The Preparation And Study Of Manganese Dioxide/carbon Composites For Aqueous Zinc-ion Batteries

With the advantages of low cost,high safety and stability and a green preparation process,zinc-ion batteries are a very promising system for storing electrical energy.The cathode material is the main factor influencing the development of zinc-ion batteries for practical production applications.Among the various cathode materials,MnO₂ has the following advantages:simple preparation,environmental protection,low toxicities and price,high theoretical discharge specific capacity,and is widely used in zinc-ion batteries.However,the MnO₂ material itself has disadvantages such as poor electrical conductivity,structural dissolution during cyclic charging and discharging,and drastic volume expansion effect,which seriously affects the actual performance of aqueous zinc-ion batteries.In contrast,the new carbon material,as an excellent conductive material,has a wide range of applications in the areas of energy storage.To better enhance the electrochemical performance of MnO₂,the introduction of carbon nanomaterials in the preparation process is an excellent improvement strategy.In this paper,three different structures of MnO₂/carbon nanocomposites were synthesized using a simple hydrothermal technique and applied as positive materials in the field of aqueous zinc-ion batteries under room temperature conditions.The study is as follows:1.α-MnO₂@g-C₃N₄ nanocomposites were prepared using a combination of high-temperature pyrolysis and hydrothermal techniques.The protonated g-C₃N₄ nanosheets were obtained by pyrolysis of melamine and further treatment in concentrated hydrochloric acid solution.Simultaneous g-C₃N₄ protonation modification duringα-MnO₂ nucleation growth showed better electrochemical performance ofα-MnO₂@g-C₃N₄ electrodes compared to MnO₂ electrodes without g-C₃N₄ materials.A discharge specific capacity near the theory value was achieved at a current density of 0.1 A·g^-1.With a current density of 1 A·g^-1,it gives a specific discharge capacity of more than 100m Ah·g^-1 after 5000 cycles,and still has a coulombic efficiency above 98.7%.The cathode based onα-MnO₂@g-C₃N₄ has a high energy density(563 Wh·kg^-1)and power energy density(2170 W·kg^-1),which is superior to the majority of currently reported manganese-based materials,providing a good cathode material for zinc ion-batteries for future life applications.2.To improve the conducting properties of the electric electrodes further and to reduce the electric resistances inside the battery,a series ofα-MnO₂/CNTs nanocomposites with various ratios were obtained by a one-step hydrothermal process.The cross-linked mesh structure was constructed by introducing CNTs,which suppressed the volume expansion ofα-MnO₂ during charging and discharging.Meanwhile,the effects of differential levels of CNTs on the structural forms ofα-MnO₂ were also investigated.The best ratio ofα-MnO₂/CNTs electrode could exhibit an initial discharge capacity of 302 m Ah·g^-1 at a current density of 0.1 A·g^-1,and maintain 116 m Ah·g^-1after 2000 cycles(2 A·g^-1),and the coulombic efficiency was always maintained above99%.3.The composite process of MnO₂ with CNTs is mainly connected by physical bonding,and this force is fragile compared to chemical bonding and therefore does not provide longer cycle life.A two-carbon（CNTs@C）synergistic strategy was used to enhance the electrochemical performance of MnO₂.CNTs@C was obtained by reacting CNTs with glucose using hydrothermal technique,while CNTs@C was introduced during the preparation ofα-MnO₂.The introduction of amorphous carbon further improved the electrical conductivity of the nanocomposite,while a large number of C-OH and O=C-OH groups were generated on the surface of the composite,which combined with the O elements on the surface of MnO₂ to enhance the bonding between them.The MnO₂/CNTs@C nanocomposite exhibited excellent cycling stability and ion transport ability.In particular,the electrode material’s multi porous structure affords a greater number of activation points for the process of a chemical interaction,which facilitates sufficient electrolyte infiltration and ion diffusion to achieve high multiplicity performance.With a current density of 2 A·g^-1 for 6000 cycles,a capacity retention of91.4%was still realized.Especially,at a high current density of 3 A·g^-1,the MnO₂/CNTs@C electrode maintained a discharge specific capacity of 107 m Ah·g^-1.In further,to intensively research the internal reaction process of the battery,the continuous intercalation reaction of Zn²⁺and H⁺in the battery was successfully demonstrated by ex-situ techniques,which promoted the subsequent research on the cathode material of zinc-ion battery.