Construction And Optimization Of Supercapacitors Based On Water-salt System Electrolytes

Energy shortage and environmental pollution are two serious challenges facing human society.The development of a green and sustainable energy system can not only fundamentally change the status of human reliance on fossil fuels,but also eliminate environmental problems caused by the use of fossil fuels.In many energy storage systems,supercapacitors have high power density,fast charging rate and excellent cycle stability,and have widely used in many fields such as electric vehicles,modern communications,aerospace and defense.The organic electrolytes used in traditional commercial supercapacitors have the insurmountable shortcomings of high cost,toxic,flammable and explosive,which directly affect the cost and safety performance of commercial supercapacitors.Aqueous supercapacitors based on water-salt systems can completely overcome the problems faced by organic supercapacitors due to their high power density,excellent cycle stability,environmental friendliness,low cost and excellent safety,and becoming a research hotspot in the field of electrochemical energy storage.However,the commercial application of aqueous supercapacitors still faces many challenges.First,the conductivity of the aqueous electrolyte decreases or even solidifies at low temperature,which causes the capacity of the supercapacitor to decay or even stop working,which limits its use in low temperature environments.In addition,due to the severe limitation of the?1.23 V thermodynamic stable potential of water,the operating voltage of aqueous supercapacitors are generally narrow,which greatly limits their energy density.In this paper,the all-climate and wide window of aqueous supercapacitors are mainly achieved through the functional regulation of the water-salt system electrolytes.First,using a deep eutectic solvent electrolyte based on hydrated salt with a very low melting point,an aqueous double-layer supercapacitor that can work stably in a wide temperature range of-40°C to 60°C was constructed.Secondly,a low-cost,high-conductivity,wide-window sodium perchlorate“water-in-salt”electrolyte was developed,and a 2.3 V wide-window aqueous double-layer supercapacitor with excellent rate performance and good cycle stability was constructed using this electrolyte.Meanwhile,2D cation-intercalated manganese oxide pseudocapacitive cathode material with wide window was scaled-up synthesized.Based on the above wide window sodium perchlorate“water-in-salt”electrolyte and cation-intercalated manganese oxide pseudocapacitor cathode material,a wide window asymmetric supercapacitor with an operating voltage of 2.4V was constructed.The main work and results are summarized as follows:Modulating the solid-liquid transition temperature of electrolyte for all-climate aqueous electric double-layer capacitor.The H₂O-Mg(Cl O₄)₂·6H₂O binary system has a very low freezing point of-69°C at its eutectic concentration.This eutectic solvent electrolyte has an ionic conductivity of 20.06 m S cm^-1 even at-40°C.Based on this simple,green,unconventional eutectic solvent,an electric double-layer capacitor with an operating voltage window of 1.8 V can be assembled.At a current density of 1 A g^-1,from room temperature to-40°C,its capacity retention rate is as high as 76%.And at-40°C,the current density of 4 A g^-1circulates 6000 cycles with almost no capacity decay and maintains almost 100%Coulombic efficiency.Adjustment of the electrochemical stability window of electrolyte for high voltage electric double-layer capacitor.In the sodium perchlorate“water-in-salt”electrolyte,almost all water molecules coordinate with Na⁺,the strong hydrogen bonds formed by free water molecules are significantly destroyed,and the activity of water is greatly limited.The window has been greatly expanded.Based on this electrolyte,an electric double-layer capacitor with a wide window of 2.3 V,excellent rate performance,and good cycle stability can be realized.Compared with the 2.3 V electric double layer capacitor based on Li TFSI“water-in-salt”electrolyte reported in the literature and the commercial non-aqueous Et₄NBF₄/CAN and Et₄NBF₄/PC electrolytes,the low-cost sodium perchlorate-based“water-in-salt”electrolyte enabled the devices achieve energy densities comparable to commercial electrolytes and exhibit the most excellent rate performance.Preparation of the wide potential window cation-intercalated manganese oxide pseudocapacitor cathode material by molten salt method and its capacitance characteristics.Using molten salt as the reaction medium,the cationic intercalated Na_0.55Mn₂O₄·1.5H₂O(Na Mn O)two-dimensional nanomaterials were prepared quickly and efficiently.The synthesized two-dimensional nanosheets have large lateral dimensions,nano-level thickness and porous structure.The presence of alkali metal ions and crystal water between the layers gives the layered nanosheets a layer spacing of 7(?).This large layer spacing can ensure the effective insertion and extraction of ions during energy storage.Quantitative kinetic analysis shows that in 1M Na₂SO₄,the Na Mn O electrode shows wide potential window and has good capacitance behavior.In electrolyte of 2 M Na OH,the Na Mn O electrode exhibits battery-like behavior.The“water-in-salt”electrolyte and the asymmetric structure cooperate to construct high voltage aqueous supercapacitor.Based on the sodium perchlorate“water-in-salt”electrolyte,the asymmetric supercapacitor was constructed by matching Na Mn O nanosheets with commercial activated carbon.Taking full advantage of the stable electrochemical windows of Na Mn O nanosheets and commercial activated carbon,the high-voltage aqueous asymmetric supercapacitor with an operating voltage of 2.4 V was assembled.The device has excellent rate performance and good cycle stability.The low-cost sodium perchlorate“water-in-salt”electrolyte provides new options for building high voltage asymmetric supercapacitors.