Modification Of Cu-Fe-Mn-Based Cathode Materials For Sodium Ion Batteries

With the rapid development of industrialization the energy crisis has become a pressing issue worldwide.Due to its abundant reserves in the crust,low cost,and simple synthetic process,sodium-ion battery（SIBs）has shown a trend of replacing lithium-ion battery（LIBs）in large-scale energy storage.In the composition of SIBs,the positive electrode material is a crucial part because it determines the energy density of the SIBs and is the key to the commercialization of SIBs.Currently,layered oxide positive electrode materials are one of the most commercially promising positive electrode materials due to their many excellent characteristics.Among the many layered oxide materials,Cu-Fe-Mn-based（Cu-Fe-Mn）layered materials are widely concerned due to their low cost and high cycle life.However,the low redox activity of copper（Cu）and the moisture sensitivity of layered materials limit their applications.In this thesis,we optimized the electrochemical performance of Cu-Fe-Mn-based layered materials through element doping and studied the moisture sensitivity of Cu-Fe-Mn-based layered materials.Finally,we further studied the surface impurity washing and performance restoration.This thesis includes the following parts:（1）The first part of this thesis focuses on the low redox activity of Cu in Cu-Fe-Mn materials,which leads to low energy density.Li doping strategy is used to modify the material from the perspective of element doping.Na Cu_0.22Fe_0.30Mn_0.48O₂（NCFM）and other Li-doped samples Na Li_xCu_0.22Fe_0.30Mn_0.48O₂（x=0.025（NLCFM-1）,0.05（NLCFM-2）,0.075（NLCFM-3））were synthesized by solid-phase sintering method.It isfound that Li doping can excite the redox activity of Fe^3+/4+after entering the transition metal layer and can suppress the redox activity of Mn^3+/4+.At the same time,due to the reduced activity of Mn^3+/4+,the corresponding Jahn-Teller effect is reduced.When x=0.075,NLCFM-3 shows a discharge specific capacity of 114.5 m Ah g^-1 at 0.1 C current density,and the capacity retention rate after 300 cycles is as high as 92.6%.（2）Based on the NCFM to NLCFM-3 samples,the second part of this thesis conducted a study on the air stability of Cu-Fe-Mn-based layered materials.The air stability of Cu-Fe-Mn-based materials was investigated through a series of storage experiments.The experimental results showed that after the storage experiment,the morphology and structure of the Cu-Fe-Mn-based layered materials changed significantly,and the electrochemical performance also deteriorated significantly.（3）In the third part of this thesis,we considered to use six cleaning methods to remove the residual alkaline compounds on the surface and improve the cathode performance.The six cleaning methods,including glycerin,acetic acid,oxalic acid,tartaric acid,acetylsalicylic acid,and ethanol,were compared through NCFM material research.Then,two optimized cleaning methods were selected to use for performance recovery experiments on the samples whose performance was damaged by humid air in the second part.The results show that for samples whose structures and properties are destroyed,the washing experiment can restore the structure of the material back to O3 phase,but it is still difficult to recover the electrochemical properties to the same level as before the storage experiment.