Preparation And Modification Of Manganese Dioxide Cathode Materials In Aqueous Zinc Ion Batteries

Manganese oxides have the advantages of low cost,non-toxicity,abundant materials and high working voltage,and can be used as good positive electrode materials for zinc ion battery.Among them,α-MnO₂ has a double-strand structure and belongs to the tetragonal system.Each unit cell contains eight MnO₂ molecules and has（1×1）and（2×2）tunnel structures.Zn²⁺can be reversibly and rapidly inserted and deserted in the tunnel with great application prospects in recent years.However,the capacity of manganese dioxide is fast decaying and the conductivity is poor,which limits its electrochemical performance,so that it cannot be used in large quantities in commercial applications.Therefore,this paper mainly improveed the electrochemical performance ofα-MnO₂ as cathode material for zinc ion batteries from the following aspects:（1）Synthesis ofα-MnO₂ cathode material by hydrothermal method.The effect of different reaction times on the properties of the materials prepared was studied.The results showed that the electrochemical properties of the material were the best with the reaction time of 6 h.The SEM image showed that the morphology of the material was a hollow sea urchin-shaped spherical particle formed by nanorods,which had a good morphology and uniform particle size with the diameter of about 5μm.The electrochemical test results showed that the specific discharge capacity of the battery was 150 mAh g^-1 at a voltage range of 0.8-2.0 V and a current density of100 mA g^-1,and the specific capacity was 50 mAh g^-1 after 50 cycles.The cycle performance was better than that of the materials prepared with the reaction time of12 h and 18 h.（2）Zr⁴⁺ion-dopedα-MnO₂（denoted as x%Zr-MnO₂）was prepared by hydrothermal method.The effects of different zirconium contents on the electrochemical properties of the samples were investigated.The SEM image showed that after doping with Zr⁴⁺ions,the morphology of the material was hollow urchin-like which was similar to that of pureα-MnO₂.The diameter of x%Zr-MnO₂composite was about 2.5μm,which was obviously smaller than the pureα-MnO₂particles.The smaller particle of x%Zr-MnO made a larger surface area,which could insert more Zn²⁺.The electrochemical performance test results showed that the electrochemical performance of the material doped with 5%Zr was the best in the current density of 100 mA g^-1 and the voltage range of 0.8-2.0 V.The first discharge specific capacity of 5%Zr-MnO₂ was reached to 300 mAh g^-1.After 50 cycles,the specific discharge capacity was 138.4 mAh g^-1,and the electrochemical performance of the material was significantly improved.The results showed that Zr⁴⁺ions were incorporated into the manganese dioxide crystals to support the structure of the material,making the tunnel structure ofα-MnO₂ more stable,and more zinc ions would be rapidly embedded in the tunnel of manganese dioxide.Morever,the electrode polarization was reduced,thereby improving the electrochemical performance of the material.（3）Theα-MnO₂ material was prepared by hydrothermal method,and the sucrose was decomposed into elemental carbon by acidic medium by using reducing property,and the carbon coating process was carried out in anhydrous ethanol to prepare carbon-coatedα-MnO2 material.（denoted asα-MnO2@C）.The SEM image showed that the morphology of theα-MnO₂@C composite was aggregated by nanowires,but the morphology was irregular and the sea urchin structure almost disappeared.The particle diameter ofα-MnO₂@C composite was about 4μm,which was smaller than that of pureα-MnO₂.After coating carbon,the material particles became smaller,resulting in the larger the specific surface area and the more material pores.The electrochemical performance test results showed that the first discharge specific capacity ofα-MnO₂@C composite was 191 mAh g^-1.After 50cycles,the specific discharge capacity was 80 mAh g^-1.Therefore,the electrochemical performance ofα-MnO₂@C composites was significantly better than that of pureα-MnO₂ materials.The results showed that the carbon coating can effectively prevent the dissolution of Mn⁴⁺ions in the MnO₂ electrode and overcome the increase of resistance caused by long-term cycling,thereby improve the electrode stability and the cycle performance of the battery.（4）A polyaniline-coated manganese dioxide composite material（denoted asα-MnO₂@PANI）was prepared by in-situ chemical oxidation polymerization of aniline.The SEM image showed that the particle diameter of theα-MnO₂@PANI composite was larger than that of the pureα-MnO₂ sample.The reason was that theα-MnO₂ material had a nucleating effect which could adsorb the PANI particles in the reaction,making largerα-MnO₂@PANI complex.The material was core-shell type.It can be seen from the XRD pattern that the main diffraction peak ofα-MnO₂@PANI composite was almost the same as that of the pure MnO₂ sample,but the strength was relatively weak.The charge and discharge diagram that the first discharge specific capacity is significantly lower than those of several subsequent cycles,and the specific capacity was reacheed the maximum at the 11 th cycle,indicating that the Zn²⁺was inserted in the composite material to activate the material.The diffusion rate of the Zn²⁺ions in the crystal structure of the material was increased,thereby lowered the polarization of the electrodes.