Molecularly Crowded Water-in-Salt Electrolyte Enabled All Climate Operational High Voltage MnO2 Pseudocapacitor

Abstract

Herein we demonstrate the fabrication of 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 anti-freezing molecularly crowding agent (MC-WiSE). MnO2 was electrodeposited onto interdigital patterned conductive LIG template, 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. Further the ion-ion and ion-solvent interaction in the electrolyte and electrode/electrolyte interfacial interplay was studied via MD simulation which validated the experimental observations of excellent thermal and electrochemical stability.