Study On Bi-based Material Modification Of Manganese Oxide Positive Electrode In Neutral Aqueous Zinc-ion Battery

Aqueous zinc-ion batteries have become the research focus of the new generation of green batteries due to its simple preparation process,low production cost,and environmental protection.Among the various cathode candidate materials,manganese-based materials have become the cathode materials for ZIBs battery systems with market development potential due to their rich resources,low production cost,high safety,wide valence distribution,and low toxicity.However,manganese-based materials have limited their large-scale applications due to poor electrical conductivity and poor cycle stability.Therefore,in order to solve the above problems,its modification has become a key research direction.Finding a simple and effective modification method and an in-depth understanding of the modification mechanism are of great significance for realizing the industrial production of neutral aqueous manganese-based zinc-ion batteries and subsequent commercial use.Firstly,the strategy and method to improve the capacity and cycle stability of the positiveα-MnO₂ of a neutral aqueous zinc-ion battery using Bi₂O₃is proposed.The efficient preparation of MnO₂-Bi₂O₃is achieved through simple physical mixing.The MnO₂-Bi₂O₃exhibits better long-term cycle stability and higher capacity than those reported in the literature.After more than 5,300 charge and discharge cycles at 3 A g^-1and 10 A g^-1current densities,the specific discharge capacity can be stabilized at around 120 and 80 m Ah g^-1,respectively.Through electrochemical kinetics test,charging-discharge mechanism study,phase and structure evolution analysis and DFT calculation,three synergistic effects of Bi₂O₃to improve the capacity and cycling performance ofα-MnO₂ are revealed:1.In the process of charge and discharge,Bi³⁺formed in situ enhances the stability of the structure by bonding with Mn-O bond of MnO₂,which effectively inhibits the dissolution of Mn²⁺.2.Competitively generate Bi₂Mn₄O₁₀,inhibit the formation of irreversible Zn Mn₂O₄,and improve the cycling performance of materials;3.Tunnel-like Bi₂Mn₄O₁₀can contribute additional capacity by inserting H⁺.The modification work can provide reference and guidance for the large-scale application of MnO₂cathode.Furthermore,the methods and effects of other bismuth-based materials to modify MnO₂are investigated.The Bi-α-MnO₂/BiOCl material is synthesized in one step under hydrothermal conditions.The microstructure and phase composition of the material are studied and analyzed by XRD and SEM.Through the electrochemical performance test,it is found that the composite precursor material still shows good cycle performance and high capacity compared with pureα-MnO₂.The specific discharge capacity of Bi-α-MnO₂/BiOCl electrode can be stabilized at 130,90 and 70 m Ah g^-1after charging and discharging more than 3500,7000 and4900 times at current densities of 2,3 and 5 A g^-1,respectively.Furthermore,three kinds of composites were obtained after heat treatment at different temperatures using Bi-α-MnO₂/BiOCl as precursors,which were denoted as BM-x（x represents the temperature of heat treatment,x=500,600,700℃）.Phase analysis shows that different heat treatment temperatures have a very important influence on the composition of the material.The composition of BM-x is Bi-α-MnO₂,Bi-α-MnO₂/Mn₂O₃,α-MnO₂/Mn₂O₃/Bi₂Mn₄O₁₀at 500,600 and 700℃,respectively.As a comparison sample,α-MnO₂after heat treatment under the same conditions is denoted as M-x（x represents the heat treatment temperature,x=500,600,700℃）,and its composition is labeled asα-MnO₂/Mn₂O₃-500,α-MnO₂/Mn₂O₃-600,α-MnO₂/Mn₂O₃-700 at 500,600,700℃,respectively.Electrochemical analysis shows that the cycle performance of BM-x is significantly improved compared to M-x and the electrochemical performance of two-phase coexisting materials reported in foreign literature.Among them,BM-700 shows the best performance,with 2500,4000,and 3000 cycles at current densities of 1,2,and 3 A g^-1,respectively,and the specific capacity is maintained at160,120,and 125 m Ah g^-1,respectively.The reason for the performance improvement of BM-x after heat treatment is attributed to the existence of bismuth which stabilizes the structure of manganese-based materials,inhibits the formation of irreversible products and facilitates the rapid migration of ions.Finally,the modification strategy of Bi₂O₃is extended to other manganese dioxide systems.The commercial Bi₂O₃is physically mixed withδ-MnO₂synthesized under hydrothermal conditions to obtainδ-MnO₂-Bi₂O₃-x（where x represents the atomic ratio of Mn to Bi,x=5,10,15,20）.Electrochemical studies have found thatδ-MnO₂-Bi₂O₃-10 shows the best excellent electrochemical performance.The specific discharge capacity is 100 m Ah g^-1after being cycled for 4900 cycles at a current density of 3 A g^-1,the capacity is stable at about 70 m Ah g^-1after being cycled for 9000 cycles at a super-high current density of 10 A g^-1.The results of CV,GITT and electrochemical impedance analysis show thatδ-MnO₂-Bi₂O₃-10has a higher ion diffusion coefficient and a lower impedance after different cycles.