Surface Modification Research Of Li And Mn-Rich Cathode Material For Lithium-Ion Battery

The widespread utilization of Lithium-ion batteries(LIBs)in portable electronics,electric vehicles and power grids requires batteries with higher energy density,stability and low cost.As a promising candidates,lithium and manganese rich layered cathode materials xLi2MnO3·(1-x)LiMO2(0<x<1,M=Mn,Ni,Co,etc.)have been intensively investigated for decades.The layered lithium-rich manganese based cathode material possess a high specific discharge capacity(～300 mAh g-1 at 2.0-4.8V),which is about 2-3 times of the commercially applied cathode material,such as:LiCoO2(140 mAh g-1),LiFePO4(130 mAh g-1),and manganese based cathode materials are relatively economical.These advantages are aligned with the pursuit of high capacity and low costs.However,the practical applications of these materials has been hindered by its poor rate performance,voltage fade and capacity decay during prolonged cycles.Although researchers have devoted numerous efforts in surface modification,doping,coating and morphology design to solve these problems,there still remians a big chanllage.Herein,we developed two effective approach to overcome the above issues of lithium-rich cathode material.To mitigate the voltage and capacity decay during electrochemical cycles,we uniformly introduced nanoscale defects into the surfaces of cathode materials via pre-activation followed by calcination of the citric acid coated cathode material at inert atmosphere.The partially irreversible redox of lattice oxygen is the main cause of capacity/voltage decay.Defects introduced by this method can improve the reversibility of anionic redox O2-/O2n-(where 1<n<3)and Li+conductivity,which leads to effectively suppressed voltage decay and capacity decline.Electrochemical tests show that the modified materials presents better performance during 200 charge-discharge cycles at 1 C rate(250 mA g-1,2.0-4.8 V).The voltage decay rate of pristine,LMCN-1,LMCN-2 and LMCN-3 electrode during 200 cycles was 3.7,1.56,1.27 and 1.36 mV per cycle,respectively.The LMCN-2 material delivers a high specific discharge capacity of 173.1 mAh g-1 after 200 cycles at 1 C rate and the retention rate is about 99.5%,as compared to the pristine material(89.3 mAh g-1 and 53.4%).Besides,the capacity of pristine and LMCN-2 changed from 120.5 and 164.8 mAh g-1 to 87.5and 141 mAh g-1 after 150 cycles at 2 C rate,with a capacity retention rate of 72.6%and 85.6%,respectively.The modified material also presents excellent rate performance,the discharge capacity of LMCN-2 at 0.5,1,2 and 5 C is about 89,80,69 and 54%of the capacity at 0.2 C rate,as compared to the pristine material(79,62,50 and 35%,respectively).These results demonstrate that the nanoscale defects on the surface of the materials can improve ion-conductivity and significantly inhibit the capacity/voltage fading.In addition,to address the discharge capacity fading and poor rate capability,we treat the cathode material with phosphomolybdic acid(H3[P(Mo3O10)4]·H2O)and anneal under air atomsphere to in-situ form Li2MoO4 coating on its surface.This coating layer can suppress the undesired interfacial reactions during the interaction of cathode material and electrolyte,therefore protect cathode material and improve electrochemical stability.As a lithium ion conductor,Li2MoO4 can also provide a three-dimensional lithium-ion diffusion path.The optimum sample(Mo-2)delivers a high discharge specific capacity of 174.5 mAh g-1 and a retention rate of 93%after 120 cycles at 0.5 C rate,which is higher than that of the pristine materials(147.6 mAh g-1 and 82.5%,respectively).Moreover,the pristine and Mo-2 material exhibits an initial discharge specific capacity of 154.6 and 150.1 mAh g-1 and a capacity retention rate of 73.7%and 99.9%,respectively,after 200 cycles at 1 C rate.These improvement mainly result from the increased interfacial stability and Li+conductivity under the protection of the Li2MoO4 layer.