Review—Superconcentrated Electrolytes for Lithium Batteries

Abstract

Ever-increasing demand for better batteries has set extraordinarily high standards for electrolyte materials, which are far beyond the realm of a conventional nonaqueous electrolyte design. Superconcentrated (or highly concentrated) solutions are emerging as a new class of liquid electrolytes with various unusual functionalities beneficial for advanced lithium (Li) battery applications. This article reviews unique features, as well as basic physicochemical properties, of highly concentrated electrolytes from the viewpoint of their peculiar solution structure, and discusses their future contributions to advanced battery technologies. © The Author(s) An electrolyte is an indispensable component in rechargeable batteries. In general, it serves as a carrier ion-conductive and electroninsulating medium facilitating ion transport between a pair of electrodes, simultaneously withstanding the strong reducing and oxidizing forces of negative and positive electrodes, respectively. From this perspective, an ion-transport property and reductive/oxidative stability are the major two metrics in a battery electrolyte design. For lithium (Li)-ion batteries, the reductive/oxidative stability is of particular importance, because they employ extremely strong reducer and oxidizer as negative and positive electrodes, respectively, to deliver the highest voltage (currently ca. 3.8 V) among various battery chemistries. To address this severe situation, an electrochemically stable nonaqueous electrolyte, rather than a highly conductive aqueous electrolyte, has been used with some compromise on ionic transport properties.The composition of nonaqueous electrolytes makes critical impacts on the performance of Li-ion batteries.1-3 This fact is seemingly paradoxical to the "rocking chair" concept of Li-ion batteries, in which electrolyte components principally do not participate in the battery faradaic reactions. This paradox arises, at least partly, from the inherent thermodynamic instability of the electrode/electrolyte interfaces; in most cases, the interfaces are stabilized in a kinetic way (by electrode passivation) based on electrolyte decomposition products, 4 and thus the nature of the interfaces strongly depends on electrolyte composition. In other words, the design of electrolytes often corresponds to the control of the interfaces where faradaic charge-transfer reactions occur, thus making significant effects on the battery performance.A nonaqueous electrolyte design seems to have infinite variations of aprotic solvents (and additives), Li salts, and their mixing ratios (i.e., electrolyte concentrations). Indeed, however, there are many restrictions in the choice of aprotic solvents and Li salts in terms of the kinetic stability of the interfaces they form. First, only specific organic carbonates (e.g., ethylene carbonate (EC)) are compatible with graphite negative electrodes. 5,6 This peculiarity results from solvent's passivation ability. Because almost all aprotic solvents are unstable at such a low potential as ∼0 V vs. Li +...

Redox Active Glyme-Li Salt Solvate Ionic Liquids Based on Tetrabromoferrate(III)

“…Unlike aprotic ILs and metal salt binary mixtures employed as typical IL-based battery electrolytes, solvate ILs can provide a high concentration of metal ions required for fast electrode reactions in batteries. 4,5 Furthermore, strong ligand binding to metal cations at extreme salt concentrations improves the electrochemical stability of the solvate IL electrolytes, similar to highly concentrated electrolytes. 6,7 Therefore, solvate ILs play a part in the blooming research field of highly concentrated electrolytes for future energy storage applications.…”

Solvation structure in dilute to highly concentrated electrolytes for lithium-ion and sodium-ion batteries

“…Recently another concept has emerged; non-aqueous electrolytes with salt concentrations significantly above 1.0 M have been reported to exhibit remarkable properties including non-volatility, raising the concept of super-concentration (up to ca. 5 M or even above) as an interesting strategy [16]. Reversible LIB operation using both graphite and lithium metal anodes, and with enhanced kinetics have been demonstrated for a number of salts and solvents [17,18].…”

Developing New Functionalities of Superconcentrated Electrolytes for Lithium-ion Batteries

“…The corrosion prevention of Al through the control of solvation states enables a new electrolyte design without relying on LiPF 6 . A possible advantage of excluding LiPF 6 from electrolyte components is to minimize electrode degradation caused by HF (i.e., a decomposition product of LiPF 6 with trace water).…”

“…1), and hence, the highly concentrated region has long been outside the major research focus of battery electrolytes. However, various new functionalities have recently been discovered in the highly concentrated region, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] which bring about vigorous research worldwide on superconcentrated (highly concentrated) electrolytes. Notably, highly concentrated electrolytes show high stabilities toward both negative and positive electrodes.…”