Supercapacitors are highly valued energy storage devices with high power density, fast charging ability, and exceptional cycling stability. A profound understanding of their charging mechanisms is crucial for continuous performance enhancement. Electrochemical quartz crystal microbalance (EQCM), a detection means that provides in situ mass change information during charging–discharging processes at the nanogram level, has received greatly significant attention during the past decade due to its high sensitivity, non-destructiveness and low cost. Since being used to track ionic fluxes in porous carbons in 2009, EQCM has played a pivotal role in understanding the charging mechanisms of supercapacitors. Herein, we review the critical progress of EQCM hitherto, including theory fundamentals and applications in supercapacitors. Finally, we discuss the fundamental effects of ion desolvation and transport on the performance of supercapacitors. The advantages and defects of applying EQCM in supercapacitors are thoroughly examined, and future directions are proposed.

Over recent years, oligomer ionic liquids (OILs), a novel class of ionic liquids, are becoming preferential electrolytes for high-performance energy-storage devices, such as supercapacitors with enhanced energy density and non-flammable lithium-ion batteries (LIBs). Herein, structures, properties, and their associations of the up-to-the-minute formulated OILs are systematically summarized and elaborately interpreted, especially for dicationic ionic liquids and tricationic ionic liquids. The physicochemical and electrochemical properties of OIL-based electrolytes are presented and analyzed, which are vitally important for supercapacitors and LIBs. Subsequently, the applications of OILs as electrolytes for supercapacitors and LIBs are summarized, with the comparisons of the energy-storage mechanisms and performance between OILs and MILs electrolytes in supercapacitors. Meanwhile, the optimization of the dynamic performance of OILs electrolytes is provided. Finally, the main difficulties and probable perspectives of OIL-based electrolytes are presented for future work. This review would contribute to a deep understanding of OILs and design optimized OIL-based electrolytes for energy storage systems.

Owing to high electrical conductivity and ability to reversibly host a variety of inserted ions, 2D metallic molybdenum disulfide (1T-MoS₂) has demonstrated promising energy storage performance when used as a supercapacitor electrode. However, its charge storage mechanism is still not fully understood, in particular, how the interlayer spacing of 1T-MoS₂ would affect its capacitive performance. In this work, molecular dynamics simulations of 1T-MoS₂ with interlayer spacing ranging from 0.615 to 1.615 nm have been performed to investigate the resulting charge storage capacity in ionic liquids. Simulations reveal a camel-like capacitance-potential relation, and MoS₂ with an interlayer spacing of 1.115 nm has the highest volumetric and gravimetric capacitance of 118 F cm⁻³ and 42 F g⁻¹, respectively. Although ions in MoS₂ with an interlayer spacing of 1.115 nm diffuse much faster than with interlayer spacings of 1.365 and 1.615 nm, the MoS₂ with larger interlayer spacing has a much faster-charging process. Our analyses reveal that the ion number density and its charging speed, as well as ion motion paths, have significant impacts on the charging response. This work helps to understand how the interlayer spacing affects the interlayer ion structures and the capacitive performance of MoS₂, which is important for revealing the charge storage mechanism and designing MoS₂ supercapacitor.