Double-safety flexible supercapacitor basing on zwitterionic hydrogel: over-heat alarm and flame-retardant electrolyte

Thermal runaway during the charging-discharging processes is always the safe issue of the flexible energy storage devices. However, the existed strategies are hard to maintain safety and good electrochemical performance simultaneously, as well as over heat alarm. Here, we report a thermoresponsive zwitterionic conductive natural polymer based hydrogel (sodium alginate/poly acrylic-acrylamide/NaCl, SPA) as electrolyte for the safe flexible supercapacitor to alarm and prevent the thermal runaway. The SPA hydrogel exhibited good flexibility (1578% strain and 0.24 MPa stress) and conductivity, and the assembled flexible hydrogel supercapacitor showed good electrochemical performance (specific capacities was 27.7 F g⁻¹ at 0.8A g⁻¹ current density) with wide working voltage ranges (1.4 V) and high energy density (7.41 Wh kg⁻¹ at the power density of 560 W kg⁻¹). Importantly, the SPA electrolyte possessed excellent controllable thermal responsive conductivity. The conductivity of SPA showed a sharp change below and above transition temperature up to one order of magnitude (2.05 mS cm⁻¹ at 25 ℃, 18.21 mS cm⁻¹ at 60 ℃). The obvious conductivity changes could be used to warn the sudden temperature raise of energy storage devices. Furthermore, SPA hydrogels also showed excellent flame retardant property, the limiting oxygen index (LOI) as high as 46%; thus, the SPA could effectively prevent the whole device from burning even in extreme situation. Combined with over-heat alarm and flame-retardant properties, a belt-and-braces strategy is provided to ensure the safety of flexible energy storage devices. Graphical abstract: [Figure not available: see fulltext.]