Research On The Electrochemical Performance And Capacity Fade Mechanism Of Spinel Manganese-Based Cathode Materials

The lithium ion battery has been considered as one of the most promising devices for electrochemical energy storage due to its high energy density,high operating voltage and long cycle life.However,capacity fade is still the main problem that limits its development.In addition,the charging cut-off potential is an important factor which affects the capacity fading of the battery significantly.If the capacity fade mechanisms of the battery at different charging cut-off potentials are investigatedsystematically,themethodforimprovingtheelectrochemical performance and suppressing the capacity fade of battery can be put forward targetedly.In this paper,capacity degradation of the spinel manganese-based cathode materials lithium manganese oxide?LiMn₂O₄?and lithium nickel manganate(LiNi_0.5Mn_1.5O₄)used in lithium ion batteries commonly at different charging cut-off potentials are investigated.The electrochemical properties such as cycle performance,coulombic efficiency and impedance are tested and compared by means of constant current charge-discharge and electrochemical impedance.The changes of electrode particles and the growth of electrode-electrolyte interface film on the cathode surface are analyzed by scanning electron microscopy?SEM?and X-ray photoelectron spectroscopy?XPS?.Firstly,long-term charge-discharge cycles are performed on LiMn₂O₄/Li half-cells at 4.3,4.4,4.5,4.6,4.7 and 4.8 V vs.Li/Li⁺to investigate the electrochemical performance and capacity fade mechanisms of LiMn₂O₄ materials with different charging cut-off potentials.When the charging cut-off potential is lower than 4.6 V,the manganese dissolution and Jahn-Teller effect will cause the phase structure of the LiMn₂O₄ electrode to be distorted or even collapsed with the severe pulverization of particles.Further,the diffusion of lithium ions in the material will be hindered with the decrease of the Li⁺diffusion coefficient(D⁺_Li)values.On the other hand,the decomposition of electrolyte can be affected by the charge voltage and the manganese catalysis,so that the surface film of the electrode surface becomes thicker continuously with the increase of the charge cut-off potentials.Therefore,when the potential reaches to 4.7 V and 4.8 V,the interface film grows up rapidly,which makes lithium ions difficult to move through the interface film and interface,causing an increase of film impedance(R_film)and charge transfer impedance(R_ct).The structural collapse of cathode material and growth of surface film will increase the polarization of the electrode and the consumption amount of recyclable lithium ions leading to the degradation of the electrochemical performance and capacity fade of the LiMn₂O₄materials.The effects of charging cut-off potentials?4.0,4.3,4.5,4.6,4.65,4.7,4.8,4.9and 5.0 V vs.Li/Li⁺?on the electrochemical performance and capacity degradation of LiNi_0.5Mn_1.5O₄ are also analyzed during long-term cycling.For cells charged below4.8 V,the materials release lower capacity but with higher capacity retention.It is found that the LiNi_0.5Mn_1.5O₄ cathode material maintains good structural stability during charge-discharge processes.And the electrolyte decomposition reactions are not obvious.When the voltage reaches above 4.8 V,the material can release normal capacity,but the capacity decreases seriously.As the battery voltage reaches to 5.0 V,the capacity degradation is further aggravated,and the elements such as P,F,Ni and Mn on the electrode surface increase significantly,indicating that the effects of the electrode surface interface film formed by the decomposition of the electrolyte on the battery capacity loss are more obvious.In summary,the deterioration of the electrode material structure and the degree of decomposition of the electrolyte are the main factors for the capacity fade of the positive electrode materials discussed above.The proposed two factors can provide clear research ideas and reasonable judgment basis for the study of the capacity fade mechanism of other cathode materials.Compared with other researches,this article pays more attention to analysis of electrode dynamics process,and studies the deintercalation process or reaction process of metal ions in materials more detailed,which has significant reference values.