Study On Controllable Preparation And Zinc Ion Storage Properties Of Manganese Dioxide Based Cathode Materials

Zinc is low cost,non-toxic,has a high theoretical capacity(820 m Ah g^-1 and 5854m Ah cm^-3)and a low standard electrode potential（approx.-0.76 V vs.Standard Hydrogen Electrode（SHE））and more importantly,zinc metal has good stability in aqueous electrolytes（alkaline,neutral and weakly acidic）.With its high specific capacity,low cost,high safety and environmental friendliness,the aqueous zinc ion battery is expected to be the most promising new energy storage device.Manganese oxides,especially manganese dioxide,have been one of the most promising high-performance cathode materials for aqueous zinc ion batteries due to their ideal electrochemical properties,rich and diverse crystal structures,high theoretical specific capacity,low cost,low toxicity and high abundance.However,the irreversible structural transformation and dissolution of the manganese dioxide cathode during the charge/discharge cycle has led to capacity degradation,which is a key issue to be addressed in the process of its commercialisation.In order to effectively alleviate the problem of irreversible structural transformation and dissolution of manganese dioxide,this thesis uses a one-step hydrothermal method to achieve the controlled preparation of modified manganese dioxide cathode materials by doping with metal ions of different valence such as K⁺and Mg²⁺.The specific research contents and results are as follows:（1）Potassium permanganate was chosen as the manganese source and oxalic acid as the reducing agent,and the layered structure ofδ-MnO₂ material was successfully prepared by a simple one-step hydrothermal method.The microscopic morphology and structure ofδ-MnO₂ material and the law of electrochemical performance were studied by a series of characterization means.The results show that when the hydrothermal reaction temperature was set at 160°C,the reaction solvent ratio was 3:2 and no acidity regulator was added,the electrochemical zinc storage performance of the laminatedδ-MnO₂ materials was superior:the initial discharge specific capacity was 248.8 m Ah g^-1at a current density of 200 m A g^-1,the reversible discharge specific capacity was 226.6m Ah g^-1 after 50 cycles,and the reversible discharge specific capacity was 226.6 m Ah g^-1 after 100 cycles.The excellent electrochemical performance is probably due to the large layer spacing（approximately 7.0?）of the layeredδ-MnO₂,which is larger than the radius of the hydrated zinc ions and allows the zinc ions to travel back and forth between the layers,resulting in a better cycling performance of the electrode material.material has a better cycling performance.（2）The tunneling typeα-MnO₂ material was successfully prepared by a one-step hydrothermal method using potassium permanganate as the manganese source,magnesium powder as the magnesium source and sulphuric acid as the acidity regulator.After the study of the hydrothermal reaction temperature on the structure,morphology and composition of the material and the law on the electrochemical properties of the electrode material.The results show that when the hydrothermal reaction temperature was set at 180°C,the reaction solvent ratio was 3:2 and the acidity regulator was added at12.5 m L,the electrochemical storage performance of theα-MnO₂ electrode material co-doped with potassium and magnesium ions was superior:the initial discharge capacity of the MnO₂ electrode material was 169.8 m Ah g^-1 at a current density of 0.2 A g^-1,and the reversible discharge ratio after 100 The excellent cycling performance may be attributed to the intrinsically stable structure of theα-MnO₂ material,the similar size of the tunneling pore ofα-MnO₂ to that of the zinc ion,which also provides more active sites for Zn²⁺and a higher specific capacity,making it a good candidate for the cathode material.During the first discharge,the Zn ions in the electrolyte can enter the tunnel structure ofα-MnO₂ and play the role of"pillars"in the tunnel structure,which not only stabilise theα-MnO₂ microstructure to prevent collapse,but also enhance the conductivity of the electrode material.These"pillars"not only stabilise theα-MnO₂ microstructure to prevent its collapse,but also enhance the electrical conductivity of the electrode material,resulting in a better cycling performance of theα-MnO₂ material.