Lean-Water Hydrogel with Multipolar Sites for Flexible and High-Performance Aqueous Aluminum Ion Batteries

Rechargeable aqueous aluminum ion batteries (AAIBs) offer a promising avenue for achieving safe, high-energy, and low-cost large-scale energy storage applications. However, the practical development of AAIBs is hindered by competitive reduction reactions in the aqueous solution, which lead to insufficient aluminum (Al) deposition and a severe hydrogen evolution reaction (HRE). In this work, an inorganic/organic hybrid hydrogel with a stable silicon-based network and multiple polar sites is successfully fabricated via an in situ sol-gel polymerization method. The preferential formation of hydrogen bonds between the polar functional groups and water molecules effectively reduces the thermodynamic reactivity of water. Furthermore, X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS) analyses confirm the formation of a stable, inorganic-rich solid electrolyte interface (SEI) layer, which kinetically suppresses undesirable side reactions. This hydrogel electrolyte exhibits a high ionic conductivity of 2.9 × 10⁻³ S cm⁻¹ at 25 °C, even under lean-water conditions. As a result, Al|hydrogel|potassium nickel hexacyanoferrate (KNHCF) full cells demonstrate excellent cycling performance, delivering a high initial discharge capacity of 74.9 mAh g⁻¹ at 100 mA g⁻¹ and achieving an outstanding capacity retention of 90.0% after 200 cycles. Additionally, pouch cells exhibit stable open-circuit voltage under various mechanical abuse conditions.