Organics-free aqueous hybrid electrolyte for high-performance zinc ion hybrid capacitors operating at low temperature

Aqueous zinc ion chemistry is the hot topic of new energy storage technologies nowadays for its high intrinsic safety as well as its advantages in terms of environmental protection and price. However, the freezing of water at low temperatures limits the practical application to a broad range of temperatures. Herein, in view of the disturbing effect of Mg(ClO4)2 on hydrogen bond of water molecules, a novel anti-freezing electrolyte (0.4 m ZnSO4 + 2.7 m Mg(ClO4)2) is developed, which features ultra-low freezing point (<−80 °C) and superior conductivity (7.50 mS cm−1 at −60 °C). Consequently, this organics-free hybrid electrolyte renders the as-built aqueous zinc ion hybrid capacitors (AZHCs) available to operate even at −60 °C with a high potential window of 2.4 V. Moreover, at −30 °C, such AZHC devices exhibit the highest specific capacity (123 mA h g−1), the maximum energy density (126 Wh kg−1 at 1050 W kg−1), an excellent rate performance (75.8 Wh kg−1 at 10495.4 W kg−1) and a stable working life over 32 000 cycles. This research points to a feasible strategy to improve the cryogenic performance of AZHCs with high energy density.