A low-temperature-tolerant and non-flammable cellulose/HEC/PVA eutectogel for flexible asymmetric supercapacitors

Asymmetric supercapacitors (ASCs), which combine the advantages of electric double-layer capacitors and pseudocapacitors, have attracted more and more research interest. However, the performance of water-based ASCs often faces the challenge of electrolyte freezing at low temperatures. To resolve the problem, a ternary deep eutectic solvent (DES) with an eutectic point of less than -100 °C was first prepared. After the DES was integrated into a polymer matrix composed of microcrystalline cellulose (MCC), hydroxyethyl cellulose (HEC), and poly(vinyl alcohol) (PVA), a flexible and non-flammable eutectogel was fabricated. The optimized eutectogel not only exhibited an ionic conductivity of 23.4 mS cm^-1 even at -20 °C, but also displayed a tensile strength value of about 0.37 MPa and an elongation value at break of nearly 600 %. The ASC with a eutectogel demonstrated a high voltage window of 0-1.8 V, an energy density of 15.13 Wh kg^-1 at a power density of 87.52 W kg^-1, and fantabulous cycling stability with ∼90 % capacitance retention after 5000 cycles. The flexible ASC with such a eutectogel could work well in a wide temperature range from -20 to 60 °C. It is expected that this work could present valuable insights for the development of wide-temperature gel polymer electrolytes in ASC applications.