Structure Design And Electrochemical Properties Of Nano Layered Manganese Dioxide

Manganese dioxide（MnO₂）is considered as an excellent alternative material for energy storage due to its abundant reserves,wide sources,environmental friendliness and high theoretical specific capacity.As a classic electrode material of electrochemistry,MnO₂suffers the problems of low electrical conductivity and slow ion transportation in the electrochemical reaction process,as well as the Jahn teller effect caused by the dissolution of Mn²⁺in the charge and discharge process.At the same time,because of the low efficiency of theoretical capacity utilization,aqueous zinc manganese battery is difficult to achieve satisfactory capacity in neutral electrolyte.On the basis of the research of energy storage,the structure and microstructure ofδ-MnO₂ were regulated by different synthesis methods.To obtain theδ-MnO₂ electrode with high ion and electronic conductivity,high capacity and high energy density,the electrode conductivity ofδ-MnO₂ were designed and optimized through the intercalation of alkali metal cations（Na⁺,K⁺,Li⁺）.And the electrochemical reaction process and charge storage mechanism were discussed and investigated.1)Firstly,the Mo⁶⁺doped layered sphericalδ-MnO₂ were synthesized by solvothermal method using ammonium heptamolybdate as the precursor of Mo⁶⁺in the reaction system of potassium permanganate and concentrated hydrochloric acid.The morphology control ofδ-MnO₂ were investigated based on the different Mo⁶⁺content.And the supercapacitive performance was evaluated relevant toδ-MnO₂ based electrode materials with different Mo⁶⁺content.On this basis,the supercapacitor performance ofδ-MnO₂ doped with Mo⁶⁺was optimized and the internal relationship between the morphology ofδ-MnO₂ and the capacitance performance was explored.When the mass fraction of Mo⁶⁺is 15 wt.%,the specific capacity ofδ-MnO₂ electrode achieved 202 F·g^-1 at the current density of 100 mA·g^-1.And the electrode has high electrochemical performance and good cycle stability.2)Secondly,the columnarδ-MnO₂ composed of nanoparticles was prepared after high-temperature calcination using potassium permanganate and medical absorbent cotton as precursor and template,respectively.The calcination route led to the formation of crystalline/amorphous interface on the surface ofδ-MnO₂ nanoparticles with good porosity,large specific surface area and interconnected pore structure.These structural advantages are conducive to shorten the diffusion path of ions and electrons and provide diffusion channels for the exchange intercalation of Na⁺and H⁺.And the capacitance ofδ-MnO₂ electrode reached 214 F·g^-1 at the current density of 100 mA·g^-1.The kinetic analysis reveals that the columnarδ-MnO₂ electrode is a charge storage process dominated by capacitance contribution.When the current density is 100 mA·g^-1,the specific capacity is 29.02 F·g^-1,and the corresponding energy density is 38.4 Wh·kg^-1.The mechanism of intercalation of Na⁺and H⁺betweenδ-MnO₂ layers is clarified in this work.3)Then,the potassium permanganate and sodium sulfate were used as precursors to prepare Na⁺dopedδ-MnO₂（Na_xKMO）formed on nickel foam by hydrothermal method.And the Na_xKMO nanosheet materials were shown as integrated,self-supported and dense three-dimensional network.The introduction of Na⁺enhances the ionic conductivity ofδ-MnO₂.As a result,Na_xKMO electrode has smaller equivalent series resistance and better capacitance performance compared with that of the countpart.As a result,the specific capacity of Na_xKMO electrode is 324.2 F·g^-1 at the current density of 0.5 A·g^-1,which is much higher than that of the unadoptedδ-MnO₂ electrode.4)Thirdly,Na⁺and K⁺dopedδ-MnO₂ were prepared by electrochemical method and hydrothermal method to improve the zinc storage properties ofδ-MnO₂ nanoflakes.Ex-situ intercalation of Na⁺can effectively enhance the ability of proton intercalation and deintercalation,and enhance the performance of zinc ion battery.The kinetic analysis shows that Na⁺intercalation increases the contribution of diffusion control and accelerates the transport capability of cations in theδ-MnO₂ interlayers.The density functional theory shows that Na⁺intercalation reduces the band gap ofδ-MnO₂ and enhances the electronic conductivity ofδ-MnO₂.The assembled zinc ion battery shows a high specific capacity of 335 mAh·g^-1 at the current density of 0.5 A·g^-1.The quasi-solid flexible zinc ion soft pack battery assembled by Na⁺modifiedδ-MnO₂ and electrodeposited Zn nanosheets possesses a capacity of 284 mAh·g^-1.On this basis,the soft pack battery shows stable open circuit voltage and can effectively drive an electronic hygrometer and 23 light-emitting diodes.5)Finally,theδ-MnO₂ nanoflakes with Li⁺ex-situ intercalation was designed by hydrothermal synthesis and electrochemical extraction.The ex-situ phase structure analysis shows that theδ-MnO₂ phase of Li⁺intercalation changes after the first charge process,and the newly formed phase has the structural stability and high-efficiency zinc storage performance during the charge/discharge reaction.The specific capacity of the zinc ion battery assembled by zinc foil and Li⁺intercalationδ-MnO₂ is 348m Ah·g^-1 at the current density of 0.5 A·g^-1 and 177 mAh·g^-1 at the current density of 6A·g^-1.After 300 consecutive charge/discharge reactions under the current density of 4A·g^-1,the capacity of the battery is 74.9%of the initial capacity.The high capacitance retention is due to the good integration performance of porosity,rich in active sites of cathode materials and conductive substrates.It provides a new design concept and method for improving the zinc storage performance of MnO₂.