Studies On Fast Ion Conductor Surface Modification Of High-Voltage Cathode Material LiNi_0.5Mn_1.5O₄

Lithium ion battery with high energy density,long cycle life,good safety performance and environmentally friendly characteristics,as one of the most widely used commercial small intelligent equipment energy storage components,is rapidly developing to the direction of new energy vehicles and large energy storage equipment.In the past 25 years,the rapid development and commercial application in portable electronic products of lithium ion batteries are obvious to all.Today,it is playing an increasingly important role in the sustainable energy sector.As far as the current lithium ion battery energy storage technology is concerned,in addition to the lack of safety and cycle life,further improvement of energy density is still the core of research.In the case where the anode material is constantly approaching the working voltage of the lithium metal,it is an effective way to increase the working voltage of the cathode material.The spinel structure LiNi_0.5Mn_1.5O₄ as representative of cathode material has received extensive attention due to the high working voltage?⁴.7 V?,high energy density(⁶50 Wh kg^-1),superior rate performance and low cost.However,high voltage cathode materials are always accompanied by more significant surface side reactions while exhibiting the advantages of a high operating voltage platform.In particular,as the number of cycles increases,the charge and discharge capacity and cyclic reversibility of the battery continue to decline,eventually leading to battery failure and even safety accidents.The main reason is that the violently surface chemical reaction of lithium ion battery cathode materials in high voltage working environment,such as irreversible surface phase transition,transition metal dissolution,John-Taylor distortion and electrolyte decomposition.In order to solve the above problemsand improve the electrochemical performance of high voltage cathode materials,a large amount of research work has carried out surface modification of materials by doping and coating to achieve the inhibition of interface side reactions and stable structures.In addition,all solid state batteries were considered as an important development aspect of next generation lithium ion batteries.The solid electrolyte has higher safety performance than the liquid electrolyte due to the use of solid state electrolytes,and it is possible to directly use lithium metal or a lithium containing compound as a negative electrode material.However,due to problems such as poor contact between the solid electrolyte and the electrodes,the all solid state battery is still in the research and exploration stage.In view of this,combined with the characteristics of the high voltage positive electrode material LiNi_0.5Mn_1.5O₄,we designed the fast ion conductor material as a modification layer to directly coat the surface of the particle,and the surface modification of the active material also solves the solid electrolyte.The interface compatibility problem with the electrode material provides an effective and feasible idea.The specific research content includes the following parts:The fast ion conductor material Li₂ZrO₃ was coated on the surface of LiNi_0.5Mn_1.5O₄ by wet chemical method.A series of physical characterizations have shown that the surface modification does not affect the bulk structure of the active material,and forms a uniform island-like?inner crystalline outer amorphous?coating on the surface.At room and elevated temperature,the modified material exhibited a better cycle stability than the pristine,and the rate performance significantly improved the diffusion kinetics of lithium ions and reduced the polarization phenomenon.In order to explore the mechanism of internal action,aging experiments were also carried out.The results show that the surface coating can effectively isolate the contact between the electrolyte and the cathode material,inhibit the occurrence of boundary side reactions,ease the dissolution of transition metals.Moreover,Li₂ZrO₃ of particular appearance could effectively improve the transmission dynamics of lithium ions at the interface while alleviating the problem of coating detachment caused by the intercalation/deintercalation process,thereby comprehensively improving the electrochemical performance of the material.The fast ion conductor LiVO₃ was selected as a coating layer to coat on the surface of LiNi_0.5Mn_1.5O₄material.By using the oxygen defect sites of LiNi_0.5Mn_1.5O₄ and the conditions of LiVO₃ material preparation,V elements are doped into the surface of LiNi_0.5Mn_1.5O₄ while modifying the surface.At the same time,electrochemical tests were carried out on materials with different coating contents.The results show that the modified materials have better cycle stability and rate performance,and the coating amount of 0.5 wt.%shows the best electrochemical performance.The XRD data and the refinement results show that the V element diffuses into the bulk phase structure,which increases the lattice parameter and realizes the double doping effect of the coating doping.The aging test results show that the surface modification relieves the occurrence of surface side reactions,effectively reduces the dissolution of transition metals and stabilizes the interface structure.On the basis of the second part,considering the addition of?PO4?^3-to improve the stability,Li₃V₂?PO₄?₃ was used as a modification layer to modify the high voltage cathode material.The physical test results show that the surface is evenly covered with an amorphous coating.After finishing comparison,it is found that the bulk crystal lattice increases linearly with the increase of the coating amount,and V ion gradient diffused into the surface lattice of the bulk phase material,which achieving the role of pinning fixation.A series of electrochemical tests show that the coated-doped integrated surface modification material has superior long-cycle stability and rate performance at room and elevated temperature.Through chemical and electrochemical aging tests,we found that the integrated modification could effectively inhibit the dissolution of transition metals,alleviate the occurrence of surface side reactions,stabilize the interface structure,and form a solid solution layer with V ions at the interface to enhance the coating layer and Li⁺transport properties between active materials.