Studies On Voltage Decay Mechanism And Modification Of Lithium-rich Manganese-based Cathode Materials

With the advantages of high energy density,long service life,small size,and environmental protection,lithium-ion batteries have been widely used in many fields such as energy storage,transportation,and consumer portable electronic devices.As the core component of lithium-ion batteries,the physical and chemical properties of the cathode material will also directly affect the overall electrochemical performance of the batteries.Li-rich Mn-based cathode materials（LRM）have become one of the most promising cathode materials due to their low cost,high discharge specific capacity(250～300 m Ah g^-1)and high operating potential（>4.5 V）.However,there are still problems regarding LRM such as low initial coulomb efficiency,poor cycling and rate performance,and severe voltage decay that hinder its large-scale commercial application.In particular,voltage decay will not only cause a rapid decrease in battery energy density,but also affect the normal operation of the battery management system.Therefore,with the help of a various of structure detection,spectral characterization,theoretical calculations,and in-situ electrochemical tests,the main causes of the LRM voltage decay were studied,and LRM was double-doped modified by co-precipitation,which effectively improved the electrochemical performance of the material.The specific research content is as follows:（1）The structural evolution of LRM and the change of valence state of transition metal（TM）in different reaction stages at different temperatures（25℃,45℃,55℃）were studied,which was effectively related to voltage decay.The results show that the irreversible structural thermodynamic evolution is the core cause of the LRM voltage decay,which worsens with increasing temperature.Electrochemical test results show that the voltage decay of LRM materials accelerates with increasing temperature,and the capacity retention rate also decreases rapidly.At high temperatures,the preactivation of inert Mn causes the LRM structure to be in non-thermodynamic stable state.Multiple phase transitions,secondary particle comminution,microcracking,and agglomeration formation are associated with the continuous migration of TM to the material surface,and the complex interfacial properties after TM migration deteriorate the Li⁺transport kinetics seriously.In addition,valence anisotropy caused by partial TM valence failure and severe interfacial reactions can accelerate the voltage decay of LRM.（2）The double doping modification of cations(Al³⁺)and polyanions(（BO₃）^3-/（BO₄）^5-)to LRM-AB was realized by the co-precipitation method.It is revealed that the inert ion Al³⁺with occupying tendency and the polyanionic group(（BO₃）^3-/（BO₄）^5-)inhibited the disordered migration of TM,which leads to a reduced phase change and mitigated oxygen loss of LRM during cycling.In addition,the interfacial properties of the modified samples are more stable and the side reaction products（Li₂CO₃,Li F,etc.）at the interface are significantly reduced,which plays a crucial role in the improvement of the structural stability and rate performance of LRM.Density function theory（DFT）calculations demonstrate that(（BO₃）^3-/（BO₄）^5-)with strong electron-absorbing groups and Al³⁺which can introduce electron holes can not only improve the poor electronic conductivity of Li₂Mn O₃,but also reduce the redox potential of the anion.After the batteries cycle test,the double-doped modified samples showed excellent electrochemical performance,and the capacity/voltage decay suppression effect was obvious.The LRM-AB has a voltage drop of only 6.0 m V per revolution over 100cycles,and the capacity retention rate is as high as 92.0%.