Study On Synthesis And Property Modification Of Li[Li_0.144Ni_0.136Co_0.136Mn_0.544]O₂ Li-and Mn-rich Cathodes Materials

With the urgent problems of energy and environment,the development of new energy materials becomes more and more important.Lithium-ion batteries?LIBs?have attracted wide attention as green and renewable energy storage devices.The key point to improve the energy density of lithium-ion batteries is to develop the high-capacity cathode materials.The specific capacities of LiCoO2,LiMn2O4,LiFePO4,Li?NixCoyMn1-x-y?O2 are less than 200 mAh g-1,while the specific capacity of lithium-excess and manganese-based cathode material?Li-rich?is higher than 250 mAh g-1.And the main element of the Li-and-Mn-rich cathode material is from Mn,which results that the price is lower than that of NCM ternary materials.Therefore it shows a good market prospect.However,Li-rich materials exist some problems,such as the large initial irreversible capacity,poor rate and cycling capacity,poor voltage attenuation,instability electrode/electrolyte surface in high voltage,lower conductivity of Li2MnO3,etc.The main ways to improve the performance are surface coating,doping,composition optimization with other materials,etc.In this dissertation,the properties of Li-rich materials are improved by adjusting the interfacial structure and bulk structure of the materials.The main research contents and results are as follows:1.Effect of ultrasonic-assisted co-precipitation method for preparing Li-rich materialsThe effect of different ultrasonic time on the morphology,structure and electrochemical performance of the material was studied by ultrasonic assisted co-precipitation method.The study found that ultrasonic can make the materials more uniform,and therefore the structure became more reasonable,which is beneficial to improve the electrochemical performance.The material with 8h time ultrasonic shows the best capacity,and the initial discharge specific capacity of 0.1C rate is 327.8 mAh g-1?higher than 265.2 mAh g-1 of the sample without ultrasonic?.The discharge capacity is 181.6 mAh g-1 at 1C rate after 50 cycles,with the capacity retention of 84.8%.After the cyclic voltammetry test and the EIS test,it was found that the composite oxide had higher current and reduced charge transfer impedance,and better rate performance.2.LiVO₃/LiV₃O₈ surface coating to improve the performance of Li-rich cathode materialsA layer of LiVO₃/LiV₃O₈ materials is coated on the surface of Li-rich manganese-based materials after annealing by thermal solvent method.LiV3O8 was evenly coated on the surface of the substrate materials.The formation of a spinel phase on the surface facilitates Li+ insertion into LiV3O8 to increase the coulomb efficiency and bulk phase migration of Li+.The initial discharge specific capacity of the composite with 5 wt.%LiV3O8-coated Li-rich electrode is 293.8 mAh g-1 at 0.1C rate,and the coulomb efficiency is 85.4%.The discharge capacity at 1C rate is 177.2 mAh g-1 after 50 cycles,with the capacity retention of 90.507%.The results demonstrate that 5 wt.07%LiV3O8-coated Li-rich electrode can effectively increase the initial coulomb efficiency and rate capacity.3.Facile fabrication of hollow porous Li-rich cathode materialThe hollow porous structure lithium-rich material is prepared by a carbon sphere template method.The formation of voids in the porous lithium-rich material was observed by electron microscopy,which increase the contact area between the electrode materials and the electrolytes,provide more electrochemical reaction sites,shorten the migration distance of Li+.The hollow porous materials exhibit a high discharge capacity of 296.6 mAh g-1 at 0.1 C rate,and cyclic stability of 185.2 mAh g-1 after 50 cycles at 1C rate.The discharge median voltage is 3.03V at 2C rate,and the specific energy is 690 Wh/Kg,and the electrode electrochemical polarization is low.Herein,we provide a facile and effective method for preparation of high-capacity hollow porous Li-rich materials.