Preparation And Electrochemical Properties Of Mg MgFe_xMn_2-xO₄ As Cathode Material For Aqueous Mg-Ion Batteries

Electrochemical energy storage has been widely used due to its advantages of being green and portable,and aqueous magnesium ion batteries are greener and more energy-efficient than current commercial lithium-ion batteries,which are expected to become the important energy storage device.However,due to the lack of suitable cathode materials of aqueous magnesium ion battery,it is far from reaching theoretical capacity.Therefore,finding a suitable magnesium ion battery cathode material is very important for the development and application of magnesium ion batteries.Spinel structure manganese oxide,as an important cathode material for magnesium ion batteries,has attracted more and more researchers.The spinel MgMn2O4 nanomaterial has the advantages of high voltage platform,three-dimensional ion diffusion channel and so on,becoming an ideal cathode material for magnesium ion batteries.However,it was found that the Jahn-Teller effect of MgMn₂O₄ nanomaterials resulted in poor structural stability of the electrode materials,which seriously affected the electrochemical cycling stability of the battery system.In addition,due to the poor conductivity of metal oxides,this seriously affects the diffusion rate of magnesium ions at the electrode material and electrolyte interface,and ultimately affects the rate performance of the battery.Both of the above problems have seriously hampered the development and application of aqueous magnesium ion batteries.Therefore,it is important to improve the structural stability of the electrode material and improve the conductivity of the electrode material.For the above two problems,the conductivity of the electrode material is mainly improved by coating reduced graphene,and the Jahn-Teller effect is suppressed by replacing some Mn elements with other metal elements,thus can increase the structural stability of the electrode material.Firstly,We prepared MgMn₂O₄ nanomaterials by sol-gel method,and then coated them with reduced graphene oxide?rGO?,finally,MgMn₂O₄@rGO composites electrode material were obtained.Then,the microstructure and morphology are first characterized by XRD,SEM,etc.In addition,the materials are subjected to a TEM test to observe the reduced graphene.Finally,they were used as cathode materials for aqueous magnesium ion batteries for electrochemical testing.It was found that the reduced graphene significantly improves the conductivity of the electrode material.Calculations showed that the coated graphene increased the diffusion coefficient of magnesium ions at the electrode material and electrolyte interface.It was found that the discharge specific capacity of the composite electrode coated with reduced graphene at the current density of 50 mA g^-1 was 211.8 mAh g^-1,which was much higher than that of MgMn₂O₄,whose discharge specific capacity is 131.4 mAh g^-1.The discharge capacity of the composite electrode was 141.8 mAh g^-1 at a current density of 1000 mA g^-1,which is 1.72 times than that of MgMn₂O₄.Subsequently,we introduced Fe elements in the MgMn₂O₄ nanomaterials to modulated the crystal structure,in order to suppress the Jahn-Teller effect and thus improve the structural stability of the electrode material.Specifically,a metal oxide nanomaterial containing both iron and manganese elements is prepared by a sol-gel method,and MgFe_xMn_2-xO₄ having different contents of iron and manganese is obtained by adjusting the contents of iron and manganese.Firstly,XRD and SEM are used to characterize the microstructure of the material.Then,the pore distribution and specific surface area are analyzed by N₂ adsorption.The study found that after the introduction of iron,different iron and manganese content have a serious impact on the electrochemical properties of the electrode material,and found that when the molar ratio of iron to manganese in the material is 2:1,it has the best electrochemical performance.The material was tested at a current density of 1000 mA g^-1 for a charge and discharge test.After a charge-discharge cycle of up to 1000 cycles,the specific discharge capacity was stable at 88.2 mAh g^-1,and its specific discharge capacity is15.7 mAh g^-11 higher than MgMn₂O₄.