Aqueous electrochemical energy storage (EES) devices are highly safe, environmentally benign, and inexpensive, but their operating voltage and energy density must be increased if they are to efficiently power multifunctional electronics, new‐energy cars as well as to be used in smart grids. This Minireview summarizes the key breakthroughs and progress in expanding the electrochemical stability window (ESW) of aqueous EES devices over the past five years. After briefly introducing the electrode engineering ways to widen ESW, we focus on four ground‐breaking electrolyte engineering strategies and classify them into two kinds from the perspective of salts and exotic solutes/solvents. The widening degree toward ESW of these emerging electrolytes is compared and the universal fundamental mechanism relating to the interactions between limited water molecules and high‐concentration salts (or large amounts of exotic solutes/solvents) is elucidated. Key challenges and perspectives for high‐ESW electrolytes as well as recent advances in low‐cost and other metal ion (sodium, potassium, zinc, etc.)‐based electrolytes for expanding ESW are also outlined.

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Over the past few decades, electric doublelayer capacitors (EDLCs) have been one of the key components with high round trip efficiency and ultralong cycling lifetime in typical energy storage systems. [1][2][3][4][5][6] Unfortunately, EDLCs are subjected to limited energy density, which are inferior to that of secondary batteries and batterysupercapacitor hybrid devices. [7][8][9][10][11] As a well-known hybrid energy storage device, lithium-ion capacitors (LICs) consist of a capacitive electrode manifesting high rate capability and a battery-type electrode with high capacity, basically achieving both high power and energy densities. [12] In most cases, LICs employ Li + insertion anode and carbon-based capacitive cathode. [13][14][15][16] For instance, the state-ofthe-art LICs assembled with a prelithiated graphite anode and an activated carbon (AC) cathode were successfully commercialized. Nevertheless, the shortage of global lithium resources will bring great pressure on the development of sustainable electrochemical systems. Hybrid supercapacitors utilizing Na + instead of Li + are emerging as a possible substitute for addressing this issue due to the wide availability of sodium resources, low cost, and similar physiochemical properties between sodium and lithium. [17][18][19] As compared to LICs, sodium-ion capacitors (SICs) are still at their infant stage and need substantial advancement. Commercial graphite that is well employed as the Li-ion intercalation anode could not be directly used for SICs. Possible reasons are the insufficient interlayer spacing of graphite and the weakest chemical binding of Na to a given substrate, compared with the other alkali metals in the same column of the periodic table. [20] To search for potential carbon anodes, specific strategies have to be implemented, such as expanding the graphite interlayer, [21] forming house-of-cards structure, [22] and introducing defects. [23,24] While these functionalized carbon and structures demonstrate promoted sodium storage, the rate performance cannot always be guaranteed. Alternatively, layered titanates (A 2 Ti n O 2n+1 , A = Na, K, and H) represent a wide range of family with large interspacing of ≈0.7 nm for sodium storage. [8b] The Sodium-ion capacitors (SICs) have attracted increasing attention for sustainable energy utilization owing to their low cost and similar intercalation electrochemistry with lithium-ion capacitors. However, the practical application of SICs is seriously hindered by the low initial coulombic efficiency (ICE) and limited redox kinetics at the battery electrode side. Herein, taking a layered sodium titanate battery anode as an example, this study reports on the synergistic combination of ether electrolyte and binder-free array architecture to simultaneously achieve superior ICE and ultrafast Na + intercalation. The resulting Na 2 Ti 2 O 5 nanosheet array anode delivers extraordinary ICE (91%), high cycle CE (≈100%), and outstanding rate performance (66% capacity retention at 120 C). The key to the superior per...

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