Preparation And Modification Of Cathode Materials For High Specific Energy Lithium-ion Batteries

With the rapid development of portable electronic devices,electric vehicles and large-scale energy storage,people have put forward higher requirements for the energy density of lithium-ion batteries.In the existing system,the development of cathode materials with high safety and high specific capacity is the key to improve the energy density of lithium ion batteries.Among many high specific energy cathode materials,nickel-rich single crystal ternary cathode materials and lithium-rich cathode materials have attracted extensive attention because of their high specific capacity and low price.However,due to the structural defects of the material itself,it leads to poor electrochemical performance and safety problems,which seriously hinders its commercial application and promotion.In order to explore the scientific problems existing in the interface of cathode materials,as well as the interaction mechanism between structure composition and electrochemical behavior,in order to promote the rapid development of large-scale preparation technology and high specific energy lithium ion batteries.Based on the problems existing in the interface and crystal structure of cathode materials for high specific energy lithium ion batteries,the following two works were carried out in this paper:（1）Preparation and Electrochemical Properties of Interfacially Modified Nickel-Rich Single Crystal Ternary Cathode MaterialsSurface coating strategy is often used to modify Ni-rich single crystal ternary cathode for its residual lithium,slow kinetics and serious interface side reactions.However,most of the coatings do not grow in situ and hinder the transfer of Li⁺at the interface,which exacerbates the problem of slow kinetics of the positive electrode of nickel rich single crystal.In this chapter,through the chemical reaction between surface residual lithium and phosphate,AlPO₄-Li₃PO₄composite coating was constructed on the surface of nickel rich single crystal ternary cathode.The ionic conductor Li₃PO₄in the coating accelerated the transfer of Li⁺at the interface,while amorphous AlPO₄relieved the stress caused by anisotropy in the single crystal during charge and discharge.The discharge capacity of the Ni-rich single crystal ternary cathode modified by AlPO₄-Li₃PO₄is up to 201.6 m A h/g at 0.1 C,and the capacity retention rate is 88.9%after 200 cycles.After the interface modification,the structural stability and diffusion kinetics of the nickel rich single crystal cathode materials are enhanced,and the electrochemical properties are significantly improved.（2）Preparation,Optimization and Electrochemical Properties of Cobalt-Free Lithium-Titanium Based Cathode Materials with High Specific EnergyIn order to solve the problem that the electrochemical performance of lithium-rich cathode materials declines due to the bad phase transition during the cycle process,we prepared a cobalt-free lithium-rich titanium based cathode material by using the co-precipitation and molten salt calcination processes.By doping titanium into the phase to form a solid solution,the interlamellar Ti-O bond can not only stabilize ions,increase the migration energy barrier,inhibit the migration of other transition metals,maintain the integrity of the layer structure,but also improve the redox activity of oxygen.After being treated with hydrazine hydrate and sintered,a stable spinel-like phase was constructed on the surface of the particles,thus accelerating the transport of lithium ions and increasing its cycling stability.The Ti atom doped stable layered structure makes the specific discharge capacity of the material reach 189.2 m A h/g at 0.05 C in the first cycle,which can maintain a good capacity retention rate in the cycle.However,with the increase of the number of cycles,the capacity of the battery decays rapidly,which may be related to the preparation process of the material.Therefore,further structural optimization of the high specific energy cobalt-free lithium-rich titanium based cathode material is needed.