Regulating The Pseudocapacitive Properties Of Layered Manganese Dioxide By Point Defect

Manganese dioxide（MnO₂）has received wide attention in the field of supercapacitors due to its rich crystal structure,low price,high specific surface area,and large theoretical specific capacitance.However,the experimental specific capacitance of MnO₂ is usually less than 100 F g^-1,which limited by their low intrinsic conductivity.In this paper,two-dimensional layered MnO₂ is used as the research object,and the electronic structure and pseudocapacitance performance are regulated by creating different defects.Furthermore,the mechanism of enhanced pseudocapacitance caused by the evolution of material’s electronic structure is explained via first-principles calculation.The main research contents are as follows:（1）In order to improve the lower conductivity of MnO₂,the transition metal（Fe,Co,Ni）doped layered MnO₂ are constructed,and the influence of heteroatom doping concentration on the pseudocapacitance performance is optimized.Structural characterization shows that the lattice distortion is generated after the introduction of transition metal,and additional active sites are formed.Compared with pure MnO₂,the doped samples possess better electrochemical performance.As results,the optimal Fe-doped sample exhibits a specific capacitance of 157 F g^-1 at 0.5 A g^-1,which is50.4%higher than that of pure MnO₂.Meanwhile,Co-doped MnO₂ shows the excellent cycle stability.The DFT theoretical calculation proves that transition metal can form an intermediate band in the forbidden band of MnO₂ or change the strength of the valence band/conducting band,enhancing the conductivity of MnO₂ and improve the electrochemical performance.（2）As a typical defect,oxygen vacancy is a shallow donor,which can increase the carrier concentration and affect the surface electronic structure of the material,significantly improving the electrochemical performance.In order to further increase the specific capacity of MnO₂,the MnO₂ with rich oxygen vacancies is prepared by simple oxidation/reduction treatment.Structural characterization proves that oxygen vacancies can delocalize adjacent atoms,changing their electronic structure,and then inducing the structural change of MnO₂.Electrochemical tests show that the reduced sample shows 181.4 F g^-1 at 1 A g^-1,and maintain an initial capacitance of 86.4%after5000 cycles.This is because the presence of oxygen vacancies improves the conductivity of MnO₂ and provides a transmission channel for ion diffusion while exposing internal atoms.Therefore,the capacitance performance and stability of MnO₂ have been greatly improved.Interestingly,the oxygen vacancies are also generated after oxidation treatment,which shows a specific capacity of 207 F g^-1 and the capacitance retention of 93.3%.By DFT calculations,oxygen vacancies can increase the density of states of electrons,increasing the carrier concentration,and then improving their capacitance performance.（3）Oxygen vacancies can effectively adjust the pseudocapacitive performance of layered MnO₂,but it can be easily filled with oxygen-containing functional groups in an aerobic atmosphere.By contrast,cation vacanc are more stable due to the higher formation energy.The cation vacancies are formed due to the absence of manganese atoms connecting the octahedral[MnO₆]octahedral center in theδ-MnO₂ structure,and other cations can usually be embedded in the lattice points of MnO₂,thereby changing the physical and chemical properties of the material,such as electronic structure,conductivity,redox properties,etc.A series of alkali metal ions（K,Na,Li）associated manganese（Mn）vacancy MnO₂ are prepared by hydrothermal reaction.Structural characterization shows that the microstructure and morphology of MnO₂are changed significantly after the introduction of Mn vacancies.In particular,by controlling the concentration of the alkali metal-ion,the position of the insertion can be controlled;it is embedded in the cation vacancies at low concentrations,and with the concentration increases,alkali metal-ions will appear in the intermediate layer ofδ-MnO₂.Accordingly,the specific capacitances of optimal K,Na,and Li associated Mn vacancy samples are enhanced about 1.9,1.6 and 1.6 times compared to pure MnO₂.Meanwhile,the rate performance has also been improved about 76%,46%and42%,respectively.In addition,DFT theoretical calculations prove that Mn vacancies will generate additional occupied states,increasing the carrier concentration.