Application Of Water-in-salt And Its Derived Electrolyte In Energy Storage Devices

With the rapid development of portable electronic products and electric vehicles,the demand for high-performance energy storage equipment is increasingly urgent.Existing energy storage systems often have some shortcomings,such as low energy density of supercapacitors,poor reversibility cycles of zinc ion batteries,high cost of flow batteries,and so on.Researchers want to improve the defects of various types of energy storage devices by designing reasonable electrolytes.Among them,high-concentration salts,as electrolytes,show an intrinsically safe and wide voltage window,so they have attracted more and more attention.In this paper,we have designed different types of high-concentration aqueous electrolytes for the energy storage mechanism and choice of electrolytes of different devices,and applied them to high-performance supercapacitors,zinc ion hybrid capacitors,and zinc-bromine batteries.The specific research content is as follows:?i?A low-cost sodium nitrate electrolyte?water-in-salt?was designed.The solvation structure of ions in different concentration solutions was studied by Raman spectroscopy and molecular dynamics simulation.Electrochemical tests show that the constructed symmetrical supercapacitor can work normally within a wide operating voltage range of 0-2.1 V,and shows a high specific capacitance of 32.68 F g^-11 at 1 A g^-1.In addition,it has been shown to have excellent rate performance and excellent cycle stability when compared with previously reported aqueous high-concentration electrolytes.It is proved that the electrolyte has good electrochemical stability,low viscosity,high conductivity and low cost,and is an ideal water-in-salt electrolyte for high-performance and ultra-stable supercapacitors.?ii?A water-in-bisalt mixed electrolyte was designed,namely:1 m Zn?ClO₄?₂-17m NaClO₄.It was confirmed that the zinc-ion hybrid capacitor assembled with the new mixed electrolyte can promote the non-dendritic deposition/stripping process of zinc at a nearly 100%Coulomb efficiency.Using molecular-scale model studies,it is shown that this excellent reversibility results from the unique solvation structure of Zn²⁺in high-concentration aqueous electrolytes.Through a series of electrochemical performance tests,it is calculated that the hybrid device can provide an energy density of 153 Wh kg^-1,the corresponding power density is 342 W kg^-1,and has a capacity retention rate of 95%after 6000 cycles.?iii?Based on the previous experiments,it was fully confirmed that the highly concentrated salt has a significant effect in widening the working voltage and increasing the energy density.Then,the third salt NaBr iscontinuously introduced into the water-in-bisalt system to form the so-called water-in-trisalt,that is:15 m NaClO₄-0.5 m Zn?ClO₄?₂-1 m NaBr.Using a reversible Br^-/Br₂ redox reaction and reversible deposition/stripping of the negative electrode Zn²⁺/Zn,a simple,scalable,low-cost,zinc-bromine battery with no ion exchange membrane was successfully assembled.It is proved that the mixed electrolyte can not only widen the working voltage of the device,but also maximize the degree of electrode redox reaction;in addition,it protects the zinc anode and greatly improves the reversibility of the Zn²⁺deposition/stripping process.