Gel polymer electrolytes containing SiO2 spheres with tuned sizes to increase rechargeability of quasi-solid-state Zn-air batteries

Rechargeable zinc–air batteries (RZABs) with high energy densities and low costs have attracted tremendous interest for meeting the ever-growing demand for flexible and portable applications. In these batteries, the quasi-solid/solid-state electrolyte plays a critical role in the battery performance, such as the cycling performance rate and power output. In this study, porous poly (vinyl alcohol) (PVA) and poly (acrylic acid) (PAA) composite gel polymer electrolytes (GPEs) were modified by incorporating SiO₂ as a water retainer. For this reason, SiO₂ spheres of different sizes were synthesized by varying the temperature (5, 20, 60, and 80°C) and reaction time (2, 6, 18, and 24 h). According to the SEM micrographs, the average size ranged from 107 to 710 nm depending on the conditions. The porous PVA/PAA–SiO₂ GPEs obtained by selecting three SiO₂ sizes (107, 425, and 630 nm) exhibited higher water retention capability than the unmodified GPE. Among them, the GPE with SiO₂-630 nm displayed the highest battery performance (1.46 V, 85 mW cm⁻² @ 0.8 V). This was achieved using CoMn₂O₄ spinel as a bifunctional electrocatalyst, with a catalyst loading of 2 mg cm⁻². The quasi-solid-state RZAB displayed twice the rechargeability of the RZAB assembled with Pt/C+IrO₂/C (120 vs. 62 cycles). According to post-mortem tests, this can be related to the improved water retention, the decrease in the size of the formed Zn dendrites, and the stability of the bifunctional material. Thus, fine-tuning of the electrocatalyst/electrolyte interface strongly affects the rechargeability.