Surface Modification Of Lithium-rich Cathode Materials For Lithium-ion Batteries

Recently,lithium-rich layered cathode material,typically formulated as xLi₂MnO₃·?1-x?LiMO₂?TM=Ni,Co,Mn,etc.?,can provide exceptionally high reversible capacity(>250 mA h g^-1),wide voltage window?2.0-4.8 V?and low coat,which have drawn great attention and have been regarded as cathodic candidate for lithium ion battery in transportation applications.However,lithium-rich layered cathode material still exist several issues.For instance,low initial coulomb efficiency,poor cycling stability and rate capability,which limit large-scale commercial application of lithium-rich layered cathode material.In this dissertation,the strategies based on surface modification including surface coating and surface pretreatment were applied to improve the electrochemistry performances of lithium-rich cathode material.And the main work are as follow:?1?Firstly,layered/spinel heterostructured Li-rich material is prepared by controlling the conditions of solvothermal method and subsequent high temperature solid phase reaction.Then amorphous LiF,Li₂TiO₃ and Li₃PO₄ was successfully deposited on the surface of layered/spinel heterostructured Li-rich material by a wet chemical method.The influences of different lithium salt coating layers on the layered/spinel heterostructured material was systematically analyzed and compared.The results show that the electrochemical properties of layered/spinel heterostructured Li-rich material can be improved by surface coating with different lithium salts,especially coated by Li3PO4.The initial coulombic efficiency of the layered/spinel heterostructured Li-rich material coated with Li₃PO₄ can reach 88.3%,and it exhibits excellent cycle performance and rate performance,the capacity retention of which is85.3%after cycling 200 times at 0.5 C and the discharge capacity at 10 C reaches148.2 mAh g^-1.?2?The pretreatment of K₂Cr₂O₇ solution with low concentration and subsequent annealing processes are applied in the modification of the layered Li-rich cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂,which caused the formation of thin spinel layer and led Cr doping on the surface of the material.The formed spinel phase with 3D Li⁺diffusion channel is favorable for the Li⁺diffusion.In addition,the surface Cr doping enlarges the interslab spacing and can suppress lattice distortion,which can further stabilize the surface structure of material in the cycle process.The materials afer surface treatement have an improved electrochemical performance.Especially,the material pretreated by 0.1 mol L^-1 K₂Cr₂O₇ solution shows a best electrochemical performance,which shows an initial coulombic efficiency of 91%,and a capacity retention rate of 78%after cycling 150 times at 1 C.Moreover,it also delivers a discharge capacity of 129.4 mAh g^-1 at 10 C,which shows an excellent rate capability.?3?The formation rate of LaPO₄ was successfully slowed down by complexation and ion exchange reaction,and the crystallographic LaPO₄ is successfully and uniformly depositing on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂.The LaPO₄ coating layer can effectively decrease the release of irreversible oxygen and effectively suppress the side reaction between cathode material and organic electrolyte,which can effectively suppress the formation of SEI film during charge and discharge processes and stabilize the surface structure of cathode material.In addition,the Li-ion conductivity LaPO₄ is beneficial to the Li⁺diffusion in the interface between electrode and electrolyte.Especially,the 2 wt.%LaPO₄ coated sample shows a best electrochemical performance,which possesses a capacity retention of 83.2%after 200cycles at 1 C and delivers a discharge capacity of 146.2 mAh g^-1 even at a high current density of 10 C.