Engineering chitosan into a recyclable and flame-resistant gel electrolyte via a dual cross-linking strategy for flexible supercapacitors

Flexible supercapacitors (SCs) using gel polymer electrolytes (GPEs) have attracted increasing attention given the popularity of portable and wearable electronics. However, the facile preparation of GPEs with not only outstanding mechanical and electrochemical properties but also recyclability, flame-resistance and sustainability is still a challenge. Herein, a dual cross-linking strategy was developed using levulinic acid (LA) and sodium borate to engineer chitosan into flame-resistant and recyclable GPEs with satisfactory mechanical properties and high ionic conductivity for application in flexible SCs. In this strategy, LA functioned as a protonation reagent to make chitosan soluble in water and a crosslinker via imine formation between its ketone groups and the amino groups of chitosan to form a crosslinked polymer network. Sodium borate functioned as not only a cross linker but also an ionic conductor and flame retardant in the GPEs. The engineered GPEs exhibited high flame-resistant ability (self-extinguishing time <1 s), desirable mechanical performance (0.87 MPa), and high ionic conductivity (24.02 mS cm−1). The assembled flexible SC exhibited a high energy density of 14.58 W h kg−1 and cycling stability over 35 000 cycles. Furthermore, benefiting from the pH sensitivity of the formed imine bonds, the GPEs could be easily degraded in acidic conditions, and then recycled for the fabrication of other GPEs. Overall, it is believed that the as-prepared eco-friendly and safe electrolyte shows great promise for applications in consumable electronics.