Strategies Of Modulating Electronic Structure Of Manganese Tetroxide For Supercapacitor Applications

Supercapacitors（SCs）have been recognized as high-efficiency and clean energy storage devices.As an ideal SCs electrode material,the practical capacitance of manganese oxide is far below its theoretical capacitance due to its low conductivity,and surface-subsurface energy storage mechanism.Also,manganese oxide has poor stability due to the John-Teller effect accompanied by the dissolution of Mn²⁺and irreversible side reactions.Therefore,it is a vital challenge to activate bulk energy storage,as well as to improve the electrical conductivity and durability of manganese oxide.In this thesis,we modulated the electronic structure of manganese oxide by constructing lattice distortion and introducing space electric field to improve its capacitive performance.The energy storage process and charge/discharge mechanism of manganese oxide electrode were thoroughly investigated by using variety operando electrochemical methods and theoretical calculation.（1）C-axis stretched lattice distorted Mn₃O₄（O_V-Mn₃O₄）was successfully prepared via simple sulfurization/desulfurization process.The prepared Ov-Mn₃O₄exhibits high Na⁺storage capacity and cycling capacity.The mechanism research indicates that Ov-Mn₃O₄exhibits delocalized electron configuration feature and further significantly accelerates electron transfer kinetics.DFT calculation shows that the c-axis stretched lattice distortion effectively broaden the ion transfer channel,significantly reduced ion diffusion energy barrier and accelerated Na⁺transfer kinetics.In addition,c-axis stretched lattice distortion in Ov-Mn₃O₄suppressed the Jahn-Teller distortion and prolong the cycle stability.An asymmetric supercapacitor by using O_V-Mn₃O₄as cathode,can deliver an energy density of 52.5Wh kg^-1at a power density of 1000 Wh kg^-1.（2）MnO/Mn₃O₄with graded junction structure was prepared via chemical precipitation and calcination method.The mechanism research indicates that the graded MnO/Mn₃O₄junction structure could induce a curved energy band,which is beneficial to charge transfer.Meanwhile,the curved energy band also affords an extended space electric field,which boosts the Na⁺transfer kinetics.The results demonstrate that the Na⁺storage capacity of MnO/Mn₃O₄could achieve 331.2 F g^-1at 1 A g^-1and 230 F g^-1at 20 A g^-1.The asymmetric supercapacitor（ASC,MnO/Mn₃O₄//AC）assembled with MnO/Mn₃O₄as cathode material achieves a maximum energy density of 50Wh kg^-1at a power density of 1000 W kg^-1.In summary,we have modulated the electronic structure of manganese oxide electrode materials via constructing lattice distortion and space electric field.The electrochemical properties of the prepared manganese oxides materials were thoroughly investigated and the relationship between the capacitive performance and electronic structure was revealed.This thesis presents useful information for preparing high performance manganese oxide based SCs electrode materials,which can certainly be extended to design and fabricate other metal oxides electrode materials.