Controllable Synthesis And Investigation Of δ-MnO₂ With Wide Operating Voltage Window For Sodium-ion Storage In Aqueous Electrolyte

Among various energy storage techniques,supercapacitors are playing a critical role because of the fast-rate capability,high power density,and excellent cycling stability.However,the application of supercapacitors is limited by their low energy density.According to the energy density formula（E=1/2 CV²）,the energy density（E）of the supercapacitors can be improved by expanding the operating voltage window（V）and increasing the specific capacitance（C）of active materials.However,the upper cut-off potential of cathode in aqueous electrolyte is always limited by the theoretical potential of water decomposition（<1.23 V vs.RHE）As an efficient strategy,the operating voltage window of the supercapacitor can be expanded by constructing an asymmetric configuration,in which the cathode and anode work in separate potential windows.Accordingly,the potential windows of previously reported asymmetric supercapacitors are limited to 2.0 V.Therefore,it is still a significant challenge to develop novel ASC with an operating voltage window beyond 2 V.Among the cathode materials,Mn O₂has attracted widespread attention due to its large specific capacitance,low cost,and high oxygen evolution potential in aqueous electrolyte（0-1.0 V）.Particularly,δ-Mn O₂with layered structure exhibits a large specific capacitance and a high upper cut-off voltage of 1.2 V（vs.Ag/Ag Cl）,which can be expected as an effective cathode material for ASC by further extending the voltage window.This thesis mainly includes following two parts:Firstly,in this thesis,uniform Mn O₂nanosheets arrays were prepared by hydrothermal method with the reactants of KMn O₄,KF·2H₂O,and H₂SO₄.It was found that the acid concentration in the system and reaction time played a key role on the final products.X-ray diffraction,Raman,and X-ray photoelectron spectroscopy systems were used to characterize the products and revealed effects of the above factors on the crystal structure of manganese oxide,the intercalation content of hydronium ions（H₃O⁺）,and the formation of interlayer Mn O₄tetrahedron.The electrochemical results indicated that theδ-Mn O₂nanosheet arrays prepared at p H 2 for 30 min（p H 2-30 min）exhibited the best electrochemical performance.At a scan rate of 5 m V s^-1,the potential window could be expanded from 0-1.2 V to 0-1.4 V with inhibited oxygen evolution reaction.A maximum specific capacity of 376 F g^-1was achieved at a current density of 1 A g^-1.In addition,high capacity retention of 88%after 2000cycles was demonstrated at a high current density of 5 A g^-1.Secondly,in order to construct an aqueous asymmetric supercapacitor,activated carbon（AC）materials were successfully prepared by chemical activation.As-prepared AC electrode exhibited a specific capacitance of 102 F g^-1at a current density of 1 A g^-1in the potential window of-1-0 V,a high rate capability(88 F g^-1at a current density of 10 A g^-1),and excellent cycling stability(capacity retention of 99.3%after 5000 cycles at 10 A g^-1).Contributed by the wide operating voltage window of cathode and highly stable anode,a 2.4Vδ-Mn O₂//AC aqueous ASC was successfully constructed,exhibiting excellent maximum energy density of 56.8 Wh kg^-1at a power density of 1208 W kg^-1in aqueous electrolyte for sodium storage.In general,this facile crystal structure engineering strategy forδ-Mn O₂was demonstrated as an efficient way to promote its electrochemical performance,which may provide a novel strategy for developing high performance aqueous energy storage devices.