The Effect Of Electronic And Ionic Conductive Coating Layer On The Physico-Chemical And Electrochemical Properties Of Li-Rich Layered Oxide

The development of lithium-ion batteries with high performance is expected to meet the needs of large-scale energy storage power stations and automotive power sources.The lower discharge capacity of the cathode material is a bottleneck problem that restricts the application of the existing lithium-ion battery technologies in the above-mentioned fields.In the existing systems of cathode material for lithium-ion battery,Li-rich layered oxide has attracted widespread attention due to its advantages of high theoretical discharge capacity(>300 mAh/g),wide discharge voltage range,and low cost.However,the practicality of Li-rich layered oxide is still facing problems of poor rate performance and poor working stability.In view of the above problems,this thesis intends to take Li1.2Mn0.54Ni0.13Co0.13O2(LMNC)as the research object,and proposes a modification strategy to optimize the microstructure and electrochemical performance of the material by preparing an electronic-ionic conduction coating layer on its surface.The specific work involves the preparation of spinel coating in situ on the surface of LMNC through a glucose-based pretreatment and the preparation of LiAlO2/C composite coating on the surface of LMNC by wet chemical method.The main findings of this thesis are as follows(1)In-situ preparation of spinel coating:Spinel coating was prepared on the surface of LMNC through a glucose-based pretreatment for the first time.The modification process can achieve the chemical activation of the lithium-rich phase Li2MnO3.The modification process significantly improved LMNC's initial discharge capacity,initial coulombic efficiency,rate performance,and cycle stability.For example,the discharge capacity and coulombic efficiency for the first cycle increase from 228.7 mAh/g and 73.4%of the pristine sample to 292.0 mAh/g and 93.4%.After 100 cycles,the discharge capacity and voltage retention increased from 73.4%and 78.7%of the pristine sample to 84.1%and 86.2%.The above-mentioned conversion of the electrochemical performance for LMNC can be attributed to the in-situ preparation of spinel coating on the surface of the material and the chemical activation of the bulk Li2MnO3 component by the glucose-based pretreatment,which also improved the electronic-ionic conductivity and optimized the evolution of the crystal structure of the material during the charge-discharge process(2)LiAlO2/C composite coating based on wet chemical method:For the first time,LiAlO2/C composite coating was prepared on the surface of LMNC based on sucrose and other raw materials.During the modification process,sucrose caused the chemical activation of the lithium-rich phase Li2MnO3 and increased cation mixing of LMNC LiAlO2 can suppress the cation mixing of LMNC.The modification process effectively improved the electrochemical performance of LMNC.For example,after 200 cycles,the discharge capacity and voltage retention increase from 68.8%and 75.9%of the pristine sample to 96%and 78.7%.This may be related to LiAlO2/C composite coating,which improved the electronic-ionic conductivity of the surface of Li-rich layered oxide and inhibited the corrosion from the electrolyte to the active material,and the chemical activation of Li2MnO3,which optimized the crystal structure evolution of the material during the charge-discharge process.