Controllable Synthesis Of Monovalent Cation Doped β-MnO₂ And Its Zinc Storage Properties

Aqueous zinc ion battery is considered as one of the potential green energy storage devices because of its low cost,simple manufacturing process,high safety,low redox potential and high theoretical capacity.In recent years,researchers have carried out a series of scientific research on positive electrode materials,electrolyte and zinc negative electrode around aqueous zinc ion batteries,and achieved some research results to promote their application and development in wearable electronic devices.As the core component of zinc ion battery,cathode material is one of the determinants of battery performance.Therefore,extensive research work has been carried out on the research of high-performance cathode materials.Among the common cathode materials for zinc ion batteries,manganese dioxide（MnO₂）has the advantages of low toxicity,high open circuit voltage,cheap and easy raw materials and simple preparation.It is considered to be an ideal cathode material for high-performance aqueous zinc ion batteries.However,in the electrochemical reaction process,with the embedding and stripping reaction of Zn²⁺,there is an irreversible structural collapse（Jiang Taylor effect）in MnO₂ electrode material,which leads to the problems of poor cycle stability and low output power.Taking MnO₂ as the research object,this paper synthesized two different cations doped MnO₂ by simple hydrothermal reaction method.Chemical doping greatly improved the structural stability and reversibility of MnO₂ electrode in the process of charge and discharge,and significantly improved the electrochemical performance of aqueous zinc ion battery.The main research contents are as follows:Synthesis and properties of sodium ion doped MnO₂（NaMO）.A simple solvothermal method for the preparation of NaMO nanorods was proposed.Sodium ion doping can not only increase the layer spacing of MnO₂ and improve the charge transfer efficiency in the electrochemical reaction process,but also stabilize the crystal structure of MnO₂ and reduce the structural collapse caused by the disproportionation reaction of Mn³⁺.The aqueous zinc ion battery assembled with NaMO as active cathode material showed good electrochemical stability and reversibility.In addition,the battery can retain 55.8 mAh g^-1 even after 1000 cycles of charge and discharge at a high current density of 4.0 A g^-1,which is much better than the zinc ion battery assembled based on traditional MnO₂ electrode materials.In addition,thanks to the increase of MnO₂ layer spacing by sodium ion doping,sodium ion doping significantly improves the embedding and stripping process of Zn²⁺in the battery reaction,and Zn//NaMO battery shows better electrochemical stability.Synthesis and properties of ammonium ion doped MnO₂（NMO）.Based on the preparation methods and performance research results of NaMO electrode materials in the previous chapter,ammonium ion doped MnO₂ electrode materials with smaller ion radius were constructed by hydrothermal reaction method by adjusting the composition of original reactants,and their effects on the embedding and stripping reactions of Zn²⁺and H⁺were explored.Compared with Na⁺,NH₄⁺doping can not only increase the layer spacing of MnO₂,but also stabilize the tunnel structure of MnO₂ by forming intermolecular hydrogen bond,so as to further improve the electrochemical stability of aqueous zinc ion battery.Firstly,the doping chemical state of NH₄⁺in MnO₂ was explored by adjusting the washing times of samples and combining solid-state NMR and mass spectrometry.Secondly,SEM,TEM and FT-IR were used to further confirm the effective residue of NH₄⁺in MnO₂（i.e.effective doping through washing process）.Finally,the aqueous zinc ion battery was assembled with NMO as active electrode material,and the electrochemical performance was tested and analyzed.Compared with NaMO and MnO₂ in the previous chapter,NMO electrode shows better electrochemical stability and reversibility.The aqueous zinc ion battery assembled based on NMO can cycle stably for 1000 cycles at a current density of 4.0 A g^-1,while maintaining a reversible specific capacity of 113.9 mAh g^-1;Even at a high current density of 8.0 A g^-1,the battery can cycle stably for 10000 cycles and maintain a reversible specific capacity of 42.7 mAh g^-1.In addition,the effects of electrolyte composition and washing times on NMO were optimized to obtain an aqueous zinc ion battery with high rate performance and long cycle life.Above all,this paper proposes a universal solvothermal method to construct different cations doped MnO₂ active electrode materials and apply them to aqueous zinc ion battery system;The doping of NH₄⁺and Na⁺solves the problems of MnO₂ structure instability and slow electrochemical reaction caused by ginger Taylor effect to a certain extent,and provides a new idea for the research of cathode materials for high-performance aqueous zinc ion batteries.