Study On The Structure Design And Electrochemical Performance Of The δ-MnO₂ Cathode For Aqueous Zinc-ion Batteries

Aqueous zinc ion batteries（ZIBs）,which possess the dominant position of low cost,high theoretical capacity,and high security,are well known as one of the most promising next-generation batteries that could replace lithium ion batteries involved in the application of electric vehicles and large-scale energy storage grid.Cathode materials as the core parts in aqueous ZIBs have always played an important role in its performance impact.δ-MnO₂ cathode materials could be one of the most ideal cathode material candidate for aqueous ZIBs owing to inherent characteristic thatδ-MnO₂ has larger interlayer between its layer structure,which simultaneously suffer from the issues over slow kinetics and Mn electrode dissolution.The study of this article would carry out around theδ-MnO₂ cathode materials to alleviate or even solve the issues suffered by this cathode,and we would optimize the structure of this cathode to improve its electrochemical performance by applying two completely different strategies of order/disorder engineering and pre-doping,respectively.（1）A novel MnO₂material,possessing the coexistence structure of crystalline and amorphous,was prepared by a facile water bath route during order/disorder engineering.This crystal/amorphous MnO₂（disordered MnO₂）have open space with more active sites.Simultaneously,the coexistence of crystalline and amorphous structures results in irregular Mn-O Bonding that could obtain more stable structure.Thus,disordered MnO₂ cathode shows an ultrahigh discharge specific capacity of 636 mAhg^-1 at 0.1Ag^-1 and improvement rate performance during charge/discharge cycle.And this electrode remains a large discharge capacity of 216 mAhg^-1 even at a high current density of 1 Ag^-1 after 400 cycles,showing better cycle stability.Furthermore,the different charge and discharge stages of this cathode are also monitored by ex-situ characterization technology,ultimately revealing that its energy storage mechanism is the reaction process of H⁺and Zn²⁺co-insertion.The superior electrochemical performance of disordered MnO₂ proves the effectiveness of ordered/disordered engineering,and providing a new way to improve material performance in the field of aqueous ZIBs.（2）Nickel dopedδ-MnO₂（Ni-δ-MnO₂）was synthesized by simple one-step hydrothermal method.Ni-δ-MnO₂ is constructed by introducing nickel ions as pillars into its interlayer without destroying the layered structure ofδ-MnO₂,which could make the overall layered structure become stronger.At the same time,the presence of nickel ions also increase active sites and conductivity for the whole frame,which can improve the reaction kinetics of zinc ions during charge and discharge.When Ni-δ-MnO2 is used as the cathode for charging and discharging cycles,the reversible specific capacity of this cathode is as high as 571 mAhg^-1 at a current density of 0.1 Ag^-1,and it shows a higher specific capacity of 371 mAhg^-1 at a current density of 0.5 Ag^-1.In addition,the capacity retention rate can reach 98%after 200 charge-discharge cycles at a current density of 1 Ag^-1,showing a relatively improved cycle stability.Therefore,the effectiveness of nickel doping strategy for improving the electrochemical performance ofδ-MnO₂ cathode fully proves that the strategy has reference and guiding significance.