Preparation And Modification Of Lithium-rich Manganese-based Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries are widely used in portable electronics and electric vehicles due to their high energy density,low self-discharge rate,no memory effect,excellent cycle performance,and no pollution.However,the current energy densities of lithium-ion batteries cannot fully meet the requirements of next-generation high-range electric vehicles,so electrode materials with higher specific capacities are urgently needed.Among the various cathode materials for lithium-ion batteries,the lithium-rich cathode materials have attracted much attention due to their advantages of both high specific capacity and high operating potential.However,lithium-rich materials also have some shortcomings such as low Coulombic efficiency in the first charge/discharge cycle,poor rate performance,structural transition,and excessively rapid discharge potential decay and capacity decay during cycling,which seriously hinder their commercial application.Among various lithium-rich cathode materials,the lithium-rich manganese-based materials have the additional advantage of abundant manganese resources.This dissertation takes the Li-rich manganese-based cathode materials Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ as the research object,and focuses on the surface modification of the materials to improve the electrochemical performance.The main research contents and results are as follows:The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂（LMNCO）cathode material was prepared by a modified sol-gel method,and then a given amount of yttrium phosphate（YPO₄）was deposited on the surface of the LMNCO particles by a chemical deposition method to modify the material with different contents（1 wt%,3 wt%and 5 wt%）of YPO₄.The effect of YPO₄ surface modification on the electrochemical performance of LMNCO material was investigated.Electrochemical studies show that during the surface modification process,the Li₂Mn O₃ conponent on the surface of the Li-rich material undergoes a pre-activation reaction of partial delithiation.Among them,the material modified with 3 wt%YPO₄（3%YPO₄@LMNCO）has the best electrochemical performance.The first discharge specific capacities of the LMNCO and 3%YPO₄@LMNCO electrodes at 0.1C rate are 263.9 and 277.7 m Ah g^-1,respectively,and the initial coulombic efficiencies（ICE）are 61.4%and 73.8%,respectively.The discharge specific capacities at 5C rate are 91.7 and 117.4 m Ah g^-1,respectively.After 150 cycles at 1C rate,the capacity retention rates are 62.2%and 75.9%,respectively.With a moderate amount of YPO₄ surface modification,the ICE,discharge specific capacity,rate performance and cycle stability of the material are effectively improved.The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂（LMNCO）cathode material was prepared by a carbonate co-precipitation method.Meanwhile,the hydroxylated graphene quantum dots（OH-GQDs）were prepared by a hydrothermal method.Then,the LMNCO material was modified with different relative contents（1 wt%GQDs,3 wt%GQDs and 5 wt%GQDs）of the GQDs coated on the surface of the LMNCO particles by a facile solvent evaporation method.The electrochemical results demostrate that the GQDs coating can effectively improve the electrochemical performance of the lithium-rich material,and the modified material with 3 wt%GQDs coating exhibits the best discharge specific capacity,rate performance and cycle stability.For example,the discharge specific capacity at 0.2C rate is 270.3 m Ah g^-1.After 150cycles at 1C rate,the capacity retention is 86.5%,and the potential decay during the long-term cycling has also been improved.These improvements are mainly attributed to the high electronic conductivity of the GQDs coatings and the enhanced charge transfer process.Besides,the uniform GQDs coating layer acts as a physical isolation layer,which effectively hinders the side reactions on the surface of the active material and alleviates the structural transformation during the cycling.The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂（LMNCO）cathode material prepared by the aforementioned carbonate co-precipitation method was employed as a control sample,and the surface modified LMNCO materials with different contents of the conductive In₂O₃（1 wt%,2 wt%and 3 wt%）were prepared by a simple solvent evaporation method.The electrochemical results reveal that an appropriate amount of In₂O₃ surface modification can effectively improve the electrochemical performance of the Li-rich material.The modified material containing 2 wt%In₂O₃（2%In@LMNCO）has the highest ICE and the best rate performance and cycling stability,which are far superior to those of the pristine LMNCO material.For example,when the current rate is increased from 0.1C to 5C rate,the discharge specific capacity of the 2%In@LMNCO is dropped from 268.2 to 109.1 m Ah g^-1,giving capacity retention of 40.7%.When cycling at 1C rate for 200 cycles,the discharge specific capacity is decreased from 176.2 to 140.9 m Ah g^-1,giving capacity retention of 80.0%.