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 Zn0.25V2O5 cathode. This study elucidates changes in the internal ion behavior that are dependent on electrolyte concentrations and pave the way for durable AZMBs.

The transference number of Zn 2+ (tZn2+) was calculated using the following equation: where ΔV is the applied voltage polarization, I0 and Is are the initial state current and steadystate current, respectively, and R0 and Rs are the initial state resistance and steady-state resistance, respectively.

Figure S1 .
Figure S1.Digital microscope image displaying the fabricated ZGPE after being cut into a disk with a diameter of 19 mm.

Figure S2 .
Figure S2.FTIR spectra of a) PVA polymer and b) SBMA repeating unit.Inset images show representative molecular structure of PVA and SBMA.

Figure S3 .
Figure S3.Mechanical stability investigation of ZGPE.Digital microscope captures depict a) ZGPE prior to the imposition of a 1 kg load, b) ZGPE subjected to compressive stress for 10 minutes, and c) ZGPE after load removal.d) A magnified image of ZGPE following the durability, revealing an absence of notable fractures.

Figure S5 .
Figure S5.Electrolyte retention rate of the pristine hydrogel, ZGPE-2, and ZGPE-5 over time at room temperature under a relative humidity (RH) of 40 and 80%, respectively.

Figure S7 .
Figure S7.Electrochemical impedance spectroscopy investigation of ZGPE-2 and ZGPE-5 at room temperature.The ionic conductivity (σ), was calculated using the following equation: =  R b , where Rb is the bulk resistance, and l and A are the thickness and area of the gel electrolyte, respectively.

Figure S8 .
Figure S8.Time-current density plot of a cell employing 2 M ZnSO4 liquid electrolyte for 400 seconds.

Figure S9 .
Figure S9.Top-view SEM image of polished Zn foil.

Figure S13 .
Figure S13.FTIR spectra of ZnSO4 aqueous solution and ZGPE with various salt concentration condition.Peaks at 1030 cm -1 and 1170 cm -1 represent characteristic peaks of S=O vibrations coming from sulfonate functional groups in SBMA, while peak at 1085 cm -1 is attributed to the sulfate contained in ZnSO4.For the peak at 1085 cm -1 , red shift occurs when the electrolyte concentration increases from 2 m to 5 m due to the intensified interaction between S=O and Zn 2+ ions.Additionally, same peak at ZGPE undergoes blue shift compared with liquid electrolyte as Zn 2+ ions favorably interact with the gel matrix, enhancing the bond strength of S=O.Meanwhile, the intensity of peak at 1170 cm -1 decreases with increment of concentration because robust interaction between ZGPE structure and internal Zn 2+ ions hinders asymmetrical vibration of S=O in zwitterionic polymer chains.

Figure
Figure S14.a, c) Synchronous and b, d) asynchronous two-dimensional Raman correlation spectra (2DCOS) of ZGPE during increasing salt concentration 0 m to 2 m.

Figure S15 .
Figure S15.The sequential order of spectral changes of ZGPE as revealed by Raman analysis with increasing salt concentration from 0 m to 2 m.

Figure S16 .
Figure S16.Current-time plots of cells employing a) ZGPE-2 and b) ZGPE-5 as an electrolyte.The inset figure shows EIS results of respective cells before and after polarization.The applied voltage polarization is 10 mV at room temperature.

Figure S17 .
Figure S17.Symmetric cell cycling result using 2 M ZnSO4 liquid electrolyte at an areal capacity of 0.5 mAh•cm -2 under a current density of 0.5 mA•cm -2 .

Figure S18 .
Figure S18.The voltage-areal capacity curves during initial Zn electrodeposition process at 0.5 mA cm -2 for Zn|Zn symmetric cells employing ZGPE-2 and ZGPE-5, respectively.The inset demonstrates an enlarged view of the area marked by the red dashed line in voltage-areal capacity curves.

Figure S19 .
Figure S19.Symmetric cell cycling results using a) 2 m ZnSO4 and b) 5 m ZnSO4 liquid electrolyte at an areal capacity of 0.5 mAh•cm -2 under a current density of 0.5 mA•cm -2 .

Figure S20 .
Figure S20.Symmetric cell cycling result using 2 M ZnSO4 liquid electrolyte at an areal capacity of 3 mAh•cm -2 under a current density of 1 mA•cm -2 .

Figure S21 .
Figure S21.Symmetric cell cycling result using 2 M ZnSO4 liquid electrolyte at an areal capacity of 2 mAh•cm -2 under a current density of 2 mA•cm -2 .

Figure S22 .
Figure S22.Transmission electron microscope images of Zn0.25V2O5 cathode active material for a) overall morphology and b) detailed image demonstrating highly ordered crystal structure.

Figure S23 .
Figure S23.Nyquist plots associated with Zn|ZVO full cells utilizing ZGPE-2 and ZGPE-5 as respective electrolytes, assessed a) prior to cell operation and b) after 10 th cycle.

Figure S25 .
Figure S25.Digital microscope images of ZGPE-5 during flammability test, with the gel electrolyte directly subjected to flame via a combustion tool.(left) Onset of the test and (right) following 10 seconds of exposure to an ignition condition.

Table S1 .
Summary of the Raman investigation for respective peak position and corresponding synchronous/asynchronous results with increasing salt concentration from 0 m to 2 m.

Table S2 .
Summary of the Raman investigation for respective peak position and corresponding synchronous/asynchronous results with increasing salt concentration from 0 m to 5 m.