Design Of Manganese Oxide Composite Electrode/electrolyte And Its Performance Research For Aqueous Supercapacitors

Aqueous supercapacitors（SC）are gaining more and more attention as an alternative or supplement to secondary batteries in high power transmission or fast energy harvesting scenarios due to their affordability,enhanced safety,long cycle life and fast charge/discharge rate.However,due to the low conductivity and sharp decrease in performance with increasing electrode thickness of manganese oxides,coupled with a narrow electrochemical window of aqueous electrolyte,its energy density is far from that of secondary batteries,so how to improve energy density of SC is the focus of the work.In this thesis,an attempt has been made to prepare high performance aqueous supercapacitors through the design of manganese oxide electrode materials and the optimization of electrolyte,and the main studies include:（1）Nanostructured birnessite（δ-MnO₂）has a high theoretical specific capacitance and virtually perfect capacitive behaviors as an essential electrode material for high-power energy storage devices.However,achieving its excellent capacitive properties is particularly challenging.Herein,Ti₃C₂T_z/CNT@MnO₂ pseudocapacitive electrode is fabricated by electrostatically assembling Ti₃C₂T_z nanosheets and CNT@MnO₂.Profited from the unique structure,strong interfacial interactions and synergistic effects between Ti₃C₂T_z nanosheets and CNT@MnO₂,TCM electrode shows a high capacitance of 384 F g^-1,excellent rate capability,and stability（92.2%retention after10000 cycles）.The outstanding capacitive charge storage of TCM originates from reversible Na⁺intercalation/deintercalation,according to electrochemical quartz crystal microbalance（EQCM）and in-situ Raman spectroscopy.Remarkably,an assembled asymmetric supercapacitor（ASC）based on TCM film and nitrogen-doped reduced graphene oxide（NRGO）exhibits an energy density of 44 Wh kg^-1,a power density of43.4 k W kg^-1.Furthermore,the ASC exhibits a high voltage of 2.4 V with an energy density of 58 Wh kg^-1 in 10 mol kg^-1（m）Na Cl O₄ water-in-salt（WIS）electrolyte.（2）WIS electrolyte have great potential for application in wide voltage EDLC.However,the expanded electrochemical stability window（ESW）of such WIS electrolytes sacrifices the advantages of low cost,high ionic conductivity,low density,and operating temperature range of aqueous electrolytes.In this study,the ESW of water is broadened to more than 3.3 V by using a nonprotonic solvent（γ-valerolactone,GVL）to modulate the solventized structure of the electrolyte at a low salt concentration of 3 m Na Cl O₄.This electrolyte is featured with high ionic conductivity(18.9 m S cm^-1),low density(1.29 g cm^-3),and wide liquid range due to its low concentration.The EDLC with the hybrid electrolyte can work in a wide temperature range of-30-70℃with excellent multiplicative performance and cycling stability at an operating voltage of 0-2.4 V broadening.Theoretical calculations indicate that GVL has a better regulation of the electronic structure of the electrolyte,thus breaking through the decomposition potential of water to broaden the ESW.