Study On The Performance Of MnO₂ Based Aqueous Zinc Ion Battery Cathode Material

MnO₂-based aqueous Zn-ion batteries（ZIBs）hold great promising for large-scale energy storage applications owing to their safe and sustainable nature.However,rapid capacity decay under high depth of discharge limits the applications of MnO₂ cathodes.In the meantime,the reaction chemistry and degradation process of MnO₂ cathodes cannot be fully understood,leading to improvement of their cycling stability lacks of robust methods.Here,ZIBs performances of MnO₂ polymorphs are investigated to disclose their detailed reaction chemistry and degradation mechanisms.It is found that R-MnO₂ shows low manganese dissolution.Besides,the electrochemical performance of MnO₂ is further optimized by doping strategy.The investigations can be described as follows:Firstly,by synthesizingα-,β-,γ-,δ-,ε-,λ-MnO₂,ZIBs performances of MnO₂polymorphs are investigated to disclose their detailed reaction chemistry and degradation mechanisms.Ex situ characterizations at different cycles exhibit evolution of active materials(original MnO₂→Mn²⁺→birnessite→ZnMn₂O₄/Mn₃O₄)and coexisted reactions from co-insertion,dissolution/deposition and chemical conversion mechanisms.Variational contributions from these intermediate products and different reaction mechanisms cause fluctuated performance during cycling.Initial performance activation is from enhanced activity of birnessite,while the degradation is caused by its conversion to electrochemically inactive ZnMn₂O₄ and Mn₃O₄.Secondly,by optimizing tunnel structures,it is found that R-MnO₂ shows low manganese dissolution with its reaction mainly achieved by intercalation/extraction of Zn²⁺/H⁺,and theoretical calculations verify its low Jahn-Teller distortion at discharged state.This specific property circumvents conversion of R-MnO₂ to metastable birnessite.The conversion of birnessite to electrochemically inert ZnMn₂O₄ and Mn₃O₄ is avoided,giving rise to a stable capacity under high-depth of discharge.Lastly,Ag-doped sea-urchin-like MnO₂ is synthesized by a simple hydrothermal method to improve the electrochemical performance of ZIB.A series of characterizations verify successful doping of Ag into MnO₂.Introducing Ag dopant not only improves the electronic conductivity of MnO₂ but also reduces the strong electrostatic repulsion between Zn²⁺and MnO₂,leading to improved ion insertion and transport kinetics.As a result,the Zn/Ag-MnO₂ battery presents high reversible specific capacity(315 mA h g^-1 at 50 mA g^-1),enhanced cyclic stability（500 cycles with the capacity retention of 94.4%）and superior rate performance.In addition,a flexible quasi-solid-state ZIB is successfully assembled using a Ag-MnO₂@graphite paper cathode,which exhibits a stable specific capacity of 171 mA h g^-1 at 1 A g^-1 over 600 cycles.