Water-based flexible batteries represent a significant advancement in energy storage technology, particularly for applications where safety is paramount. Traditional lithium-ion batteries rely on organic electrolytes, which are flammable and pose risks of thermal runaway. In contrast, aqueous batteries use water-based electrolytes, eliminating flammability concerns while maintaining reasonable energy density. This makes them ideal for wearable devices designed for children and elderly users, where safety cannot be compromised.

Aqueous zinc-ion and lithium-ion chemistries are the most prominent water-based systems. Zinc-ion batteries utilize zinc metal as the anode and a water-based electrolyte, often containing zinc salts like zinc sulfate. The cathode materials typically include manganese oxides or vanadium-based compounds, which allow for reversible zinc ion insertion and extraction. Zinc-ion systems offer advantages such as low cost, high abundance of zinc, and inherent safety due to the non-flammable electrolyte. Their operational voltage ranges between 1.2 and 1.8 volts, suitable for low-power wearable electronics.

Aqueous lithium-ion batteries, while less common, replace organic solvents with water-based electrolytes containing lithium salts. These systems often use lithium manganese oxide or lithium iron phosphate as cathode materials. The main challenge is the narrow electrochemical stability window of water, which limits the voltage to around 1.5 volts to prevent electrolysis. Despite this, aqueous lithium-ion batteries benefit from the well-established lithium-ion intercalation chemistry, providing stable cycling performance.

Encapsulation is critical for flexible water-based batteries to prevent leakage and maintain performance under mechanical stress. Common encapsulation methods include thin-film polymer coatings such as polyethylene terephthalate (PET) or polydimethylsiloxane (PDMS), which provide flexibility while acting as a moisture barrier. Multilayer encapsulation, combining hydrophobic and hydrophilic layers, further enhances durability. Some designs integrate self-healing polymers to automatically seal minor cracks, extending battery lifespan in wearable applications.

Safety testing for water-based flexible batteries focuses on mechanical robustness, leakage resistance, and electrochemical stability under deformation. Standard tests include bending cycles, where the battery is repeatedly flexed to a small radius while monitoring capacity retention. Puncture tests assess the risk of electrolyte leakage under mechanical damage. Thermal stability is evaluated by exposing the battery to elevated temperatures, ensuring no thermal runaway or gas generation occurs. Unlike organic electrolyte systems, aqueous batteries show no fire or explosion risks even under severe abuse conditions.

Target applications for these batteries include health-monitoring wearables for children and elderly users. Examples are smart patches for continuous glucose monitoring, wearable ECG sensors, and temperature-tracking devices. The flexibility and safety of water-based batteries allow seamless integration into textiles or adhesive patches, ensuring user comfort and reliability. Unlike rigid conventional batteries, these systems conform to body movements without compromising performance.

Contrasting with organic electrolyte systems highlights the trade-offs. Organic electrolytes, such as those using lithium hexafluorophosphate in carbonate solvents, enable higher energy densities (exceeding 200 Wh/kg) and wider voltage ranges (up to 4.2 volts). However, they are highly flammable and require stringent safety measures, including flame-retardant additives and robust battery management systems. In wearables, a leak or rupture could expose users to toxic and flammable substances, making them less suitable for vulnerable populations.

Water-based batteries, while safer, have lower energy densities (typically 50-100 Wh/kg) due to the limited voltage window of aqueous electrolytes. They also face challenges like hydrogen evolution at the anode and cathode dissolution over long-term cycling. However, ongoing research focuses on optimizing electrode materials and electrolyte formulations to improve energy density and cycle life without compromising safety.

In summary, water-based flexible batteries offer a compelling solution for wearable devices targeting children and elderly users, where safety outweighs the need for high energy density. Aqueous zinc-ion and lithium-ion chemistries provide non-flammable alternatives to conventional systems, with encapsulation methods ensuring durability under flexing. While they may not yet match the performance of organic electrolyte batteries, their inherent safety and flexibility make them ideal for next-generation wearable applications. Advances in materials and design will further bridge the performance gap, enabling wider adoption in safety-critical markets.