Surface Modification Of Layered Lithium-rich Cathode Materials For Lithium Ion Batteries

With the rapid development of the electric vehicles,the commercialized cathode materials are difficult to meet the requirements of energy density for the extended cruising range.So it is essential to develop new cathode materials with high specific energy.Nowadays,the layered lithium-rich cathode materials have attracted a lot of attention due to its high specific capacity?>250 mAh/g?,low cost,good thermal stability and environmental friendliness.After years of research,however,the commercial application of lithium-rich cathode still has many challenges,including:the large irreversible capacity loss in the initial cycle,severe capacity and voltage attenuation,poor rate performance,etc.Therefore,the modification of layered lithium-rich cathode materials has become a research hotspot.The main methods of modification include optimizing synthesis conditions,synthesizing micro-nano secondary particles with special morphology,material pretreatment,surface modification,and cation and anion doping,etc.In this work,the layered stoichiometric lithium-rich cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂?symbolized as LrMNC for simplicity?is selected as the research object,and a series of surface coating studies are carried out to improve the electrochemical performance of the material.The main research work and results are as follows:?1?LrMNC is synthesized by traditional solid phase method,and then it is coated with Al₂O₃ and SiO₂,respectively.The results indicate that both coatings can enhance the electrochemical properties of LrMNC by structure,morphology and electrochemical characterization.But the improvement focuses on different aspects for different coating:the surface modification with SiO₂ is more effective in terms of increasing discharge capacity,but Al₂O₃ coating is better for stabilizing structure and improving cycle stability.?2?LrMNC is coated by Al₂O₃/SiO₂ composite?the molar ratio of Al₂O₃ to SiO₂is 1:1?with different mass fractions?1 wt.%,2 wt.%and 3 wt.%?.By comparing the electrochemical properties of different samples,the results show that when the coating amount is 1 wt.%,the cycle stability of the sample is still poor;when the coating amount is 3 wt.%,the sample presents poor rate capability due to the thick coating layer;and when the coating amount is 2 wt.%,the overall electrochemical performance of the sample is the best.?3?Under the premise of 2 wt.%coating amount,LrMNC is modified by nAl₂O₃/SiO₂ composite with different molar ratios?Al₂O₃:SiO₂=1:1,1:2,2:1?.By comparing the electrochemical properties of each sample,it is found that the coating has the best effect on the improvement of electrochemical performance when the molar ratio is Al₂O₃:SiO₂=2:1.The mechanism of the improvement is analyzed by various characterization methods?XRD,SEM,HR-TEM,XPS and FT-IR?:Al₂O₃/SiO₂ can combine with a small amount of Li₂O extracted from the surface layer of the bulk material during the coating process,and a new amorphous coating layer like Li_x[Al_ySi_zO₄]is formed.On the other hand,the surface layer of the bulk material undergoes structural transformation after losing a part of Li₂O to form an intermediate transition layer,which has a lithium-deficient spinel-like structure.The newly formed double-layer structure can accelerate Li⁺diffusion,and it can also prevent the erosion of electrolyte decomposition products,inhibit the release of lattice oxygen and structural evolution.Therefore,the surface coating method can effectively improve the cycle stability,rate performance and can aslo significantly suppress voltage attenuation.