The Polarization Characteristics Of Lithium-rich Manganese-based Cathode Materials

Layered lithium-rich manganese-based cathode material xLi2MnO3·(1-x)LiMO2(0<x<1,M=Mn,Co,Ni)is one of the most promising new-generation lithium-ion battery cathode materials because of its high lithiation and delithiation potential,high specific capacity and environmental friendliness.However,the lithium-rich materials exhibit obvious voltage hysteresis between the charge and discharge process,and the voltage continuously decays during cycling.Besides,the materials show significant differences of electrochemical performance at different current densities and different temperatures.These challenges limit their application in lithium-ion batteries.In order to provide theoretical guidance for further improving the electrochemical performance of lithium-rich materials and promote their application in high specific energy lithium ion batteries,this article systematically studies the electrochemical characteristics of lithium-rich manganese-based cathode materials and discusses the influence factors.The main contents and results of this article are as follows:The electrochemical properties of the materials were analyzed by the galvanostatic charge and discharge test,galvanostatic intermittent titration technique(GITT),and electrochemical impedance test(EIS).X-ray diffraction(XRD),scanning electron microscope(SEM)were used to characterize the structural changes and surface characteristics of the material during cycling.The results show that the lithium-rich manganese-based cathode materials exhibit significant voltage hysteresis between charging and discharging process,which is caused by two reasons.On the one hand,the charge compensation reaction of oxygen has irreversible characteristics due to the changes of material structure between charging and discharging process.On the other hand,the polarization of the electrochemical reactions is different between charging and discharging process.Compared with the charge transfer process of nickel/cobalt,the charge transfer process of oxygen has a larger polarization.And the lithium ion diffusion coefficient is lower in the charging high-voltage range and discharging low-voltage range,which further exacerbates the polarization of the material.The influence of the charge-discharge current density on the electrochemical characteristics of the lithium-rich manganese-based cathode material was further analyzed.Galvanostatic charge and discharge measurements were carried out on the materials at different current densities of 0.01C,0.05C,0.1C,and 0.5C,and the specific capacity of the material decreases with increasing current density.The discharge specific capacity at 0.01C is 239.6 mAh/g,while the discharge specific capacity at 0.5C is 159.1 mAh/g.It is found that the specific capacity contribution of charge compensation reaction of oxygen significantly reduces when the current density increases,GITT is used to analyze the electrochemical polarization value of the materials under different current densities.The results show that the polarization of the oxygen charge compensation reaction increases significantly when the current increases,which is caused by the poor kinetics of the oxygen/manganese ion charge transfer process.Charge-discharge cycle measurements of the material were performed at 0.1C,and the evolution of electrochemical performance were analyzed.It is found that the specific capacity and mid-voltage of the material gradually decrease during cycling.The specific capacity and mid-voltage of the material at the tenth and fiftieth cycles are 209 mAh/g,183 mAh/g and 3.68V,3.38V,respectively.And the open circuit voltage corresponding to the charge compensation reaction of oxygen shows significant voltage decay during cycling.In addition,compared with the voltage region of the reaction of nickel/cobalt,the increase polarization value in the voltage region of the charge compensation reaction of oxygen changes is large upon cycling.After 50cycles,the ohmic impedance(Rb),interface film impedance(RSEI)and charge transfer impedance(Rct)were measured by EIS.During cycling,the value of Rb continues to increase,while that of RSEI and Rct increase significantly.In addition,RSEI and Rct show a larger increase in the voltage range of the charge compensation reaction of oxygen,compared with the charge compensation reaction of nickel/cobalt.From the analysis of surface,it can be concluded that the morphology of the interface film has changed.According to the analysis,the voltage decay of the lithium-rich manganese-based material during cycling is due to the attenuation of equilibrium voltage and increased polarization in the charge compensation reaction of oxygen.The capacity decay is mainly due to the increase of the charge transfer impedance of the oxygen charge compensation reaction and the increase of the impedance of the interface film.This paper also analyzed the effect of temperature on the electrochemical performance of lithium-rich manganese-based cathode materials.The results show that the specific capacity of the material increases with increasing temperature,while the polarization value decreases with increasing temperature.The variation of electrochemical characteristics in different voltage ranges at different temperatures were analyzed.The results show that temperature has a great influence on the charge compensation reaction of oxygen.The charge specific capacity of the material in the high voltage range(4.1V to 4.8V)during charging increases from 57.9 mAh/g at 5? to 94.4 mAh/g at 45?,the discharge specific capacity of the material in the low voltage range(3.53V to 2.0V)during discharging increases from 44.7 mAh/g at 5? to 124.4 mAh/g at 45?.And polarization value has larger changes in the voltage range corresponding to the charge compensation reaction of oxygen.According to the changes of lithium ion diffusion coefficient,interface film impedance,and charge transfer impedance with temperature,the apparent activation energy was calculated.The results show that the activation energy of the charge transfer process calculated by the charge transfer impedance is the highest in all voltage ranges,the apparent activation energy of the interface ion transport process calculated by the interface film impedance is followed by,and the activation energy of the lithium ion solid phase diffusion process calculated by the lithium ion diffusion coefficient is the lowest.It indicates that the electrochemical polarization of the lithium-rich manganese-based cathode material mainly depends on the charge transfer process and the interface ion transport process.The apparent activation energy of the charge transfer process of the oxygen charge compensation reaction is larger than that of the nickel and cobalt charge compensation reaction,indicating that the charge compensation reaction of oxygen is critical process affecting the electrochemical performance of the lithium-rich manganese-based.