Bound Water-Mediated Fast Ion Transport in Viscoelastic Solid-State Electrolyte Boosting Performance of Solid-State Zinc-Ion Batteries

Efficient ion transport represents a key challenge in the development of solid-state zinc-ion electrolytes. While the introduction of bound water is known to enhance ionic conductivity, the regulation of its content and its precise role in the ion transport mechanism remain inadequately understood. Herein, we systematically investigate the role of water content (0–19.87 wt%) in a poly(ethylene glycol) (PEG)-based viscoelastic solid-state electrolyte (V-SSE) in modulating Zn2⁺ transport and enhancing the performance of Zn||I2 batteries. Through positron annihilation lifetime spectroscopy, Fourier transform infrared spectroscopy, and molecular dynamics simulations, we observe that the free volume expands from 152 Å3 to 163 Å3 with increasing water content up to 9.16%, facilitating ion mobility. Beyond this level, free volume slightly decreases, yet ionic conductivity continues to rise, suggesting alternative promoting mechanisms. At a critical H2O-to-ether oxygen (EO) molar ratio of 1:1, a threefold increase in the ion diffusion coefficient occurs compared to that at H2O:EO = 1:3, stemming from shortened transport distance and enhanced diffusion kinetics. Consequently, the bound water-mediated V-SSE enables a high specific capacity of 170–208 mAh g−1 in Zn||I2 full cells, approximately twice that of the anhydrous system (94 mAh g−1), and maintains stable cycling over 3000 cycles. This study elucidates the multirole mechanism of bound water in enhancing ion conduction without compromising electrochemical stability, providing valuable insights for the design of high-performance solid-state electrolytes.