Layered Fe2(MoO4)3 assemblies with pseudocapacitive properties as advanced materials for high-performance sodium-ion capacitors

Abstract Sodium-ion capacitors (SICs) show great potential for large-scale energy storage devices due to their high energy-power density, long cycling life, and low cost of sodium. Nevertheless, a daunting challenge to this technology is the sluggish sodium ion diffusion kinetics of the faradaic anode materials, which impede the widespread development of SICs. Herein, uniformly layered Fe2(MoO4)3 (L-FMO) assemblies are prepared though a facile solvothermal-assisted route and used as anode material of SICs for the first time. Benefiting from the improved extrinsic pseudocapacitive contribution, the L-FMO assemblies exhibit fast kinetics, high sodium storage capacity as well as long-term lifetime. Moreover, an initial intercalation-subsequent conversion sodium storage mechanism of L-FMO assemblies is further demonstrated though a series of techniques including in-situ X-ray diffraction, ex-situ transmission electron microscopy as well as ex-situ X-ray photoelectron spectroscopy. When the L-FMO assemblies was applied into SICs, the assembled devices achieve a high-energy-power density (227.2 Wh kg−1, 20.05 kW kg−1) along with good cycling stability (a low capacity decay of only 0.0062% per cycle during 3000 cycles at the current density of 1 A g−1). The present work pioneers alkaline earth metal molybdates for SICs anodes and will push the development of energy storage systems.