Synthesis And Energy Storage Mechanism Of Ammonium Cations Pre-Intercalated Manganese Oxide Cathode Materials

Due to its advantages of low cost,high safety,and simple assembly conditions,aqueous zinc-ion battery have broad application prospects in the emerging large-scale energy storage fields.However,there are still some inevitable problems and challenges in aqueous zinc-ion batteries.Especially for the cathode material,because the active water in the aqueous electrolyte will react with the cathode material,resulting in the dissolution of the cathode material and the generation of byproducts during the process of charge and discharge.At the same time,the bivalent Zn²⁺has a high charge density,which will produce strong electrostatic interaction with the cathode material during the battery charging and discharging process,and affect the structural stability of the cathode material.In addition,the energy storage mechanism of cathode materials in aqueous zinc-ion batteries is still unclear.To address the above problems,this thesis has proposed a new method for structural modification of layered manganese dioxide cathode materials and discussed the energy storage mechanism,and achieved excellent electrochemical performance of aqueous zinc ion battery.Specific work is as follows:（1）Ammonium cation（NH₄⁺）,tetramethylammonium cation（TMA⁺）,tetramethylammonium cation（TEA⁺）and tetramethylammonium cation（TBA⁺）with different sizes were selected,and ammonium cations pre-embedded layered manganese dioxide materials（δ-MnO₂）were prepared by self-assembly.X-ray diffraction（XRD）tests confirmed that the embedding of different ammonium cations can be used as the"pillar"between layers to expand theδ-MnO₂ layer spacing.Scanning electron microscopy（SEM）,X-ray energy dispersive spectroscopy（EDS）,infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）tests demonstrated that the ammonium cations were successfully pre-embedded and uniformly distributed in theδ-MnO₂ layer.Then various electrochemical tests were carried out and the specific capacity,electrochemical impedance and cyclic performance of above materials were compared.TMA-MnO₂ materials with the best electrochemical performance were selected and further characterized and tested.（2）The energy storage mechanism of TMA-MnO₂ material was explored.Cyclic voltammetry tests forδ-MnO₂ and TMA-MnO₂ materials showed that the reaction kinetics can be improved effectively with larger layer spacing.In situ X-ray diffraction（In operando XRD）was used to analyze the time of by-product formation and disappearance,and it was found that H⁺intercalation in TMA-MnO₂ material was relatively small.In situ p H test and the analysis of H⁺and Zn²⁺embedding content during the discharge process show that the larger layer spacing of TMA-MnO₂ material is more conducive to the embedding of Zn²⁺,so as to improve the competitiveness of Zn²⁺and inhibit the embedding of H⁺.Finally,in situ X-ray absorption spectroscopy（In operando XAS）showed that the local structure of TMA-MnO₂ material had a large deformation during the discharge process,indicating that it was greatly affected by Zn²⁺with a higher charge density,which also indicated that TMA-MnO₂ material could intercalated more Zn²⁺.In conclusion,we successfully synthesizedδ-MnO₂ with ammonium cationic pre-embedding,and selected the TMA-MnO₂ material with the best electrochemical performance through a series of characterization and testing.At the same time,the energy storage mechanism of TMA-MnO₂ cathode material was explored,and it was found that the large layer spacing of TMA-MnO₂ can effectively improve the competitiveness of Zn²⁺intercalation and inhibit H⁺intercalation.This work also provides a new idea for studying the energy storage mechanism and modification strategy of layered cathode materials.