Study On Electrochemical Properties Of Mg_xMnO₂ Cathode Material For Zinc-ion Batteries

Among various potential battery systems,aqueous zinc-ion（Zn-ion）batteries have attracted extensive attention due to their low cost,high safety,and environmental friendliness.Manganese-based oxides,vanadium-based oxides,and Prussian blue analogs have been reported as the high-performance aqueous Zn-ion battery cathode materials.Among them,manganese-based oxides show good application prospects due to their low price,rich oxidation states,and diverse crystal structures.However,the reported mechanism of aqueous Zn-ion batteries is still controversial,especially for the manganese-based system.So far,the research on the mechanism is still limited to a certain phase of manganese oxides,and there is rarely analysis on whether the phases with similar crystal structures have similar or identical mechanisms.Therefore,in this thesis,we systematically and comprehensively reveal the relationship between the tunnel structure of cathode and the reaction mechanism in Zn-ion batteries.Besides,we provide a new research direction for optimizing and improving the electrochemical performance of manganese-based oxides.The main research contents of this thesis are as follows:（1）The Mg_xMnO₂ nanowires with 3×3 tunnel structure and the reference sample K_xMnO₂ with 2×2 tunnel structure were prepared by hydrothermal method and simple ion exchange.The above two materials were assembled into zinc-ion batteries,and their electrochemical properties were tested.Compared with K_xMnO₂,the larger tunnel structure of Mg_xMnO₂ makes the surface electrochemical reaction more active and the kinetic process faster.Therefore,after 130 cycles at 100 m A g^-1,the capacity of Mg_xMnO₂ increases to 370 m Ah g^-1.And when the current density increases to 2000m A g^-1,it still has a capacity of 125 m Ah g^-1.Through comparative analysis,it is found that the reaction extent/kinetics can be regulated by phase engineering with the unchanged redox characteristics（2）Based on the difference between the electrochemical behaviors of Mg_xMnO₂and K_xMnO₂,a detailed analysis of the reaction mechanism was completed through a series of in-situ and ex-situ tests.It is found that the reaction mechanism of manganese oxides with different tunnel structures includes surface faradaic dissolution/deposition coupled with intercalation mechanism of cations.The deposition/dissolution reaction on the surface of the electrode material is not completely reversible,and there is an accumulation of layered manganese oxides on the cathode electrode.The large tunnel structure of Mg_xMnO₂,on the one hand effectively alleviates the volume expansion caused by ion intercalation,and on the other hand induces the deposition of layered manganese oxides during the charging process to provide higher capacity.