A Supercapacitor Architecture for Extreme Low-Temperature Operation Featuring MXene/Carbon Nanotube Electrodes with Vertically Aligned Channels and a Novel Freeze-Resistant Electrolyte

The electrochemical performance of supercapacitors drops precipitously at extreme low temperatures due to a multitude of reasons, which includes electrolyte freezing, sluggish ion transport in the electrode and electrolyte, and high charge transfer resistance at electrode–electrolyte interfaces. To address high interface resistance, a new supercapacitor architecture is reported, in which MXene/carbon nanotube electrodes with vertically aligned channels are synthesized to reduce tortuosity and maximize the electrode–electrolyte contact area. These electrodes are fabricated using a directional-freezing strategy, generating direct and fast ion transport pathways. Further, a freeze-resistant electrolyte which shows high ionic conductivity is synthesized by designing a double-crosslinked polymer network in a binary solvent consisting of ionic liquid and water, which exhibits an ultralow freezing temperature of −54 °C. An all-in-one supercapacitor is assembled by an integrated polymerization strategy to minimize interfacial resistances. The resulting device delivers a specific capacitance of 231 F g −1 at 2 mV s −1 and a maximum energy density of 10.17 Wh kg −1 , while maintaining a capacitance retention of 92%, even at an extreme low temperature of −50 °C. The supercapacitor architecture developed in this study, demonstrates the feasibility of electrochemical energy storage at extreme low temperatures.