Solvation Structure Design And Performance Of Aqueous Potassium-ion Electrolytes

With the rapid development of electric vehicles,electronic products,and grid energy storage,the requirements for electrochemical energy storage devices with high safety and high energy density are soaring.Organic electrolytes have many disadvantages such as combustible,non-aqueous environments and high cost.Therefore,it is necessary to develop a new electrolyte with low cost,high ionic conductivity and safe.High-concentration water-in-salt(WIS)electrolytes are nonflammable,low cost and low toxicity,having a wide electrochemical stability window(ESW).WIS electrolytes are intrinsically safe.Besides,the wide ESW of WIS moves toward the high energy density devices.However,most of the reported aqueous electrolytes are based on lithium ion,which are inherited from the organic lithium-ion systems.In the organic system,the ionic radius of lithium ion(0.094 nm)and the solvation radius of lithium ion(0.382 nm)are the smallest among alkali metals.Moreover,the charge-to-mass ratio of lithium ion is greater than that of sodium and potassium,then Li+based electrochemical energy storage devices can realize high energy density and high-power density.However,the Stokes radius of potassium ion(0.331 nm)is smaller than that of lithium ion in the aqueous electrolyte,and the element abundance of potassium ion in the crust(1.5 wt%)is much greater than that of lithium ion(0.0017 wt%),which suggests that potassium ion electrochemical energy storage devices is low cost.Therefore,the energy storage devices based on the potassium ion WIS electrolyte have great potential in large-scale power grid energy storage.However,the outstanding performance of WIS electrolyte depends on the high concentration of salt,which leads to concerns about cost and sustainability.To solve this issue,this thesis intends to build a high-performance potassium ion aqueous electrolyte through optimizing the microstructure of the aqueous electrolyte with additives.The details are as follows:1)To address the problem of high cost and high viscosity of WIS electrolyte,protonation additives are used to promote the formation of regular solvated shell structure of potassium ions in WIS electrolyte.In this chapter,we show a new type of hybrid electrolyte containing additives.Because of its low viscosity,high ESW,extremely low freezing point and excellent flame retardancy,trimethyl phosphate(TMP)is widely used as a flame-retardant additive in organic electrolyte.Here,we introduce TMP as a protonation additive,revealing the interaction between TMP and water molecules,which breaks the hydrogen bond network between water molecules,makes the WIS electrolyte forming a regular solvent shell structure at the micro level,and expands the ESW to 3.4 V at a low concentration of 1.6 M KOTF.Through the characterization of Raman spectrum,FTIR spectrum,NMR hydrogen spectrum and MD simulation,the isolation phenomenon caused by the TMP molecules and the strong coordination between potassium ions and water molecules were revealed.Combining this electrolyte with commercial activated carbon material YP-80F,the supercapacitors have achieved a working window of 2.4 V,excellent rate capability,good cycle stability and a wide operating temperature range(-20 to 60℃).In addition,we assembled an aqueous potassium ion battery based on hybrid electrolyte.By combining Prussian blue analog positive electrode and organic negative electrode,this low-cost and environment-friendly aqueous potassium ion full battery can work stably at a current density of 0.2-4.0 C,and provide 66.5 Wh kg-1 energy density at a current density of 0.2 C.The capacity retention rate reaches 84.5%after 600 cycles at a current density of 0.8 C.2)To solve the problem of narrow working voltage window of aqueous electrolyte,an electrolyte modified with ionic liquid with the functionalities of enhancing the interface was designed.An aqueous potassium ion electrolyte with excellent kinetics,operating temperature range and cycle stability is prepared by adding the ionic liquid EMIM NTf2 into 18 M KOTF electrolyte.Combining this electrolyte with commercially available activated carbon material YP-80F,the aqueous supercapacitor has been designed to achieve a working window of up to 2.4 V,excellent magnification performance,good cycle stability,and a wide operating temperature range(-10 to 140℃).In addition,we further assembled an aqueous hybrid ion supercapacitor by using a CuHCF as the positive electrode and an YP-80F as the negative electrode in this hybrid the electrolyte through combining.This aqueous hybrid ion capacitor with the advantages in low-cost and environmentally friendly,can operate stably at a current density of 0.05-5 A g-1,and provide a specific capacity of 20.4 mAh g-1 at a current density of 1 A g-1.The capacity retention rate reaches 92.7%after 1000 cycles at a current density of 1 A g-1.