Study On Synthesis,Microstructure And Lithium Storage Performance Of Mg-Mn Spinel Anode Materials

Due to the advantage of high energy density,low self-discharge and long cycle life,lithium ion batteries dominate the application market of portable electronic devices and electric vehicle energy storage systems.Anode materials largely determine the performance of the whole electrochemical system because it is a key component of lithium ion batteries.Graphite,currently still used as the main anode material for commercial lithium ion batteries,cannot meet the increasing energy and power density requirements of modern society due to its low theoretical capacity and low lithium ion transport rate.Therefore,the research and development of a new generation of highperformance and low-cost anode materials is an important research direction for lithium ion batteries.Mg-Mn spinel oxide due to its advantages of high theoretical capacity,low cost and environmental friendliness,has become one of the most promising anode materials for lithium ion batteries.However,some inherent defects of spinel metal oxide limit the application of Mg-Mn spinel in the field of lithium ion battery,such as poor conductivity,poor rate performance and large volume changes during charging and discharging.In order to overcome the above shortcomings,in this thesis,different synthesis routes were designed to regulate the morphology and structure of Mg-Mn spinel oxide materials（Mg₂MnO₄ and MgMn₂O₄）,metal ion doping substitution and oxide coating modification to improve the electrochemical performance of the materials.The specific research contents of this paper include the following aspects:（1）The cubic spinel Mg₂MnO₄ materials with porous sponge structure and porous microsphere morphology were prepared by sol-gel and solvothermal method,respectively.The effect of annealing temperature on the microstructure of the materials was studied.Mg₂MnO₄ was used as anode material for lithium ion battery for the first time.The influence of different morphology on its lithium storage performance was studied,and the relationship between cation distribution and electrochemical performance of the material was discussed.The results show that the Mg₂MnO₄ porous microsphere material obtained by solvothermal method at 800℃ annealing temperature exhibits excellent lithium storage performance due to the large pore structure and low cation mixing degree on the structure.Since the porous microsphere structure provides more lithium ion transport channels,it promotes the migration of lithium ions in the electrode material and increases the lithium storage sites.At a current density of 100 mA g^-1,the first discharge capacity of the porous microsphere electrode material is 835.3 mAh g^-1,the first coulombic efficiency is 59.12%,and the discharge capacity after 100 cycles is 406.5 mAh g^-1.Rate tests show that the material can maintain good cycle stability and provide an average lithium storage capacity of 255.0 mAh g^-1 even at a current density of 1000 mA g^-1.In addition,CV and EIS results show that the porous microsphere electrode material has excellent electrochemical reversibility and high lithium ion diffusion coefficient.（2）Tetragonal spinel MgMn₂O₄ materials with different morphologies were synthesized by three methods（sol-gel,reverse coprecipitation and template method）.The effects of different experimental conditions on the phase,morphology and cation distribution of the materials were investigated.The electrochemical properties of MgMn₂O₄ spinel anode materials with different morphologies were studied.The lithium storage mechanism of the materials was analyzed,and the relationship between the microstructure and electrochemical properties of MgMn₂O₄ materials with different morphologies was discussed.The results show that the MgMn₂O₄ materials obtained by different preparation routes are mixed spinel structures.The spinel inversion parameters of the sheet like nanomaterials（obtained by sol-gel method with citric acid（chelating agent）/anhydrous ethanol（solvent）as the synthesis conditions）,the porous microsphere materials（obtained by reverse coprecipitation method annealing at 700℃）and the graded porous hollow microsphere materials（obtained by MnCO3 as self-sacrificing template）are less than 0.5.Electrochemical tests show that different morphologies have a significant effect on the performance of MgMn₂O₄ anode materials.Among them,the electrode material with graded porous hollow microsphere structure has the best lithium storage performance.The initial discharge capacity is as high as 1429.5 mAh g^-1 at a current density of 100 mA g^-1.After 200 cycles,it can still maintain a discharge capacity of 670.7 mAh g^-1,and provide an average discharge capacity of 295.5 mAh g^-1 at a high current density of 1000 mA g^-1 and maintain good cycle stability.The excellent lithium storage performance of MgMn₂O₄ electrode material is due to its graded porous structure,which increases the contact area between the electrode surface and the electrolyte and accelerates the migration of lithium ions.（3）The cation substitution strategy is considered to be an effective method to generate defective structures in the spinel structure to improve the lithium storage performance of MgMn₂O₄ electrode materials.MgMn_2-xCo_xO₄（x=0,0.5,1,1.5,2）and MgMnMO₄（M=Ni,Co,Ga）materials were synthesized by hydrothermal and sol-gel method,respectively.The effects of metal ion doping on the microstructure and electrochemical properties of MgMn₂O₄ materials were studied.The results show that the doping ion type and doping concentration have a significant effect on the microstructure and properties of the material.All doped materials are mixed spinel structure,Mn/Co/Ni/Ga ions exist in the spinel structure with multiple valence states.Co ion doping can significantly increase the oxygen vacancy concentration in the material and improve the conductivity of the material.Ni ions reduce the degree of cation mixing in the spinel structure due to their octahedral selectivity.The lithium storage performance of the material was studied.It was found that the lithium storage performance of the Ni-doped MgMnNiO₄ electrode material was greatly improved.The first lithium insertion capacity was as high as 1635.3 mAh g^-1,and the high cycle capacity of 760.5 mAh g^-1 was maintained after 280 cycles.Electrochemical impedance test confirmed that the Ni-doped MgMn₂O₄ material had a high lithium ion diffusion coefficient.The rate test showed that it had cycle stability and good capacity recovery at high current.（4）N-doped Co₃O₄ was coated on the surface of MgMn₂O₄ microspheres by direct coating method with PVP as additive,and N-C03O₄ uniformly coated MgMn₂O₄ microspheres composites were obtained（MMO@N-Co₃O₄）.The electrochemical test results show that the N-Co₃O₄ coated MgMn₂O₄ composite electrode exhibits excellent electrochemical performance.The initial discharge capacity of MMO@N-Co₃O₄ is 1269.2 mAh g^-1 at a current density of 100 mA g^-1,which is higher than the initial lithium storage capacity of MgMn₂O₄ material before coating(637.0 mAh g^-1).The composite electrode can still maintain a high lithium storage capacity of 800.2 mAh g1 after 200 cycles.The rate performance test results at different current densities show that the MMO@N-Co₃O₄ electrode has good rate performance and capacity recovery.The EIS results confirm that the N-Co₃O₄ coating layer is beneficial to the MgMn₂O₄ material to form a stable solid electrolyte interphase film structure and significantly improve the lithium ion diffusion coefficient of the electrode material,and promote the electrochemical reaction kinetics process.