Reserach Of Surface Modification Of Na_0.44MnO₂ Cathode Material For Sodium Ion Battery By Magnesium-based Compounds

With the continuous shortage of lithium resources in recent years,sodium ion batteries are expected to replace lithium ion batteries as the next generation of commercial secondary batteries..Sodium and lithium are in the same main group of the periodic table and have similar physical and chemical properties.Sodium is one of the most widely distributed elements on the earth.The raw materials used in the preparation of sodium ion batteries are easily available and low in cost.However,due to the large radius of sodium ions,the crystal structure will be distorted due to the extraction/intercalation of sodium ions during the charging and discharging cycle of the battery,so that the capacity of the material will rapidly decline.Therefore,searching for a cathode material with stable structure and favorable sodium ion transmission is one of the current research focuses of sodium ion batteries.The tunnel-type sodium ion cathode material Na_0.44MnO₂ has become a hot research topic in sodium ion batteries,because its structural model is a unique tunnel type,can stabilize the structure well during the sodium ion deintercalation process,and at the same time is beneficial to the transportation of sodium ions.However,the main problems of Na_0.44MnO₂ during charging and discharging are as follows:（1）During the first week of charging,the material and the electrolyte have a serious and complicated side reaction,which produces a large amount of reversible capacity while generating the CEI film,（2）Like the currently known manganese-based transition metal oxide cathode materials,during the cycling process,the Mn³⁺ ion of Na_0.44MnO₂ will undergo the Jahn–Teller effect,distorting the material lattice,（3）In order to promote the commercialization of sodium ion batteries,it is urgent to improve their electrochemical window,（4）The electronic conductivity of Na_0.44MnO₂ material is not ideal(about 10-6 S cm^-1),which also limits its electrochemical performance.In order to solve the above problems and further improve the physical properties and electrochemical performance of Na_0.44MnO₂ cathode materials,bulk doping and surface coating are currently used in more research methods.Bulk phase doping mainly achieves the suppression of the phase transition process by replacing the position of Mn ions with other metal ions,thereby improving the stability of the structure;the surface coating does not change the bulk structure,and the barrier effect of the coating layer is avoided The direct contact between the cathode material and the electrolyte can effectively reduce the side reactions on the surface and achieve the effect of inhibiting the growth of the CEI film on the surface of the particles.However,the existing research on surface coating mainly focuses on the low cut-off voltage.Therefore,in this work,magnesium-based compounds MgF₂ and Mg₃（PO₄）₂ were used to modify the surface of the sodium ion battery Na_0.44MnO₂ cathode material.The surface coating layer maintained the Na_0.44MnO₂ tunnel structure while enhancing its high-voltage window Electrochemical performance.First,the cathode material of the tunnel-type Na_0.44MnO₂ sodium ion battery was prepared by the molten salt method,and then the surface was modified by MgF₂ and Mg₃（PO₄）₂ respectively.The work is as follows:1.A simple wet chemical method is used to coat the surface of the tunnel-type Na_0.44MnO₂ material with a layer of MgF₂（1 wt.%,2 wt.% and 3 wt.%）.X-ray diffraction（XRD）,field emission scanning electron microscope（SEM）,transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）and Raman spectroscopy（Raman）etc.of physical characterization confirmed that the coating layer did not change the original material bulk and surface structure.Constant current charge-discharge cycles,rate,cyclic voltammetry（CV）,electrochemical impedance（EIS）etc.of the electrochemical test results show that Na_0.44MnO₂ modified by MgF₂ coating shows superior performance compared to the pristine material.Under the high voltage window of 2.0-4.5 V,especially the 2 wt.% coating sample after 60 cycles,the reversible specific capacity is 88.8 m Ah g-1,the retention rate is 86.5%,and the original material is only 68.3 m Ah g-1 and 71.8% retention rate.The MgF₂ coating layer not only suppresses the Jahn-Teller effect of Mn³⁺ ions,stabilizes the surface structure of the electrode material,but also suppresses the surface side reactions and the growth of the CEI film during the electrochemical cycle.2.The surface of Na_0.44MnO₂ was coated with different contents of Mg₃（PO₄）₂（magnesium phosphate）using the same wet chemical method（2 wt.%,3 wt.%,4 wt.%）.By comparing the physical characterization（XRD,SEM,TEM,XPS,etc.）and electrochemical characterization（long cycle,rate,EIS,etc.）of the cathode material before and after coating,the results confirmed that the bulk structure of the material remained intact after coated,and the surface becomes rough with the increase of the coating amount.3 wt.% as the optimal coating amount not only suppresses the growth of the CEI film,but also suppresses the complex phase transition during the cycle,thereby maintaining the body during the cycle The stability of the phase structure improves the reversibility of the capacity.