Preparation And Electrochemical Performances Of Electrode Materials For Aqueous Mg-Ion And Na-Ion Batteries

Due to the safety problems of organic-based lithium ion batteries and the increase of production costs caused by the shortage of lithium resources,aqueous magnesium-ion batteries and sodium-ion batteries have become new research hotspots in the field of energy storage.These two aqueous batteries have the advantages of low cost,intrinsic safety and environmental friendliness.In this paper,the spinel phase Mg Mn₂O₄with high theoretical capacity and the NASICON type compound Na Ti₂（PO₄）₃with high structural stability were selected as the research objects.To address the problems of low actual discharge specific capacity and serious capacity decay of Mg Mn₂O₄and low electronic conductivity of Na Ti₂（PO₄）₃,spray drying technology was introduced to control the particle size and morphology of the product and increase the contact area between electrode material and electrolyte.Therefore,the long cycle performance,specific discharge capacity and electrical conductivity of Mg Mn₂O₄and Na Ti₂（PO₄）₃were improved.The main research contents are as follows:（1）The tetragonal spinel phase MgMn₂O₄was prepared by sol-gel method,chemical co-precipitation method and chemical co-precipitation assisted spray drying methods,and the crystal structure,surface morphology and electrochemical properties of the three materials were compared and analyzed.Among them,the spherical micron particles of Mg Mn₂O₄with particle size within 10μm were obtained by chemical co-precipitation assisted spray drying method.Compared with the electrode materials prepared by the other two methods,the electrochemical performance of Mg Mn₂O₄spherical micron particles is better.Controlling sintering time and temperature.Mg Mn₂O₄spherical micron particles with high specific capacity,long cycle life and excellent rate performance were obtained under the condition of sintering temperature 600℃and sintering time 7.5 h.The reduction of diffusion impedance of magnesium ions promoted the migration of magnesium ions in the electrode material and improved the electrochemical performance of the material.The highest discharge specific capacity of 204.6 m Ah/g was achieved at a current density of 100 mA/g.At a current density of 1 A/g,cycling nearly 5000 turns,it still has 79.5%capacity retention and its specific capacity is above 100 m Ah/g.After resuming a current density of 100 m A/g after a high current density（2 A/g）discharge,it still has a discharge specific capacity of over 200 m Ah/g.（2）Spherical NaTi₂（PO₄）₃particles with hollow structure were prepared by spray drying method.To improve the defect of low electrical conductivity of Na Ti₂（PO₄）₃materials,an excessive urea was added during the synthesis process,and finally the carbon content of Na Ti₂（PO₄）₃particles was at 23.23%,which improved the electrochemical performance of Na Ti₂（PO₄）₃electrode material to some extent.The initial discharge specific capacity was66.49 m Ah/g at a current density of 0.1 C,and there was 89%capacity retention after 100cycles of stable cycling.Optimization of pole-sheet preparation process.By using graphene aqueous slurry to replace the original conductive agent activated carbon with a 1:2replacement ratio or 40%replacement amount,the conductivity of the Na Ti₂（PO₄）₃negative electrode sheet can be further improved,and the charge transfer resistance of the electrode sheet can be reduced.Thus enhancing the electrochemical performance.The Na Ti₂（PO₄）₃negative electrode sheet has an initial specific capacity of 82 m Ah/g at a current density of 0.1C and can be stably cycled for 100 cycles at a capacity retention rate of 87%or more.