Stable Thermochromic Hydrogel for a Flexible and Wearable Zinc-Ion Yarn Battery with High-Temperature Warning Function

Hydrogel electrolytes are considered to be excellent substitutes for liquid electrolytes due to their safety and portability and are widely used in flexible and wearable electronics. In this work, we constructed hydrogels based on polyacrylamide (PAM), cotton cellulose nanofibers (CNFs), and carboxymethyl cellulose IV (CMC) with a semi-interpenetrating cross-linked network structure with excellent mechanical properties and high ionic conductivity for a thermochromic aqueous zinc-ion yarn battery, which exhibits excellent thermochromic cycling stability and outstanding electrochemical performance. It exhibits excellent thermochromic features during repeated cycling from 25 to 60 °C, delivers a high capacity of 383 mAh g–1 at 0.5 A g–1, and shows excellent cycle stability with 98% and 81% of the initial capacity retained after 1000 and 3500 cycles, respectively. More importantly, it has a very fast response speed to high temperature, which can intuitively provide the wearer with a high-temperature warning and effectively avoid low temperature scalding while meeting the energy supply. It offers great potential for practical wearable applications in high-temperature warning. Therefore, this work provides an approach and application for developing reliable, durable, and wearable smart batteries.