Sodium Alginate-Based Multifunctional Hydrogel Systems for Aqueous Flexible Energy Storage: Ion Transport Regulation, Interfacial Optimization and Device Stability Enhancement

Aqueous flexible energy storage is regarded as a highly promising safe technical route for next-generation wearable electronic devices. The central theme of this review is sodium-alginate-based hydrogel electrolytes for aqueous flexible energy storage. Among various application scenarios for such electrolytes, zinc-based flexible devices constitute the dominant research case, while supercapacitors and derivative energy-sensing integrated systems are also included for comparative analysis and extended discussion. Conventional gel electrolytes are plagued by intractable bottlenecks, including the inherent conflict between mechanical and electrochemical performances, rampant zinc dendrite growth, detrimental interfacial side reactions, narrow water-decomposition voltage window and inadequate adaptability under broad temperature conditions. Sodium alginate (SA), a cost-effective biomass polysaccharide extracted from marine brown algae, possesses distinctive structural superiority originating from the G-block “egg-box” metal-ion chelation effect for constructing high-performance hydrogel electrolytes. This review systematically summarizes three dominant modification strategies for SA-based hydrogel electrolytes and elaborates their performance enhancements together with unavoidable trade-offs. Multi-scale synergistic mechanisms are interpreted from ion-transport modulation, electrode–gel interfacial compatibility, homogeneous zinc deposition and long-term cycling stability. This work further compares SA with cellulose and chitosan, and demonstrates their applications in supercapacitors, zinc-ion hybrid energy storage, fibrous micro-devices and energy-sensing integrated systems. Critical challenges in molecular rational design and prospective large-scale manufacturing are emphasized. An integrated development paradigm of “precise molecular programming–closed-loop process control–device performance mapping” is proposed, which may offer guidance for future fundamental exploration, whereas practical engineering deployment of degradable flexible energy-storage hydrogels still requires substantial further validation.