Synthesis And Coating Modification Of MnO₂ Cathode Material For Zinc-ion Battery

Higher requirements are put forward for the vehicle power system in terms of energy conservation and emission reduction with the rapid development of green transportation.Due to the special environment in ship operation,such as high temperature,humidity and turbulence,it is necessary to develop safer power batteries.Recently,aqueous zinc-ion batteries have great development potentialities in transportation applications,such as vessel and automobile,owing to the low cost,high safety and environmental friendliness.As one of the ideal cathode materials for batteries,manganese dioxide has been extensively studied due to its high theoretical capacity,working voltage and its advantages of abundant resources,low price and low toxicity.However,the current materials still have problems such as poor conductivity and excessive capacity decay.To address the above-mentioned issues,micro-nano structure design and oxide coating have been proposed to effectively improve the electrochemical performance of manganese dioxide materials,and the coating mechanism have been explored.The main research contents are as follows:（1）The synthesis and performance study of micro-nano MnO₂.Nanorodα-MnO₂with length of about 1μm and width of about 40-50 nm was synthesized by hydrothermal method at 180℃,using potassium permanganate and manganese sulfate as reactants.Micro-nano spherical R-MnO₂composed of nanosheets and nanoneedles of about 6μm was synthesized by thermal method at 140℃,using manganese sulfate and ammonium persulfate as reactants.Micron sphericalε-MnO₂of about 2μm composed of nanoparticles was synthesized by liquid-phase precipitation method and calcination at 400℃for 5 hours,using manganese sulfate and ammonium bicarbonate as reactants.The results reflect that theε-MnO₂has the best electrochemical performance.When the current density is 0.1 A/g,the maximum discharge capacity can attain 280.3 m Ah/g,and the capacity retention rate is 79.8%after 60cycles.（2）The study on particle size control and performance ofε-MnO₂.Regulate the particle size of the material by changing the concentration of reactants.When the concentration increases from 0.14 mol/L to 0.56 mol/L,the particle size of the microsphere ranges from 2μm to about 0.8μm.With the reduction of particle size,the capacity of the material increases from 182 m Ah/g to 220.4 m Ah/g,and the rate performance of the material is also enhanced.（3）The study on the synthesis and modification ofε-MnO₂oxide coating.The composite materials coated with the insulator Al₂O₃and the ion conductor V₂O₅were prepared by the thermal decomposition method,and the composite material coated with the semiconductor In₂O₃was synthesized by the hydrothermal method.The results show that the coating effect of the zinc ion conductor V₂O₅is the best.The material capacity of MnO₂@V₂O₅can achieve 321.7 m Ah/g at a current density of 100 m A/g,which is obviously higher than that of MnO₂material（213.5 m Ah/g）,MnO₂@Al₂O₃material（222.4m Ah/g）and MnO₂@In₂O₃material（240.1 m Ah/g）.Moreover,V₂O₅coating also enhances the rate performance of the material.The discharge capacity can still possess 160.7 m Ah/g under a high current density of 1.6 A/g.（4）The study on optimization of V₂O₅coating amount and related mechanism.With the increase in the amount of V₂O₅coating（3%,5%and 10%）,the initial capacity of the material increases（249.9 m Ah/g,312.2 m Ah/g and 371.9 m Ah/g）.However,the material has the best cycle stability when the coating amount is 5%.Through the CV study of different scanning speeds,it is found that the contribution of the material capacitance increases from 58.6%to76.9%,and from the GITT test results,it is found that the Zn²⁺diffusion coefficient of MnO₂@V₂O₅is 3.78×10^-5-4.17×10^-9cm²s^-1under charge and discharge conditions,which is significantly higher than that of uncoated MnO₂materials.The above results indicate that the presence of the vanadium pentoxide layer not only improves the surface reaction activity of manganese dioxide,but also enhances the zinc ion diffusion capacity of the material.In addition,by comparing the microscopic morphology changes of the coating material before and after the cycles,it is observed that the V₂O₅coating on the surface of MnO₂can prevent the direct contact of MnO₂with the electrolyte,thereby improving the cycle stability of the material.