A New Zinc Salt Chemistry for Aqueous Zinc-Metal Batteries
Figure 1. Structure and properties of the designed salt Zn(BBI)2 and its aqueous solution. a) Key factors for zinc salt design. b) Atomic structure of BBI− anion group. C atoms in black, H atoms in light-grey, O atoms in red, N atoms in blue, and S atoms in yellow. c) XANES of 1M ZnSO4, 1 M Zn(TFSI)2, and 1 M Zn(BBI)2. d) 1H NMR of 0.1 M, 0.5 M, and 1 M Zn(BBI)2. Inset of (d): Atomic position of (A)-H, (B)-H, and (C)-H. Comparisons of e) pH, f) chemical shift δ of 1H NMR, g) Raman spectra, h) water activity (h) at 25 °C, and i) LSV curves of 1 M Zn(BBI)2, 1M Zn(TFSI)2, 1M Zn(OTf)2, and 1 M ZnSO4.
Aqueous zinc-ion batteries (ZIBs) are attractive energy storage options because of their low cost and high level of safety, but they are subject to electrochemical and chemical deterioration that is strongly tied to the electrolyte. New zinc salt design was provided and a drop-in replacement for aqueous ZIBs with long cycle lives. The salt Zn(BBI)2 has a unique amphiphilic molecular structure that optimally combines the advantages of hydrophilic and hydrophobic groups to form an anion group N-(benzenesulfonyl) benzenesulfonamide(BBI).
Fig. 1c inset demonstrates how 1 M Zn(BBI)2 XANES pre-edge moves to a higher energy than that of 1M ZnSO4 and 1M Zn(TFSI)2, showing that the former has a greater effective valence of Zn2+ (which can be quantitatively compared by the energy at 0.5 normalized intensity). It implies that whereas ZnSO4 and Zn(TFSI)2 are both strongly solvated and their Zn2+ solvation sheaths in 1M aqueous solutions are primarily made of neutral H2O molecules, Zn(BBI)2 is poorly solvated and has more anion-derived solvation sheath.
In conclusion, a novel amphiphilic zinc salt with a novel molecular structure for aqueous ZIBs was developed. Zn(BBI)2 has a poor solvability due to the correct pairing of the hydrophilic, hydrogen-bonding active -SO2-N-SO2- group and hydrophobic, weakly electron-withdrawing -Ph group, and its 1M solution significantly reduces water activity and stabilizes the Zn metal/H2O interface. The electrochemical performance of Zn||Cu cells, Zn||Zn symmetric cells, and Zn||PANI full cells is significantly improved by our new low-salt-concentration electrolyte 1M Zn(BBI)2, which is superior to reference cells cycled with commercial 1 M ZnSO4 electrolyte and other electrolytes using organic salts (such as 1 M Zn(OTf)2 and 1 M Zn(TFSI)2).
L.Q. acknowledges the support by the National Key Research and Development Program (2021YFB2400300), the National Natural Science Foundation of China (52027816), and the Fundamental Research Funds for the Central Universities (2021GCRC001). W.H.K. acknowledges the support by the National Natural Science Foundation of China (U1930102). J.L. acknowledges the support by Samsung Advanced Institute of Technology. The authors thank Scientific Gear Service and Synchro-Ultra for arranging the XANES measurement in Taiwan Light Source (TLS) of the National Synchrotron Radiation Research Center (NSRRC).