Modified MnO₂ Cathode Material Based On Interlayer Regulation Strategy And Its Zinc Storage Performance

Aqueous zinc-ion batteries（AZIBs）are increasingly developed due to their low cost,high safety,and simple fabrication process.Manganese dioxide has the advantages of high theoretical capacity,abundant natural reserves,and diverse crystal structures（α,β,γ,δ）,enabling it to be a very competitive cathode material for AZIBs.However,the practical application of manganese dioxide is hindered,first of all,its low working capacity and its own structure（tunnel/layered）is accompanied by severe collapse and damage during discharge/charge.In addition,its lack of intrinsic conductivity results in slow ion transport rates.In view of the above problems,from the perspective of regulating the microstructure,manganese dioxide was modified by a metal ion（Na⁺/Ag⁺）pre-intercalation strategy to increase the conductivity and stabilize the layer structure.To this end,two research works were carried out,combining various characterization and testing techniques to analyze the samples,and to explore the reasons why the interlayer pre-intercalation of metal ions improves the battery performance by analyzing the electrochemical kinetic process.The main research contents are as follows:（1）Aiming at the problems of low specific capacity and poor cycle stability of layered manganese dioxide.The pristine MnO₂ was prepared by a hydrothermal method,followed by a Na⁺pre-intercalation strategy and a chemical reduction method under ultrasonic conditions to successfully synthesize the Na⁺pre-intercalated manganese dioxide electrode material（Na-MnO₂）containing oxygen vacancies.The experimental results show that the introduction of Na⁺reduces the interlayer spacing by 0.01 nm and improves the stability of the MnO₂ layered structure.The combined action of Na⁺and oxygen vacancies entering the interlayer causes the decrease of the average oxidation state of the Mn element,which increases the conductivity of the electrode material and improves the ion transport rate.At the same time,the generated oxygen vacancies additionally increase the active sites for ion intercalation,thereby enhancing the specific capacity of the electrode.Electrochemical test results show that the Na-MnO₂ electrode has excellent zinc storage capacity,increasing the specific capacity from 210.5 m Ah g^-1 to 247.6 m Ah g^-1 at 200 m A g^-1,and dis/charged 2000 Circle at 1000 m A g^-1,the capacity of Na-MnO₂ is improved to 140.1 m Ah g^-1 compared to pristine MnO₂(91.6m Ah g^-1).Furthermore,GITT and EIS calculations show that Na-MnO₂ has lower charge-transfer electrons and faster ionic diffusion coefficients compared to pristine MnO₂.（2）On the basis of studying the pre-insertion of inactive ions between the manganese dioxide layers and the introduction of oxygen vacancies leading to interlayer changes and capacity enhancement,the effect of the introduction of electrochemically active ions on the structure and properties of manganese dioxide was also explored.Ag⁺interlayer mediated MnO₂（Ag-MnO₂）was prepared by a simple one-step hydrothermal method and compared with the pristine MnO₂.The introduction of Ag⁺expands the interlayer spacing by 0.02 nm,which effectively alleviates the electrostatic interaction between the layers,thereby enhancing ion mobility.In addition,through the ex-situ characterization technique,it was found that the Ag⁺between the MnO₂ layers would migrate during the first several discharge/charge processes,and the irreversible electron transfer would form the Ag element.The interlayer Ag⁺ions and the in situ generated Ag metal synergistically enhance the electrical conductivity and structural stability of MnO₂.Comparing the performance of the two electrodes,it was found that the Ag-MnO₂ electrode achieved a higher capacity.At 100 m A g^-1,the highest capacity of Ag-MnO₂ is 286.0 m Ah g^-1,with no decay after 150 cycles.However,the capacity of the MnO₂ electrode after 150 cycles is only 45 m Ah g^-1,which is only 24.3%of the highest capacity.At 1000 m A g^-1,Ag-MnO₂ was dis/charged more than 600 times,and the capacity was 161.7 m Ah g^-1（retention rate,98.8%）.The capacity of MnO₂ remains only 39.1 m Ah g^-1（retention rate,33.0%）.This suggests that the electrochemically active metal ions pre-intercalate will participate in the redox process,thus providing a higher specific capacity.