Study On Modification Of Cobalt-free Layered Transition Metal Oxide Cathode Materials

The multivariate transition metal layered oxide cathode material Li_xMO₂（M=Ni,Mn,Fe,Al,etc.,1≤x≤1.2）has a high energy density.Due to its complex phase structure and multiple controllable components,it shows different properties with the change of phase structure and components.Among them,cobalt-free Li₂Mn O₃ is a key component of high energy density lithium-rich manganese-based materials in this system.Previous studies have shown that the voltage attenuation of the discharge platform of lithium-rich manganese-based materials is closely related to Li₂Mn O₃.On the other hand,currently,the commercialized cathode material in the ternary system is the high nickel layered cathode material.Compared with Li₂Mn O₃,this material has a relatively stable discharge platform voltage.However,the secondary spherical particles composed of nanoparticle aggregates prepared by the traditional coprecipitation technology are easy to crack during cycling,which affects its cycling stability.In this paper,the modification of Li₂Mn O₃ materials and the preparation of high-nickel materials are studied.Firstly,pure Li₂Mn O₃ and oxygen ion conductor(Zr_0.92Mg_0.08O₂（ZMO）and Zr_0.92Sr_0.08O₂（ZSO）)coated Li₂Mn O₃cathode material were synthesized by liquid phase method.The effect of oxygen ion conductor on the crystal structure,particle morphology and electrochemical performance of Li₂Mn O₃ is studied.The results show that the oxygen ion conductor only acts as a coating layer on the surface of the particles,and does not change the crystal structure of the original material.In addition,due to the oxygen ion migration property,the irreversible release of oxygen is inhibited by the oxygen ion conductor,and part of the oxygen ions return to the body,which makes the generated spinel phase with electrochemical reversibility and prevents the continuous collapse of material structure in the late cycle,thus alleviating the attenuation of discharge platform voltage and capacity.In the 2.0-4.8 V voltage window and12.5 m A·g^-1 current density,the coulombic efficiency of the first cycle of LMO@1ZSO cathode material is 65.86%,and that of pure LMO is only 62.18%.For 100 cycles at 250 m A·g^-1,the capacity retention of LMO@1ZSO is 82.7%,while that of pure LMO is only 44.3%.Secondly,the co-free Li Ni_0.9Fe_0.05Al_0.05O₂ cathode material was synthesized by starch assisted liquid phase method.The effects of starch addition on the crystal structure,particle morphology and electrochemical properties of NFA cathode materials were investigated.The results show that the addition of starch can increase the viscosity of the solution,which is beneficial to reduce the cation disorder,make more Al and Fe occupy the Ni position and make the synthesized material conform to the stoichiometric ratio.In addition,the modified material particles behave as irregular polyhedral nanoparticles and disperse more uniformly,no larger aggregates,and lower Li⁺/Ni²⁺mixing.This facilitates the electrolyte to fully wet the particles and provide a good lithium ion diffusion channel,thus improving the charge-discharge capacity and alleviating the voltage attenuation of the discharge platform,and further improving the cycle stability.When the voltage range is 2.7-4.5 V and the current density is 18 m A·g^-1,the initial discharge capacity of S-NFA cathode material is 204.27 m Ah·g^-1,while that of NFA is only 187.13 m Ah·g^-1.When the current density is 180 m A·g^-1 for 150 cycles,the capacity retention of S-NFA is 92.6%,while that of the NFA is only 77.97%.