| The Lithium-rich layered ternary cathode material x Li2Mn O3·（1-x）Li MO2（M=Ni,Co,Mn,0<x<1）has the characteristics of high capacity（up to 300 m Ah·g-1 or more）and low cost,which is considered as the most important next-generation high-energy-density lithium-ion battery cathode material.And it is also a research hotspot for the development of Lithium-ion batteries currently and in the future.However,due to its low coulomb efficiency,poor cycle performance,and poor rate performance,the application and development of Lithium-rich ternary cathode materials has been seriously hindered.In this paper,solvothermal technology was used to prepare spherical precursor particles with regular and uniform morphology.And we studied the influence of solvothermal temperature on the morphology of the precursor.After optimization,the cathode material Li1.2Mn0.52Ni0.2Co0.08O2 was obtained.At the same time,in view of the current poor rate performance and poor cycle stability of cathode materials,we carried out research work on the gas-phase Phosphating surface modification and the combination of Carbon-coating and surface Phosphating,which effectively improved the electrochemical performance of the Lithium-rich ternary cathode material.Furthermore,XRD,SEM,TEM,XPS and constant current charge-discharge,cyclic voltammetry,AC impedance analysis have been used to characterize and analyze the materials intensively,and the improvement mechanism of the electrochemical performance for the cathode material has been explored.The main research contents and achievements are as below.（1）First of all,solvothermal technology is used to prepare precursor materials of Li-rich materials,and the influence of solvothermal temperature on the morphology of the precursor were intensively investigated.By optimizing the addition amount of lithium,the best composition of the cathode material Li1.2Mn0.52Ni0.2Co0.08O2,was confirmed.Then we studied the influence of different solvothermal temperatures on the electrochemical performance of the cathode material.It is found that when the solvothermal temperature T=180℃and the mixed lithium content Li/M=1.50,the cathode material exhibited the best layered structure and electrochemical performance.Its first-cycle discharge specific capacity at 0.1 C is 241.2m Ah·g-1.The highest discharge specific capacity at 1 C reached 172.1 m Ah·g-1,and the capacity retention was 75.8%after 400 cycles.（2）Aiming at the problems of the low specific discharge capacity,poor rate performance,and insufficient cycle performance for the lithium-rich cathode material Li1.2Mn0.52Ni0.2Co0.08O2,the gas-phase Phosphating surface treatment strategy was adopted to modify the cathode material Li1.2Mn0.52Ni0.2Co0.08O2 surface.The constant current charge-discharge results reveled that when the mass ratio of the cathode material to Sodium Hypophosphite was 1:3,the material obtained by calcined at 350℃at a rate of 2℃/min for 3h in an argon atmosphere exhibited the best electrochemical properties.Its discharge specific capacity at 0.1 C is up to 290.7 m Ah·g-1,and its highest discharge specific capacity at 1 C reached 205.6 m Ah·g-1.The capacity retention rate after 300 cycles also increased from 78.8%of the untreated material to 80.1%.Based on the above investigation,we intend to find the reason for the improvement of electrochemical performance:During the gas phase Phosphating process,the P element enteres the surface layer of the material,which enlarges the distance between the Li layers.At the same time,the PH3 reacts with active oxygen on the surface of LRM due to its reducibility,forming Oxygen vacancies on the surface of LRM.Thereby gas-phase Phosphating could greatly improve the diffusion rate of Li+and the stability of the material structure.（3）The surface modification strategy of the combination of Carbon-coating and surface Phosphating was adopted to modify the surface of the cathode material Li1.2Mn0.52Ni0.2Co0.08O2.It is revealed by constant current charge-discharge test results that when using ethyl cellulose for 3 wt%Carbon coating,combined with the optimal surface Phosphating treatment,the cathode material exhibited better electrochemical performance than the only Carbon-coating material and/or the only Phosphating material.It is found that there is synergistic effect existed between Carbon-coating and surface Phosphating.Carbon coating could optimize the layered structure of the cathode material,increase the conductivity of electrons,and alleviate the corrosion of the electrolyte to the cathode material under high voltage.The surface Phosphating treatment could increase Li layers spaces and increase the diffusion rate of Li+.At the same time,Carbon-coating and surface Phosphating could activate more Li2Mn O3 phases and provide higher discharge specific capacity for the cathode material.Therefore,the cathode material co-processed by Carbon-coating and surface Phosphating achieved excellent electrochemical performances.Its specific discharge capacities are up to 297.5 m Ah·g-1 and216.2 m Ah·g-1 at the current densities of 0.1 C and 1 C.And the capacity retention at 1 C after300 cycles is up to 83.5%. |
PDF Full Text Request