Polyacrylamide-based hydrogel electrolyte for modulating water activity in aqueous hybrid batteries

Abstract

While zinc-ion and hybrid aqueous battery systems have emerged as potential substitutes for expensive lithium-ion batteries, issues like side reactions, limited electrochemical stability, and electrolyte leakage hinder their commercialization. Due to their low cost, high stability, minimal leakage risks, and a wide variety of modification opportunities, hydrogel electrolytes are considered the most promising solution compared to liquid or solid electrolytes. Here, we synthesized a dual-function hydrogel electrolyte based on polyacrylamide and poly(ethylene dioxythiophene):polystyrene (PPP). This electrolyte reduces water content and enhances stability by minimizing side reactions while swelling in a binary ethylene glycol and water solution (EG 10%) further stabilizes the battery system. The developed hydrogel exhibits relatively good ionic conductivity (1.6 × 10^-3 S cm^-1) and excellent electrochemical stability, surpassing 2.5 V on linear sweep voltammetry tests. The PPP-based system reached a value of 119.2 mA g^-1, while the aqueous electrolyte reached only 80.4 mA g^-1 specific capacity. The rechargeable PPP hydrogel electrolyte-based hybrid aqueous battery with zinc anode achieved more than 600 cycles. Coulombic efficiency (CE) remained at 99%, indicating good electrochemical reaction stability and reversibility. This study highlights the potential of polyacrylamide-based hydrogel electrolytes with dual functionality as the electrolyte and separator, inspiring further development in hydrogel electrolytes for aqueous battery systems. This study highlights the potential of polyacrylamide-based hydrogel electrolytes with dual functionality as the electrolyte and separator, inspiring further development in hydrogel electrolytes for aqueous battery systems.

Fig. 1. (a) Schematic illustration of the one-pot PPP hydrogel polymerization and chemical and physical cross-link formation. (b) Schematic of the hydrogel electrolyte with an ethylene glycol additive working principle preventing side reactions and modulating water activity. (c) Photos of prepared PPP hydrogel.

Fig. 2. (a) FTIR spectroscopy of acrylamide monomer, comparative PAM and PPP hydrogels. (b) SEM images (average pore size is 2.15 μm. Scale: 10 μm) of freeze-dried PPP hydrogel. EDS mapping (scale: 10 μm) of PPP hydrogel after soaking the polymer in ZnSO₄ and Li₂SO₄ solution with the EG additive. (c) Demonstration of thickness and mechanical flexibility of the PPP hydrogel (thickness = 0.188 mm). (d) Water Uptake (WU) and Swelling Ratios (SR) of PPP and comparative PAM hydrogel electrolytes. (e) Binding energies of different PPP hydrogel sites and zinc with water molecules.

Fig. 3. (a) Non-covalent interactions and (b) reduced density gradient of PPP hydrogel sites.

Fig. 4. (a) The pilot line for pouch cell preparation. (b) Preparation of the cathode. (c) Schematic illustration of the symmetric Zn‖Zn (left) and full Zn‖LiFePO₄ (right) pouch cell assembly.

Fig. 5. (a) EIS spectra of PPP and PAM hydrogel in symmetric Zn‖Zn cell. (b) LSV curves of the PPP hydrogel and aqueous symmetric Zn‖Zn cells at 1 mV s⁻¹. (c) Zn plating and stripping behavior of PPP hydrogel and aqueous Zn‖Zn symmetric cells at different current densities (0.1–2 mA cm⁻²). (d) Zn plating and stripping long cycling of PPP hydrogel and aqueous electrolyte-based Zn‖Zn symmetric cells at 0.1 mV h⁻¹. (e) An enlarged demonstration of the zone is indicated on the above Zn plating/stripping long cycling behavior test.

Fig. 6. (a) CV of the PPP hydrogel and aqueous electrolyte-based full Zn‖LFP pouch cells at 0.1 mV s⁻¹. (b) Discharge capacity and coulombic efficiency, and (c) charge–discharge curve comparison PPP hydrogel and aqueous electrolyte-based hybrid Zn‖LFP pouch cell battery system tested. (d) SEM images of the Zn anode surface after 100 charge–discharge cycles.

Fig. 7. (a) Long cycling performance and (b) galvanostatic charge/discharge curves of PPP hydrogel-based Zn‖LFP hybrid pouch cell battery at 0.1C. (c) Discharge capacity and (d) voltage/capacity curves of the PPP hydrogel and aqueous electrolyte-based Zn‖LiMn₂O₄ hybrid pouch cell batteries.

Fig. 8. Images of LED light applications of the PPP-based hybrid pouch cells in clear polyethylene packaging under various deformations (bending and folding).