Flexible and safe batteries, coupled with high performance and low cost, constitute a radical advance in portable and wearable electronics, especially considering the fact that these flexible devices are likely to experience more mechanical impacts and potential damage than well-protected rigid batteries. However, flexible lithium ion batteries (LIBs) are vastly limited by their intrinsic safety and cost issues. Here we introduce an extremely safe and wearable solid-state zinc ion battery (ZIB) comprising a novel gelatin and PAM based hierarchical polymer electrolyte (HPE) and an α-MnO₂ nanorod/carbon nanotube (CNT) cathode. Benefiting from the well-designed electrolyte and electrodes, the flexible solid-state ZIB delivers a high areal energy density and power density (6.18 mW h cm⁻² and 148.2 mW cm⁻², respectively), high specific capacity (306 mA h g⁻¹) and excellent cycling stability (97% capacity retention after 1000 cycles at 2772 mA g⁻¹). More importantly, the solid-state ZIB offers a high wearability and an extreme safety performance over conventional flexible LIBs, and performs very well under various severe conditions, such as being greatly cut, bent, hammered, punctured, sewed, washed in water or even put on fire. In addition, flexible ZIBs were integrated in series to power a commercial smart watch, a wearable pulse sensor, and a smart insole, which has been achieved to the best of our knowledge for the first time. These results demonstrate the promising potential of ZIBs in many practical wearable applications and offer a new platform for flexible and wearable energy storage technologies.

Over the past two decades, a series of aqueous rechargeable metal-ion batteries (ARMBs) have been developed, aiming at improving safety, environmental friendliness and cost-efficiency in fields of consumer electronics, electric vehicles and grid-scale energy storage. However, the notable gap between ARMBs and their organic counterparts in energy density directly hinders their practical applications, making it difficult to replace current widely-used organic lithium-ion batteries. Basically, this huge gap in energy density originates from cell voltage, as the narrow electrochemical stability window of aqueous electrolytes substantially confines the choice of electrode materials. This review highlights various ARMBs with focuses on their voltage characteristics and strategies that can effectively raise battery voltage. It begins with the discussion on the fundamental factor that limits the voltage of ARMBs, i.e., electrochemical stability window of aqueous electrolytes, which decides the maximum-allowed potential difference between cathode and anode. The following section introduces various ARMB systems and compares their voltage characteristics in midpoint voltage and plateau voltage, in relation to respective electrode materials. Subsequently, various strategies paving the way to high-voltage ARMBs are summarized, with corresponding advancements highlighted. The final section presents potential directions for further improvements and future perspectives of this thriving field.

It remains a challenge to render aqueous batteries operating at subzero temperatures properly, not even to mention the maintenance of their flexibility and mechanical robustness. This fundamentally arises from the freezing of hydrogel electrolytes under such low temperature, resulting in performance deterioration and elasticity loss. Here we propose an intrinsically freeze-resistant flexible zinc manganese-dioxide battery (Zn-MnO₂-B) comprising a designed anti-freezing hydrogel electrolyte which can preclude the ice crystallization of the hydrogel component and maintain a high ion conductivity even at −20 °C. Benefiting from exceptional freeze resistance, the fabricated anti-freezing Zn-MnO₂-B (AF-battery) exhibits excellent electrochemical stability and mechanical durability at subzero temperatures. Even at −20 °C, the specific capacity of the AF-battery can retain over 80% with Coulombic efficiencies approaching ∼100%, compared to the thorough performance failure of the Zn-MnO₂-B with traditional polyacrylamide (PAM) hydrogel electrolyte. More impressively, the flexibility of batteries can also be well maintained even under severe mechanical stresses at subzero temperatures, such as being bent, compressed, hammered or washed in an ice bath. Furthermore, the AF-battery sealed in an ice cube can be integrated in series to power a wristband of an electronic watch, LED lights and a 72 cm² electroluminescent panel. It is believed that this work opens new perspectives to develop anti-freezing batteries and would play the role of a model system for developing new hydrogel aqueous electrolytes for flexible batteries in extremely cold environments.