Surface Modification And Properties Of Li-rich And Mn-Based Cathode Material Li_1.18Mn_0.52Co_0.15Ni_0.15O₂

Li-ion batteries have the advantages of high energy density,high power density,and long cycle life,which have been widely used in consumer electronics and electric vehicles,and so on.Li-rich and Mn-based cathode material,which can be written the notation of Li1+xM1-xO2 or xLi2MnO3-(1-x)LiMO2(M=Ni,Co,Mn,0<x<1),is likely to be the cathode material for the next generation of Li-ion batteries,due to its high discharge specific capacity,low cost,and environmental benignity.However,it suffers from some problems,such as high initial irreversible capacity,inferior rate performance,fast capacity and voltage fade.In addition,serious side reactions between the cathode material and electrolyte further lead to the deterioration of electrochemical performance during cycling,which limits its commercial application.In this dissertation,we take Li1.18Mn0.52Co0.15Ni0.15O2 cathode material as the research object,and try to construct stable surface/interface to improve the surface and bulk structural stability by adopting an in situ coating method,thus enhancing the electrochemical properties of cathode material.(1)The precursor of Li1.18Mn0.52Co0.15Ni0.15O2 cathode material was synthesized by a spray drying method firstly,and then(NH4)2HPO4 was coated on the surface of the precursor by a simple wet chemical method,obtaining the Li3PO4 coated Li-rich and Mn-based cathode material after high temperature calcination.A three-dimensional network with fast ion diffusion channels is constructed by the formation of the Li3PO4 coating layer on the surface of the primary particles,the Li+diffusion coefficient increases from 9.3×10-14 cm2 s-1 to 1.1 ×10-12 cm2 s-1(2.6 wt%Li3PO4 modified sample).At the same time,the Li3PO4 surface modification layer also acts as a protective layer to effectively suppress side reactions between the electrode and electrolyte,thus stabilizing the electrode and electrolyte interface.Therefore,the electrochemical performance of the Li3PO4 coated material is significantly improved.After Li3PO4 modification,the initial coulombic efficiency of the 2.6 wt%Li3PO4 modified sample increases from 78%to 86%,and the initial discharge capacity increases from 260.9 mAh g-1 to 280.1 mAh g-1.After 197 cycles at 1 C rate,its discharge capacity reaches 171.1 mAh g-1 and the corresponding capacity retention is 76.2%,while that of the pristine sample is only 47.3%.Moreover,the rate performance of the modified sample has also been significantly improved.The discharge capacities of the 1.3wt%Li3PO4 coated sample are as high as 206.5 mAh g-1 and 184.1 mAh g-1 at 3 C and 5 C rate,respectively,while those of the pristine sample are only 180.4 mAh g-1 and 148.4 mAh g-1.(2)An integrated Li3PO4 and spinel modified Li1.18Mn0.52Co0.15Ni0.15O2 was successfully synthesized by a facile synchronous route.A three-dimensional network of fast ion diffusion channels is constructed by the coating of Spinel/Li3PO4 on the primary particle surface,which improves the electrochemical reaction kinetics of cathode material.On the one hand,the Li3PO4 coating layer acts as a protective layer,which can significantly stabilize the interface between the electrode and electrolyte by retarding the undesirable side reactions On the other hand,the inner spinel coating layer effectively enhances the structural stability of cathode material by mitigating the structural transformation from layered structure to spinel-like structure.As a result,the Spinel/Li3PO4 modified samples show excellent electrochemical properties.The initial coulombic efficiency of the modified samples increases from 78%to 88%.After 97 cycles at 0.2 C rate,the discharge capacity of the modified sample with the best comprehensive performance is as high as 246.1 mAh g-1(capacity retention of 92.7%),while that of the pristine sample decrease to 154.6 mAh g-1(capacity retention of 62.3%).Meanwhile,the rate performance of the modified samples is also significantly improved.Specifically,the discharge capacities of the modified sample with the best rate performance reach 219.2 mAh g-1 and 186.4 mAh g-1 at 3 C and 5 C rate,respectively,while those of the pristine sample are only 180.4 mAh g-1 and 148.4 mAh g-1.(3)An integrated Li3PO4 and Fe2O3 modified Li1.18Mn0.52Co0.15Ni0.15O2 was successfully synthesized by a facile synchronous route.The Fe2O3/Li3PO4 heterogeneous modification layer is coated on the primary particle surface.On the one hand,the Li3PO4 component in the heterogeneous modification layer acts as a fast ion conductor,which contributes to the diffusion of lithium ions,so that the rate performance of the modified sample is not greatly affected.On the other hand,the Fe2O3 component in the heterogeneous modification layer acts as an HF scavenger,which can consume HF during cycling,suppressing the dissolution of transition metals resulting from the continuous attack by HF species.Moreover,the Fe2O3/Li3PO4 surface modification can enhance the structural stability of cathode material.The electrochemical test demonstrates that the modified samples have excellent cyclic stability.After 97 cycles at 0.2 C rate,the capacity retention rate of the modified sample with the best comprehensive performance is as high as 95.1%.Its discharge capacity decreases by 12.2 mAh g-1,while that of the pristine sample decreases by 93.7 mAh g-1.