Preparation And Electrochemical Performance Of Manganese Oxides As Cathode Materials Of Zinc Ion Batteries

Aqueous zinc ion batteries（ZIBs）hold great potential to become one of important energy storage devices.Compared to lithium ion battery and lead-acid battery,ZIBs had several unique advantages in performance.Firstly,the use of mild,cheap aqueous electrolytes,instead of the flammable,expensive organic electrolytes or strongly acidic/alkaline aqueous electrolytes,makes ZIBs safe,low-cost and eco-efficient.In addition,aqueous ZIBs were insensitive to water and oxygen,need no use strictly water/oxygen-controlled assembly line,which can further save their production costs.Among all potential cathodes of aqueous ZIBs,manganese oxides（MnO_x）were considered to be one of the most promising candidate materials.The preparation of MnO_x is generally carried out by hydrothermal and coprecipitation method.While achieving encouraging properties,these preparation methods are generally time-consuming,rigid,or complex,which would substantially limit their practical applications in massive production of ZIBs’cathode materials.In view of this problem,two simple and efficient methods were adopted to prepare mesoporous manganese oxides as cathode materials of ZIBs with high Zn²⁺storage ability.Firstly,mesoporous manganese oxides（MnO_x）electrode materials were prepared by a one-step,cost-efficient solution combustion process.The influence of fuel/oxidizer ratio on phase composition,morphology and Zn²⁺storage ability of products is investigated.Secondly,a uniform cladding of MnO₂ was deposited in situ on the surface of carbon fiber by electrodeposition.The effects of different deposition time on the electrochemical performance of MnO₂ were systematically investigated.Then,the influence of different deposition time on the electrochemical properties of MnO₂ was systematically studied.The brief research contents of this paper are showed as follows:1.Using citric acid（C₆H₈O₇）and manganese nitrate（Mn（NO₃）₂）aqueous solution as starting materials,mesoporous manganese oxides（MnO_x）were prepared by a one-step,cost-efficient solution combustion process.The influence of fuel（C₆H₈O₇）/oxidizer（Mn（NO₃）₂）ratio on phase composition,morphology and Zn²⁺storage ability of products was investigated.Experimental results demonstrated that a proper fuel/oxidizer molar ratio ensures a violent combustion mode with intensive gas evolution,resulting in fluffy products.The porous structure of the fluffy products not only facilitates the penetration of electrolyte into the bulk of the materials,but also shortens the transport distance of Zn²⁺during the electrochemical reactions.When used as cathode materials of aqueous zinc ion batteries（ZIBs）,the optimized product MnO_x III,with C₆H₈O₇/Mn（NO₃）₂ molar ratio of 0.9/1,exhibited a high specific capacity of 261mAh g^-1 at 0.5 A g^-1 after 120 cycles.This facile combustion process maybe suitable for mass production of MnO_x electrode materials for aqueous ZIBs.2.To further research the electrochemical performance of MnO_x prepared by different methods,a homogeneous deposited layer was in situ deposited on the surface of carbon fiber by electrodeposition in aqueous solution Mn（NO₃）₂.The influence of different deposition time（10,20,40 and 80 min）on phase composition and morphology of deposited layer was systematically studied.Analysis showed that a homogeneous MnO₂ cladding was deposited on the surface of carbon fiber.The deposited layer of samples became thicker and then broke with the increase of deposition time.The electrochemical test showed that sample with deposition time of 10 min delivers a high reversible capacity of 493mAh g^-1 at 100 mA g^-1 at the first cycle,and 252.6 mAh g^-1 after 300charge-discharge cycles.The sample also showed an outstanding rate capability.Electrochemical impedance spectroscopy（EIS）results indicated that sample with deposition time of 10 min had lowest internal resistance and ion diffusion resistance among all samples.