All-climate high-voltage MnO2 pseudocapacitor enabled by a molecularly crowded water-in-salt electrolyte

Abstract

Herein we demonstrate the fabrication of a laser induced graphene (LIG) supported MnO₂ symmetric pseudocapacitor with unprecedented electrochemical performance enabled by 17 m NaClO₄ water-in-salt hybrid electrolyte with ethylene glycol (EG) as the anti-freezing molecularly crowding agent (MC-WiSE). MnO₂ was electrodeposited onto an interdigital patterned conductive LIG template by a fast and facile binder free approach that eliminates the requirement for a separator or metal current collector, effectively minimizing inactive dead mass in the device. The depressed freezing point and suppressed electrochemical activity of water in the MC-WiSE enabled stable cell operation even at −40 °C within a broad voltage window of 0–2.5 V. Under these conditions, the device delivered a high areal capacitance of ∼25.1 mF cm⁻² (∼0.32 mA cm⁻²), corresponding to an energy density of 21.8 µWh cm⁻² at a power density of 396.8 µW cm⁻², along with excellent cycling stability. The cell also showed exceptional thermal response with an increased energy density of ∼75 µWh cm⁻² at a power density of ∼1984 µW cm⁻² via activating bulk pseudocapacitance at 60 °C. Furthermore, the ion–ion and ion–solvent interactions in the electrolyte and electrode/electrolyte interfacial interplay were studied via MD simulation which validated the experimental observations of excellent thermal and electrochemical stability.