High‐Voltage Aqueous Na‐Ion Battery Enabled by Inert‐Cation‐Assisted Water‐in‐Salt Electrolyte

Abstract

Water‐in‐salt (WiS) electrolytes provide a new pathway to widen the electrochemical window of aqueous electrolytes. However, their formulation strongly depends on the solubility of the chosen salts, imposing a stringent restriction on the number of possible WiS systems. This issue becomes more severe for aqueous Na‐ion batteries (ANIBs) owing to the relatively lower solubility of sodium salts compared to its alkaline cousins (Li, K, and Cs). A new class of the inert‐cation‐assisted WiS (IC‐WiS) electrolytes containing the tetraethylammonium (TEA+) inert cation is reported. The Na IC‐WiS electrolyte at a superhigh concentration of 31 mol kg–1 exhibits a wide electrochemical window of 3.3 V, suppresses transition metal dissolution from the cathode, and ensures singular intercalation of Na into both cathode and anode electrodes during cycling, which is often problematic in mixed alkali cation systems such as K–Na and Li–Na. Owing to these unique advantages of the IC‐WiS electrolyte, the NaTiOPO4 anode and Prussian blue analog Na1.88Mn[Fe(CN)6]0.97·1.35H2O cathode can be coupled to construct a full ANIB, delivering an average voltage of 1.74 V and a high energy density of 71 Wh kg−1 with a capacity retention of 90% after 200 cycles at 0.25C and of 76% over 800 cycles at 1C.

Interface Concentrated‐Confinement Suppressing Cathode Dissolution in Water‐in‐Salt Electrolyte

“…[ 20,22 ] Also, theoretically, a large radius TEA cation is difficult to form a hydrated ion with water due to its weak charge localization. [ 23 ] On the contrary, the sodium hydration of Na(H 2 O) 6 can be easily realized. In the 9 m WIS system, the concentration of Na cation is relatively high to 9 m, which can bound almost all of the water to form Na(H 2 O) 6 , thereby reducing water activity.…”

Modulating electrolyte structure for ultralow temperature aqueous zinc batteries

“…More importantly, the Raman peak value of Zn(H 2 O) 2 Cl 4 2and ZnCl + are exhibited in Fig. 2h and the blueshift of peaks unveil the enhanced ion interactions with increased C ZnCl 2 38 , which is also demonstrated by the total electrostatic interaction potential energy among cations, Original water network linked by H-bonds can easily transform to ice network at 0°C. After adding ZnCl 2 , the H-bond network is broken by the strong interaction between ions and water, while the ion interactions are enhanced.…”

“…The physical characteristics of the solution are also plagued by such concentrated content. First, the strengthened interactions between cations and anions bring about high viscosity, leading to a detrimental process of mass transfer and high T t 36,38,39 . Meanwhile, the solubility of metal-salts decreases greatly with temperature reducing, and the tendency of expelling salt severely limits the liquid temperature range.…”

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…Many strategies implemented for the energy storage systems (e.g., aqueous metal-ion batteries, [344][345][346] chloride ion batteries, [238] supercapacitors [347] ) to improve the stability of Faradaic electrodes will be instructive for CDI electrodes. For example, structural design of electrode material by particle nanostructuring, morphology optimization, or constructing composites with other materials can help accommodate the large volume changes during the ion capture/release processes; and strategies like surface protection using coatings or adding surface stabilizers [341,348,349] can help alleviate the issue of electrode dissolution, thus improving the cycle life. In addition, some advanced material engineering and functionalization approaches like changing the crystalline phase, 278 defect engineering, 279 and element doping 280 have been demonstrated to be effective to improve electrochemical performance, which should also be tried for the design and optimization of desalination electrode materials to improve long-term desalination/salination stability.…”