Water-in-Salt Electrolyte-Based Extended Voltage Range, Safe, and Long-Cycle-Life Aqueous Calcium-Ion Cells

Abstract

The narrow electrochemical stability window (1.23 V) of an aqueous electrolyte hinders the practical realization of calcium-ion chemistries of high-energy-density and long-cycle-life batteries. Furthermore, developing an aqueous electrolyte that is low cost, is environmentally friendly, and has a wide voltage window is essential to designing safe, high-energy-density, and sustainable calcium-ion batteries. A calcium-based water-insalt (WISE) aqueous electrolyte surpasses the narrow stability window by offering a 2.12 V wide window by suppressing the hydrogen evolution at the anode and minimizing the overall water activity at the cathode. A comprehensive theoretical study predicts the preferential reduction of salt aggregates over water to form a passivation layer at the electrode−electrolyte interface and enhance the electrolyte stability window. Additionally, Raman spectroscopy reveals that the calcium ion coordination number, which is the number of nitrate ions surrounding the calcium ions in the aqueous electrolyte, gradually increases with an increase in the electrolyte concentration, leading to a gradual decrease in the hydration number of the calcium ions. A full cell in WISE was demonstrated to exhibit an excellent rate capability and cycling stability with negligible capacity loss (0.01 per cycle), maintaining 80% capacity retention over 1800 cycles with ∼99.99% Coulombic efficiency. The full cell provides an energy density of 232 Wh kg −1 at a power density of 69 W kg −1 and a current rate of 0.15 A g −1 . Even at a higher current rate of 5 A g −1 , the battery delivers an energy density of 182 Wh kg −1 (based on the active mass of the anode). This is one of the best performances to date of all previously reported full-cell aqueous calcium-ion batteries. A fundamental understanding of the storage mechanism and a electrode degradation study was achieved. This work suggests and expands new avenues for the practical realization of low-cost, safe, eco-friendly, and high-performance aqueous calcium-ion batteries for future large storage applications.

Reverse Dual‐Ion Battery Enabled by Reversing the Cation/Anion Storage Mechanism in an Aqueous ZnCl2 Water‐in‐Salt Electrolyte

“…Consequently, the ionic conductance and diffusivity of ions may decrease, leading to an increased potential gap for cation/anion (de)insertion. [46][47][48] Furthermore, the theory of closed-ion pairs formation at higher concentration also reported by Shoolery et. al., [49] where the 1 H-NMR peak shifts linearly with increasing concentration, suggesting alterations in ion solvation structure and inter-ionic forces.…”

“…According to the Debye-Huckel theory of strong electrolytes, the salt is ionized but not completely dissociated, owing to the appreciable inter-ionic attraction forces. [46][47][48] This results in the formation of "ionic doublets", where both the cation and anion remain in closed ion pairs (Figure 5c). Consequently, the ionic conductance and diffusivity of ions may decrease, leading to an increased potential gap for cation/anion (de)insertion.…”

“…In contrast, at the highest electrolyte concentration (10 m), the hydration shell size becomes highly compact due to the significantly reduced number of water molecules present (Figure 5c). According to the Debye‐Huckel theory of strong electrolytes, the salt is ionized but not completely dissociated, owing to the appreciable inter‐ionic attraction forces [46–48] . This results in the formation of “ionic doublets”, where both the cation and anion remain in closed ion pairs (Figure 5c).…”

Aromatic Organic Small-Molecule Material with (020) Crystal Plane Activation for Wide-Temperature and 68000 Cycle Aqueous Calcium-Ion Batteries

“…(f) Cycling performances at 1000 mA g –1 . (g) Comparison of the Ca 2+ storage performance of PTCDI with those of the reported electrodes for ACIBs. ,,,− …”

K3v2(Po4)3/C as a New High‐Voltage Cathode Material for Calcium‐Ion Batteries with a Water‐In‐Salt Electrolyte

“…Different from organic electrolytes, aqueous Ca electrolytes have high ionic conductivity, and the shielding of Ca 2+ by the polar H 2 O molecules facilitates fast diffusion of Ca 2+ . [20,21] In addition, aqueous batteries are safer, lower cost, and more sustainable. [22,23] Therefore, aqueous calcium-ion batteries (ACIBs) have the potential to be a low-cost, high security, and high-performance electrochemical energy storage device.…”