An interfacially coupled Mott–Schottky heterojunction encapsulated in graphitized nanofibers for an ionic-liquid hybrid supercapacitor operable from −25 °C to 60 °C

Developing high-energy supercapacitors operable under extreme temperatures remains challenging due to the limited voltage window and poor thermal stability of aqueous electrolytes, as well as the difficulty in constructing uniform heterostructured electrodes. Here, we report a terminal-group-directed self-assembly strategy to construct a uniform 2D-2D MXene–SnSe (MX-SS) Mott–Schottky heterojunction, followed by electrospinning and carbonization to embed the heterostructure within graphitized carbon nanofibers, yielding a binder-free MX-SS@CNF-800 electrode with strong interfacial coupling and hierarchical porosity. Building on this architecture, the as-designed MX-SS@CNF-800₍₊₎//CNF-800₍₋₎ coin-cell device using PVDF-HFP/[EMIM][BF₄] as the ionic-liquid gel electrolyte operates stably at ∼3.5 V and delivers IR drop corrected energy densities of 191.0, 194.0, and 192.5 Wh kg⁻¹ at −25, 25, and 60 °C, respectively, representing a 2.5-fold improvement over its aqueous counterpart. The device retains 91%, 97%, and 94% of its initial capacitance after 20k cycles at −25, 25, and 60 °C, respectively, owing to the thermally robust [EMIM][BF₄] electrolyte and the highly conductive MX-SS heterointerface protected within a graphitic matrix. Nevertheless, the device reliably powers multiple LEDs for nearly 1.8 hours at both −25 °C and 60 °C after only 1 minute of solar charging, demonstrating real-condition applicability across a wide temperature spectrum. These findings highlight ionic-liquid-based hybrid supercapacitors as a promising platform for high-energy and thermally robust energy storage.