The Effect Of L-ascorbic Acid-based Treatment On The Properties Of Li-rich Mn-based Cathode Materials For Li-ion Batteries

Li-ion batteries with a relatively long cycling life and high erergy density have been widely used as energy storage technologies.The next-generation Li-ion batteries with higher energy/power density,long life,and low cost are being further developed for electric cars.Developing high performance cathode materials for Li-ionbatteriesisofgreatimportance.Li-richlayeredoxides,xLi₂MnO₃·（1-x）LiMO₂ with relatively high discharge capacity,wide operating voltage,and low cost,have attracted significant attention.However,they still suffer from some drawbacks such as a relatively large irreversible capacity loss during the first charge/discharge cycle,and poor rate and cycling performance.Based on the experimental results and theoretical analysis,the above problems in Li-rich layered cathode materials are mainly related to their structural stability and surface physicochemical states during charge/discharge.In this work,the effect of an L-ascorbic acid-based treatment on the physicochemical and electrochemical properties of Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has been studied.The main contents are listed as below:（1）The relationship among the L-ascorbic acid-based treatment,the physicochemicalproperties,andtheelectrochemicalperformanceof Li_1.2Mn_0.6Ni_0.2O₂ was studied systematically.The modification process involves the mixing and stirring of a proper amount of Li_1.2Mn_0.6Ni_0.2O₂ with a solution of L-ascorbic acid for specified durations of time,followed by a thermal treatment.The results show that:Clear corrosion cavities with spinel coating were observed over the majority of particle’s surface.Surface morphology evolution heve been ascribed to the metal cations leaching from the surface of the Li-rich layered oxide during the mixing process by the L-ascorbic acid solution,and the rearrangement of the material’s surface structure caused by subsequent thermal treatment.Modification process cannot cause variation of cell volume.Furthermore,a special region without obvious traces of etching is observed over a small part of the surface of the modified samples,which arises from the enrichment of Ni²⁺at the rim of the particle.Compared to the pristine oxide,the modified samples exhibit significantly higher initial discharge capacity/Columbic efficiency,remarkably enhanced rate performance.These improvements are attributed to the formation of spinel coating layer and improved chemical activation of the Li₂MnO₃ compound.（2）The properties of Li-rich layered cathode materials are closely related to their components.The role of L-ascorbic acid-based treatment on the physicochemical and electrochemical properties of another common material,Li_1.2Mn_0.54Ni_0.13Co_0.13O₂,has been systematically studied in this work.The modification process is similar to that of Li_1.2Mn_0.6Ni_0.2O₂.The results show that:The modified samples exhibited a volumetric shrinkage,the shrinkage in lattice volume can be attributed to the extraction of Li⁺and lattice oxygen from Li₂MnO₃phase during the treatment.Surface morphology of modified samples is similar to that of Li_1.2Mn_0.6Ni_0.2O₂.But less amount of spinel phase has formed in the modified samples as compared to that of Li_1.2Mn_0.6Ni_0.2O₂,this phenomenon is related to Co incorporation.The enhancement in electrochemical performance of the L-ascorbic acid treated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ is attributed to the co-contribution of chemical activationofLi₂MnO₃phase,exposureofhighproportionof Li-conducting-favorable faces at the particle surface,increase in specific surface area and in-situ formation of minor spinel phase under the surface.The different response of structural variation and electrochemical behavior to the L-ascorbic acid treatment was observed in these two kinds of materials.