Effect of Cross-Linking and Solvent Exchange on the Structure and Electrochemical Properties of PVA/Zinc Borate Electrolytes

Abstract

Inorganic fillers are widely employed in solid polymer electrolytes to regulate the coordination environment, Lewis acidity, and interfacial reactivity. Among them, borate-based fillers are particularly attractive due to their dual Lewis acidic sites and strong affinity toward oxygen-containing ligands. However, in poly(vinyl alcohol) (PVA)-based electrolytes, zinc borate (ZB) fillers readily form reversible borate ester linkages with PVA hydroxyl groups, passivating Lewis acid sites and limiting filler functionality. Herein, to overcome the situation and to improve the performance of the filler, competitive esterification, cross-linking, and solvent coordination are employed. Citric acid (CA) cross-linking preferentially consumes PVA hydroxyl groups and modifies PVA–ZB interactions, potentially increasing the availability of borate-associated Lewis acidic sites. A subsequent solvent exchange with diethyl carbonate replaces the coordinated water molecules, further stabilizing the borate-coordination framework. Spectroscopic and electrochemical analyses confirm the cross-linking and solvent coordination, correlating with enhanced ionic conductivity and interfacial stability. A stable lithium plating-stripping reaction with very low polarization potential provides evidence of uniform interfacial lithium deposition. While extended cycling reveals gradual capacity decay associated with inorganic-rich interphase formation, the study highlights the critical role of coordination chemistry and the solvent environment in governing the functionality of borate fillers in polymer matrices.