Study On The Modification And Electrochemical Properties Of Lithium-Rich Ternary Cathode Materials

With the rapid development of consumer electronics,electric vehicles,and clean energy storage,higher energy density,longer cyclic life,and higher safety are required for the lithium-ion batteries（LIBs）.As one kind of new cathode material for LIBs,lithium-rich ternary cathode material has attracted wide attention due to its advantages of high energy density,low cost,and friendly environment.However,it has a lot of problems to be solved,such as poor cycle performance and serious voltage decay.In this work,we prepared lithium-rich ternary cathode material,that is Li_1.2Ni_0.133Co_0.133Mn_0.533O₂（p-LR）,with high crystallinity and uniform morphology by low temperature combustion method.Then,this material was modified by surface engineering and composition design to improve its cyclic ability and suppress the voltage decay.In order to solve the problem of poor cycle performance of Li_1.2Ni_0.133Co_0.133Mn_0.533O₂,a uniform and thin Al₂O₃ layer was coated on the surface of Li_1.2Ni_0.133Co_0.133Mn_0.533O₂.With Al（NO₃）₃·9H₂O as the aluminum source,we used ammonia to adjust the p H value,and then Al₂O₃ layer on the surface is synthesized by subsequent annealing at high temperature.The effects of the amount of Al₂O₃ layer,p H value of the solution,and annealing temperature on the structure and properties of the final Al₂O₃@p-LR sample were explored by orthogonal experiment.The optimal coating parameters were obtained as follows:heat treatment temperature was 500℃,coating amount of Al₂O₃ was 2%,and the p H value was 5,while the annealing temperature demonstrated the greatest influence on coating Al₂O₃ layer on p-LR.XRD pattern of the optimal Al₂O₃@p-LR sample showed that the phase composition of the product after Al₂O₃ coating was not changed,indicating that the coating process does not damage the materials,and the mixing degree of Li⁺/Ni²⁺was small.TEM image indicates that a uniform Al₂O₃ layer with a thickness of about 6 nm was observed on the surface of Al₂O₃@p-LR.By comparing the morphology of the Al₂O₃@p-LR after 200 cycles,it can be seen that the Al₂O₃ layer can slow down the corrosion of electrolyte on the p-LR to a certain extent,reduce the rate of p-LR inactivation,and thus improve the performance of the material.Galvanostatic charge/discharge measurement showed that the discharge capacity of Al₂O₃@p-LR in the cladding group after 200 cycles（1 C）was 162.3 m Ah g^-1.The capacity retention rate was 84.8%,which was 12.9%higher than that of the p-LR,indicating the cycle performance was greatly improved.In order to solve the problem of voltage decay,the high voltage material Li Ni_0.5Mn_1.5O₄（LNMO）was introduced to composite with p-LR.In situ synthesis and composition of LNMO were conducted by low-temperature combustion method.Firstly,the composite quantity is controlled by material ratio.Then,the pretreatment temperature is regulated to conduct in-situ hybridization of LNMO,and finally the high-temperature heat treatment is carried out.The effects of composite quantity,pretreatment temperature,and final heat treatment temperature were explored by orthogonal experiment.The optimal parameters were obtained as follows:composite quantity was 5%,pretreatment temperature was 600℃,and final treatment temperature was 750℃.The composite quantity had the greatest influence.XRD pattern showed that the composite LNMO/p-LR had a distinct layered structure,and temperature reaction stage,newly generated LNMO will directly adhere to the p-LR to form compact composite secondary particles.This kind of dense structure can reduce the corrosion of the electrolyte to the p-LR,slow down the transition from the layered structure to the spinel structure,and thus reduce the voltage decay.The EIS spectrum showed that LNMO/p-lr has a smaller charge transfer impedance and a higher Li⁺diffusion coefficient than pure p-LR.This indicates that the electrode has higher ionic conductivity and electron conductivity after the combination with LNMO.After 200 cycles at 1 C,the discharge capacity of LNMO/p-LR was 148.4m Ahg^-1,with a high capacity retention of 93.5%.The discharge medium voltage decreased from 3.49 V to 3.27 V,which was 0.27 V higher than that of the pure p-LR electrode.This showed that the voltage decay of Li-rich ternary cathode was greatly improved by the composition of high-voltage materials.