Study On Preparation And Its Zinc-ion Batteries Performance Of MnO₂ Electrode Material

With the excessive consumption of fossil fuels and the increasing demand for clean energy,it is urgent to develop effective electrochemical energy storage devices.However,the commercialized lithium-ion batteries are limited in energy storage due to the flammable electrolyte and high cost.Recently,aqueous zinc-ion batteries have attracted extensive attention from researchers because of Zn-rich resources,high security and environmental friendliness.Among the many candidates,MnO₂ is recognized as one of the most promising cathode materials because of the large theoretical capacity,high working voltage and cost effectiveness.Unfortunately,the irreversible manganese dissolution and poor conductivity result in sluggish reaction kinetics and inferior electrochemical performance during charge/discharge process.In order to overcome these issues,some effective means,such as Mn²⁺addition in electrolyte,pre-intercalation,defect engineering and architecture construction,have been carried out to modify the cathode materials,which demonstrate excellent electrochemical performances.Research shows that,doping can improve the inherent conductivity of the material by introducing active atoms into the main material and changing the lattice parameters.In addition,the changes in the bond energy caused by heteroatom can alleviate the volume expansion of host structure during Zn²⁺ion（de）intercalation,thus enhancing the structural stability.However,the metal ion doping of manganese dioxide is mostly focused on mono-metal doping and dual metal ions doping is rarely reported.In addition,doping is also a effective approach to improve the properties of materials,among which oxygen defect is a common anionic defect.Electrode materials rich in defects can induce more active sites,promote ion diffusion and maintain the structural stability.While most modification methods for introducing defects focuse on post-processing on the basis of non-defective materials,which not only consume time,but also are not conducive to commercialization.In this work,we improve the electrochemical performance of manganese dioxide by doping dual ions and introducing oxygen vacancies.The main research contents are as follows:（1）Herein,we synthesis the Co/K co-dopedδ-MnO₂ ultrathin nanosheets by one-step hydrothermal method on a carbon cloth substrate.The vertical growthδ-MnO₂ is conducive to the diffusion of electrolyte and can expose richer active sites,thus,shorten the transmission path of Zn²⁺.The synergistic effect of K⁺and Co²⁺effectively reduces the charge transfer resistance and speeds up the reaction kinetics.Surface capacitive control can alleviate the volume expansion of host structure and ex-situ XRD further demonstrated the stability of the material structure during charging and discharging.Electrochemical measurements show a specific discharge capacity of 424 m A h/g at a current density of 1A/g and 331 m A h/g at a current density of 5 A/g,the capacity retention is as high as 70%after charged-discharged up to 1000 cycles.Compared with the K-MnO₂,performance and cycle stability are improved largely.Finally,we assembled it into a button battery,which successfully lit 74 LEDs and worked for nearly two hours.The dual-metal-ion doping strategy proposed in this work opens up a new opportunity for manganese-based aqueous zinc ion batteries in the field of portable electronic devices.（2）We prepare the MnO₂ nanoparticle rich in oxygen vacancies by liquid phase method at room temperature.The nanosized structure of MnO₂ NDs not only have large surface area,but also can effectively reduce the volume expansion during charging/discharging.At the same time,EPR and XPS confirmed that the introduction of oxygen vacancy in MnO₂ nanoparticles.EIS test also showed that oxygen vacancy effectively reduced the charge transfer resistance and accelerated the ion diffusion rate,thus achieve a higher specific capacity at high current density.Additionally,the H⁺/Zn²⁺co-insertion energy storage mechanism was discussed by employing various electrochemical kinetics analyses.Originating from its unique electronic structure,the as-prepared MnO₂ possesses a high discharge capacity of 127 m A h/g at a current density of2 A/g,and the capacity retention rate is 70.9%after 1000 cycles.Besides,the exploration of different synthesis processes further confirmed that the excellent electrochemical properties of MnO₂ nanomaterials can be attributed to the unique nanoscale structure and rich oxygen vacancy.The strategy of introducing oxygen vacancy into manganese dioxide cathode material at room temperature proposed in this work provides an idea for commercialization of manganese based aqueous zinc ion battery.