A free-sealed high-voltage aqueous polymeric sodium battery enabling operation at −25°C

An increasing demand for electric vehicles and flexible electronics focuses attention on developing a safe, high-energy, and sustainable battery that can work under severe conditions. Emerging high-voltage aqueous batteries based on highly concentrated salts and molecular crowding electrolytes are likely to be hampered by their poor low-temperature performance because of a high freezing point and salting out at low temperature. Inspired by the antifreezing ionogel electrolyte for transport measurements at subzero temperatures, we design a water-in-ionogel electrolyte with a low salt-concentration (2m NaTFSI) and high operational voltage (3.0 V) by changing the hydrogen bonding and introducing fluoride additives for low-temperature operation. A full cell with a P2-type Na2/3Mn2/3Co1/3O1.98F0.02 cathode and hard-carbon anode could deliver high energy densities of 109 and 23.4 Wh kg−1 at room temperature and −25°C. This eco-friendly aqueous polymeric battery could be free sealed and perform in water. This work opens an avenue for designing high-energy, free-sealed aqueous batteries for low-cost, sustainable energy storage, enabling subzero temperature operation.