the materials.This will provide a new research idea for the co-modification and optimization of element proportion for layered cathode materials.