Rechargeable batteries, regarded as one of the most efficient energy storage technologies, have experienced tremendous research interest over the past decades.(1,2) The simple configuration with basic components of cathode, anode, electrolyte, separator, and current collectors makes them easy to manufacture and use, where the success of Li-ion batteries (LIBs) is the best example. However, most research work focused on the exploration of new electrode materials and new battery chemistries; the importance of the electrolyte was not recognized until it was found that ethylene carbonate (EC) is able to prevent the exfoliation of the graphite to render improved LIBs.(3) Similarly, ether electrolyte can cointercalate with Na+ into graphite to realize the storage of sodium into the graphite anode.(4) Recently, it has been widely accepted that the future development of battery technology will overwhelmingly depend on the further design and discovery of tailored electrolyte systems. In fact, the electrolyte, which is sandwiched between the highly oxidative cathode and highly reductive anode, plays a key role in transferring ionic species while insulating electronic conducting internally to ensure the normal operation of batteries. In addition to solid electrolytes, most electrolytes used today are in the liquid state because of the easy access with electrode materials and the low impedance across the electrode/electrolyte interface.(5) The typical electrolytes are usually formed by dissolving salts in polar solvents, where cations and anions of salts are dissociated by aqueous or nonaqueous solvents via solvation sheaths. To obtain a good electrolyte formulation, comprehensive parameters ranging from physical properties (wide liquidus range, low viscosity, high ionic conductivity, good thermal stability, low cost, environmentally benign, etc.), chemical characteristics (simple synthesis, inert toward inactive or active components, etc.), and electrochemical requirements (wide electrochemical stability window, thin and stable solid electrolyte interphase, etc.) should be taken into account. Thus, electrolytes significantly influence the overall performance of batteries including practical capacity, rate capability, cycling stability, intrinsic safety, and so on. Early efforts in optimization of electrolyte properties mainly lie in adjusting the electrolyte composition, such as varying the combinations of salts and solvents, mixing different types of solvents or salts, adopting diversified functional additives, and so on. However, the regulation of electrolyte concentration did not arouse tremendous interest because of the so-called “1 molarity (M) legacy” of nonaqueous electrolytes where the maximum of ionic conductivities almost always occurs near the salt concentration of 1 M for most systems. Several research works on electrolyte concentrations open a new field of research on electrolytes and pave the way for the further development of battery technologies. The magic of electrolyte concentration was perhaps first noticed by McKinnon and Dahn in 1985 who reported the cointercalation of propylene carbonate (PC) with Li+ into the ZrS2 layered material can be circumvented with a saturated solution of LiAsF6 in PC, which is not possible in the 1 M electrolyte.(6) The confinement of electrolyte concentration was first broken by Angell et al.(7) in 1993 who mixed lithium salts with small quantities of polymers and found the decreasing trend of ionic conductivity was reversed when increasing the salt concentration beyond a certain threshold. They defined the new ionic conductors as “polymer-in-salt” electrolytes. In 2003 Inaba et al.(8) examined the effects of electrolyte concentration on the interfacial reactions between graphite and PC-based solutions and demonstrated that the poor compatibility between graphite and PC could be improved without the need for a film-forming agent such as EC as long as the concentration of lithium salts is sufficiently high. In 2004 Chen et al.(9) mixed two solids of LiTFSI and acetamide to form a room-temperature molten salt which was later known as the concentrated electrolyte. The tested superior physicochemical properties indicate the potential application in lithium batteries. In 2010 Watanabe and co-workers(10) started to investigate the glyme-based superconcentrated electrolytes, which they regarded as a new family of room-temperature ionic liquids (later named solvated ionic liquids) because of the similar physicochemical properties. In 2013, Hu et al.(11) proposed that if either the weight or volume ratio of salt to solvent exceeds 1.0 then the new class of electrolyte can be denoted as “solvent-in-salt (SIS)” with unusual properties to distinguish it from traditional electrolytes. Meanwhile they demonstrated the benefits of this SIS electrolyte in a Li||S battery, including the low solubility of polysulfides, a high Li+ transference number, as well as the suppression of Li dendrite growth. This is the first demonstration that the superhigh concentrated electrolyte has a positive effect on stabilization of Li metal and opens a new way to improve the reversibility of Li anode and suppress the lithium dendrite growth. Afterward, this SIS concept and/or concentrated electrolytes became widely extended to many other battery chemistries, including but not limited to Li–O2/air batteries, Li/Na/K-ion batteries, multivalent cation batteries, and aqueous batteries. Superior battery functions achieved through the SIS concept are well-summarized in recent review articles.(12−14) It is worth mentioning that in 2015 Xu, Wang, and co-workers(15) chose special solvent water and prepared a “water-in-salt (WIS)” electrolyte, expanding the narrow electrochemical stability window of water from 1.23 to 3.0 V in aqueous LIBs. Yamada et al.(16) soon independently reported a similar system termed “hydrate melt” to further improve the stability of water via employing a second salt at even higher concentrations in aqueous LIBs. Both works triggered interest in further expanding the electrochemical stability window of water.(17,18) Even though the superconcentrated electrolytes could bring tremendous benefits for battery performance, the high cost necessitated by the high salt concentration could be a disadvantage for practical applications. Zhang et al.(19) proposed “localized high concentration electrolyte,” maintaining the solvation structure of a superconcentrated electrolyte in the local environment while keeping the parent electrolyte in a dilute state. Very recently, Hu, Lu, and co-workers(20) found it is not necessary to employ a high-concentration electrolyte in every battery system; they employed “ultralow-concentration electrolyte” for Na-ion batteries to further reduce the cost and expand the operating temperature range. While it is not possible to discuss all the related research work in this Editorial, representative work on the electrolyte concentration is shown in Figure 1a.(21−25) Figure 1. (a) Representative research work on the electrolyte concentration for rechargeable batteries (Source: Web of Science, Clarivate Analytics, accessed 2020-10-16).(6−11,15,19−25) (b) Solvation and interfacial structures of superhigh and ultralow concentrated electrolytes. Besides the above-mentioned initial trials to tailor the electrolyte concentration from superhigh to ultralow, many efforts were also dedicated to unravel the mystery of electrolyte concentration. As part of the study, two particular structures, viz., solvation structure and interfacial structure, are deemed to influence the electrolyte properties as well as electrode/electrolyte interfaces that together can affect the battery performance (Figure 1b).(5,26−28) The solvation structure illustrates the interactions among the cations, anions, and solvent molecules, such as the cation–anion Coulomb interaction (Li+–X–) and the ion–dipole interaction (Li+–solvent). The competition between these two interactions directly influences the existence of the solvates, mainly in the form of solvent-separated ion pairs (SSIPs), contact ion pairs (CIPs), and cation–anion aggregates (AGGs), which in turn defines a series of parameters such as viscosity, solubility, reactivity, ionic conductivity, ionic transport mode, and so on. The interfacial structure existing at the inner-Helmholtz layer describes the correlations between the electrolyte components and charged electrode surfaces, and hence, it determines the SEI.(26) At present, most of the research work is also focused on the origin of the interfacial chemistry involving the electrolyte from dilute to high concentration, switching from solvent molecules to anions. However, a claim has been made that an aqueous SEI does not necessarily require an anion to provide the chemistry source.(29) The interphase chemistry still remains an important area worthy of future investigations. The intensified investigations of electrolyte concentration have indeed helped to create new electrolyte systems and provided a deeper understanding of the fundamental science. The investigations will continue to advance our understanding of both electrolyte nature and interfacial electrochemistry. We believe this field will receive increasing attention in the coming decades as we expect unusual properties have yet to be discovered that will further improve the performance of batteries. Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. The authors thank Mr. Yuqi Li for drawing Figure 1. This article references 29 other publications.