Promoting Homogeneous Zinc-Ion Transfer Through Preferential Ion Coordination Effect in Gel Electrolyte for Stable Zinc Metal Batteries

Abstract

Aqueous zinc metal batteries (AZMBs) are emerging energy storage systems that are poised to replace conventional lithium-ion batteries owing to their intrinsic safety, facile manufacturing process, economic benefits, and superior ionic conductivity. However, the issues of inferior anode reversibility and dendritic plating during operation remain challenging for the practical use of AZMBs. Herein, a gel electrolyte based on zwitterionic poly(sulfobetaine methacrylate) (poly(SBMA)) dissolved with different concentrations of ZnSO₄ is proposed. Two-dimensional correlation spectroscopy based on Raman analysis reveals an enhanced interaction priority between the polar groups in SBMA and the dissolved ions as electrolyte concentration increases, which establishes a robust interaction and renders homogeneous ion distribution. Attributable to the modified coordination, zwitterionic gel polymer electrolyte with 5 mol kg^-1 of ZnSO₄ (ZGPE-5) facilitates stable zinc deposition and improves anode reversibility. By taking advantage of preferential coordination, a symmetrical cell evaluation employing ZGPE-5 demonstrates a cycle life over 3600 h, where ZGPE-5 also exerts a beneficial effect on the full cell cycling when assembled with Zn_0.25 V₂ O₅ cathode. This study elucidates changes in the internal ion behavior that are dependent on electrolyte concentrations and pave the way for durable AZMBs.

Keywords: aqueous rechargeable batteries; ion coordination; preferential coordination; zinc metal anodes; zwitterionic gel electrolytes.

Figure 1

Working mechanism and characterization of the ZGPE. a) Schematic illustration of the zwitterionic gel electrolyte for Zn metal batteries. b) Synthesis procedure of the ZGPE from the zwitterionic repeating unit and polymeric materials, undergoing polymerization of pre‐gel solution and immersion in ZnSO₄ aqueous solution sequentially. c) FT‐IR spectra of the fabricated zwitterionic hydrogel before electrolyte soaking. d) Electrolyte retention rate of ZGPE‐5 over time at room temperature under a relative humidity of 60%.

Figure 2

Effect of the gel concentration on electrochemical behavior. a) Chronoamperometry results of cells employing ZGPE‐2 and ZGPE‐5. b) Schematic description for atomic diffusion behavior on Zn anode surface indicating vertical growth and uniform deposition of Zn, respectively. Top‐view SEM images of deposited Zn electrodes using 2 m ZnSO₄ liquid electrolyte under the areal capacity of c) 1, d) 3, and e) 5 mAh cm⁻². Top‐view SEM images of deposited Zn electrodes using ZGPE‐2 under the areal capacity of f) 1, g) 3, and h) 5 mAh cm⁻². Top‐view SEM images of deposited Zn electrodes using ZGPE‐5 under the areal capacity of i) 1, j) 3, and k) 5 mAh cm⁻².

Figure 3

Investigation on the working mechanism of ZGPE‐5. a) 1D Raman analysis results of ZGPE with increasing salt concentration from 0 to 5 m, featuring positions between 1100 and 3100 cm⁻¹. b,d) Synchronous and c,e) asynchronous 2D Raman correlation spectra of ZGPE during increasing salt concentration 0–5 m. f) The sequential order of spectral changes of ZGPE as revealed by Raman analysis with increasing salt concentration from 0 to 5 m.

Figure 4

ZGPE‐based Zn symmetric cell investigation. a) Long‐term symmetric cell cycling performance for cells using ZGPE‐2 and ZGPE‐5 under an areal capacity of 0.5 mAh cm⁻² and a current density of 0.5 mA cm⁻². Top‐view SEM images of Zn anode after 100 repetitive plating/stripping processes with b) ZGPE‐2 and c) ZGPE‐5. Nyquist plots of cells employing d) ZGPE‐2 and e) ZGPE‐5 at the 10th and 100th cycles, respectively, where the ZGPE‐5 further shows the impedance analysis result of the 500th cycle. Inset figures illustrate the equivalent circuit model of the proposed system. Symmetrical cell cycling results for ZGPE‐2 and ZGPE‐5 based cells f) under an areal capacity of 3 mAh cm⁻² and a current density of 1 mA cm⁻², and g) under an areal capacity of 2 mAh cm⁻² and a current density of 2 mA cm⁻².

Figure 5

Electrochemical characterization of Zn|ZVO cells. A) XRD pattern of Zn_0.25V₂O₅ cathode active material. B) Full cell cycling performance of ZGPE‐2 and ZGP‐5 based cells at a current density of 0.5 mA g⁻¹. Voltage profiles of c) ZGPE‐2 and d) ZGPE‐5 for the 1st, 10th, 100th, and 200th cycle, respectively. e) EIS analysis results of Zn|ZVO cells with ZGPE‐2 and ZGPE‐5 as an electrolyte after 200 repetitive cycles. f) Change in the normalized capacity (C/C ₀, where C ₀ indicates an initial capacity) of assembled pouch cells under different mechanical deformations (flat, 90° bent, and 180° folded). g) Digital microscope images of cells connected in series to light an LED bulb operated in different mechanical states.