Intrinsic Reaction Mechanisms Of Different Manganese Oxides (MnO_x) Used In Cathode Materials For Aqueous Zinc Ions Batteries

The emergence of aqueous zinc ion battery fundamentally solves the safety problems of the conventional lithium ion battery,which is flammable,explosive and volatile.It becomes very necessary to seek cathode materials with ideal energy density and working voltage,and manganese based materials can meet the above requirements to a certain extent.Therefore,on one hand,it is urgent to seek more potential manganese-based cathode materials,and on the other hand,clarifying the internal electrochemical reaction mechanism is also extremely important for future research.In this paper,representative aqueous ZIBs manganese oxide anode materials were selected,and novel structures with different characteristics were designed and constructed by wet etching method,controllable temperature method,one-step hydrothermal method and chemical vapor deposition method.Their zinc storage properties and the advantages brought by their structures are revealed and their intrinsic electrochemical energy storage mechanisms also are proved through a series of in-situ and ex-situ tests.The details are as follows:（1）The hollow hierarchical structure MnO₂ was successfully prepared by a facile wet chemical oxidation and etching of pre-prepared Mn CO₃ cubes.The as-prepared MnO₂ is hollow and nanosheets uniformly distributed on the surface.Benefiting from this novel structure,the MnO₂ electrode supplies superior charge-discharge performance when used as a cathode material for aqueous ZIBs(210 m Ah g^-1 after 400 cycles at 500 m A g^-1,110 m Ah g^-1at 2500 m A g^-1).Furthermore,a co-intercalation mechanism of H⁺and Zn²⁺is justified,which is accompanied by the phase transition of the structure and the generation of by-products.（2）The Mn₂O₃ material with a hollow multi-shell structure was successfully prepared by calcining precursor synthesized by hydrothermal route.The obtained Mn₂O₃ material has a shell structure of at least 4 layers,and when the electrode is used as an aqueous ZIBs cathode material,it can exhibit a reversible capacity of 230 m Ah g^-1 at 500 m A g^-1,and 201 m Ah g^-1even after 400 cycles.It shows a splendid specific capacity of 103 m Ah g^-1 even the current density of 2500 m A g^-1.Its intrinsic electrochemical reaction mechanism and kinetic properties were proved to be similar to MnO₂.（3）The nano-octahedral Mn₃O₄ material was successfully prepared by a facile one-step hydrothermal method.The advantage brought by the nanoscale of the material enables the Zn/Mn₃O₄ battery system to have a high capacity(200 m Ah g^-1 at 500 m A g^-1).In addition,through a series of characterizations,it is also proved that H⁺and Zn²⁺participate in the behavior of ion intercalation/deintercalation.Unlike the above two,the host material does not undergo an irreversible phase transition during the initial activation process.（4）The CVD surface modification technology was explored and researched.Using absolute ethanol as the carbon source and Mn CO₃ as the precursor,the phase transformation of Mn CO₃to MnO and the uniform carbon deposition on the surface of the MnO unit were realized.Benefiting from the carbon modification strategy,the obtained MnO@C composite still exhibits excellent reversible capacity and great cycling stability(347 m Ah g^-1 after 400 cycles at 500 m A g^-1).Even at 1000 m A g^-1,it still exhibits a respectable specific capacity of 194m Ah g^-1 after 200 cycles.In addition,the superior energy density of 351 Wh kg^-1 at 625 W kg^-1,and the greater rate capability(149 m Ah g^-1 at 2500 m A g^-1 current density),both indicate that the feasibility and advantages of MnO@C cathode as energy storage in aqueous ZIBs.At the same time,the energy storage mechanism of H⁺and Zn²⁺co-intercalation of MnO and Zn Mn₂O₄ on the electrode was elucidated.Finally,by sorting out and in-depth analysis of the intrinsic reaction mechanism of these representative materials of manganese oxides with different atomic ratios in the electrochemical reaction process,a general mechanism strategy suitable for manganese-based material cathodes in aqueous ZIBs system is proposed,namely H⁺and Zn²⁺co-intercalation/de-intercalation mechanism.The hollow hierarchical structure,multi-shell structure and CVD surface modification technology designed in this paper can be applied to the preparation of the other new manganese-based materials.At the same time,the electrochemical mechanism of universal manganese oxide used in aqueous ZIBs cathodes is proposed for future researchers.Our further exploration provides a powerful reference.