Structure, Preparation And Electrochemical Performance Of Ternary Manganese-based Lithium-rich Cathode Materials

With the development of science and technology and the increase demand of energy usage,challenges and opportunities appeared in the field of energy storage conversion.As an energy storage device,lithium ion secondary battery has been widely used and well developed Due to its special energy storage performance.However,the specific capacity,rate performance 'and cycle performance of the electrode materials depend on the materials adopted and the synthetic methods,which in turn has an impact on the overall performance of the battery.To reveal the structure-performance relationship and to provide some guideline for material design,the present thesis is aim at combining theoretical and experimental techniques to investigate some issues existing in the Mn-based cathode compounds.Firstly,the structural stability,electronic structure and theoretical voltage of LiMn2O4 cathode material are calculated by using the plane wave pseudo potential method based on density functional theory.The results show that the crystal has a good thermodynamic stability.In order to study the surface effects of LiMn2O4 materials,14 different surface models were constructed,and the corresponding surface energy and surface Grand potential were calculated.The calculation results show that along with the chemical potential changes,the morphology of the particles will be changed regularly.Using first principle calculation method,displacement of Li+ calculated after effects on electronic structure and bonding characteristics of LixNi0.5Mn1.5O4 stability and the calculation results show that the thermodynamic stability of LiNi0.5Mn1.5O4 and Ni0.5Mn1.5O4 are better than the traditional LiNiO2 and LiCoO2 electrode materials.To investigate the influence of Li2MnO3 phase on the structure and properties of xLi2MnO3·(1-x)Li(Mn1/3Co1/3Ni1/3)O2 materials,different theoretical models were constructed.The computational results shown that lithium ions located between the TM layers will be extracted first during the charging process,accompanied by the oxidation of the TM ions.With the increase of L2MnO3 content.the calculated formation enthalpics decreased gradually.To investigate the electrochemical performance of the electrode materials,solid-state synthesis,co-precipitation,solvothermal,and molten salt methods were applied to synthesize 0.5Li2MnO3·0.5Li(Mn1/3Co1/3Ni1/3)O2.To control the microscopic structure and improve the capacity,a solvothermal method was applied first to synthesize MnO2 microspheres.Then molten salt,lithium,and transition metal sources were introduced.After calcination,lithium-rich cathode materials with hierarchical structures were obtained.Our results showed that the material has a discharge capacity of about 199.2 mAh·g-1 when LiNO3 was added as lithium sources.After introduction of NaCl,the discharge capacity was improved significantly and reached 284.7 mAh·g-1.Furthermore,by using Li2CO3 and NaCl as lithium source and molten salt respectively,the discharge capacities of the materials were 226.1 mAh·g-1 and 259.6 mAh·g-1,respectively.To investigate the effect of Li2MnO3 content on the structures and performances,x Li2MnO3·(1-x)Li(Mn1/3Co1/3Ni1/3)O2 materials with x=0.18,0.25,and 0.40 were successfully prepared by using LiNO3 and NaCl as lithium sources and molten salt respectively,and the XRD,SEM,Raman characterizations,and corresponding electrochemical tests were also performed consequently.Furthermore,our calculations also suggested that with the increase of Li2MnO3 content,M-O bond covalency and the structural stability of the materials will be enhanced gradually,which is favorable for the improvement of the cycling stability and rate capability of the materials.