The nature of electrolyte solutions is dominated by the three factors of Li salts, solvents, and their mixing ratios (salt concentrations). Conventionally, the selections of Li salts and solvents ...have been considered of prime importance for Li-ion battery electrolytes, while the salt concentrations have been always fixed to approximately 1 mol dm−3 based on maximized ionic conductivities. Recently, however, the salt concentrations are increasingly recognized as a key to developing new functionalities for battery electrolytes in the wake of various unusual interfacial/bulk properties discovered in superconcentrated (highly concentrated) electrolytes. For example, highly concentrated electrolytes i) passivate effectively negative electrodes, ii) facilitate rapid Li+ intercalation reactions, iii) show high oxidative stabilities, iv) prevent the corrosion of Al current collectors, and v) suppress the dissolution of transition metals from positive electrodes, all of which are beneficial for battery applications. This article discusses those unique functionalities of highly concentrated electrolytes from the viewpoint of their ion-solvent and ion-ion coordination structures.
The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation advanced batteries (e.g., Li–O2 systems). ...Acetonitrile (AN) solutions are one of the most promising electrolytes with remarkably high chemical and oxidative stability as well as high ionic conductivity, but its low stability against reduction is a critical problem that hinders its extensive applications. Herein, we report enhanced reductive stability of a superconcentrated AN solution (>4 mol dm–3). Applying it to a battery electrolyte, we demonstrate, for the first time, reversible lithium intercalation into a graphite electrode in a reduction-vulnerable AN solvent. Moreover, the reaction kinetics is much faster than in a currently used commercial electrolyte. First-principle calculations combined with spectroscopic analyses reveal that the peculiar reductive stability arises from modified frontier orbital characters unique to such superconcentrated solutions, in which all solvents and anions coordinate to Li+ cations to form a fluid polymeric network of anions and Li+ cations.
Solid electrolyte interphase (SEI) is an ion conductive yet electron‐insulating layer on battery electrodes, which is formed by the reductive decomposition of electrolytes during the initial charge. ...The nature of the SEI significantly impacts the safety, power, and lifetime of the batteries. Hence, elucidating the formation mechanism of the SEI layer has become a top priority. Conventional theoretical calculations reveal initial elementary steps of electrolyte reductive decomposition, whereas experimental approaches mainly focus on the characterization of the formed SEI in the final form. Moreover, both theoretical and experimental methodologies could not approach intermediate or transient steps of SEI growth. A major breakthrough has recently been achieved through a novel multiscale simulation method, which has enriched the understanding of how the reduction products are aggregated near the electrode and influence the SEI morphologies. This review highlights recent theoretical achievements to reveal the growth mechanism and provides a clear guideline for designing a stable SEI layer for advanced batteries.
The nature of the solid electrolyte interphase (SEI) formed on battery electrodes governs the performance and safety of current and post‐Li‐ion batteries. However, it has been unclear how reaction products of an electrolyte interact and aggregate to form the final SEI. The morphological growth mechanism of the SEI, as revealed by state‐of‐the‐art theoretical simulation methodologies, is highlighted.
The development of high-rate lithium-ion batteries is required for automobile applications. To this end, internal resistances must be reduced, among which Li+ transfer resistance at ...electrode/electrolyte interfaces is known to be the largest. Hence, it is of urgent significance to understand the mechanism and kinetics of the interfacial Li+ transfer. This Spotlight on Applications presents recent progress in the analysis and mechanical understanding of interfacial Li+ transfer. First, we review the reported activation energies (E a) at various solid/liquid interfaces. On this basis, the mechanism and rate-determining step of the interfacial Li+ transfer are discussed from the viewpoints of the desolvation of Li+, the nature of the solid electrolyte interphase (SEI), and the surface structural features of electrodes. After that, we introduce promising strategies to reduce the E a, highlighting some specific cases that give remarkably low E a. We also note the variations in frequency factors or pre-exponential factors (A) of the interfacial Li+ transfer, which are primarily dominated by the number of Li+ intercalation sites on electrode surfaces. The current understanding and improvement strategies of interfacial Li+ transfer kinetics presented herein will be a foundation for designing high-rate lithium-ion batteries.
Concentrated electrolytes of LiN(SO2F)2 (LiFSA) and organic phosphates (e.g., trimethyl phosphate, TMP) are receiving intense attention for safe and long-lasting lithium-ion batteries, because of ...their nonflammable character and unusual passivation ability toward negative electrodes. However, their high viscosity and low ionic conductivity have hampered their practical application. In this work, a low-dielectric diluent, 1,1,2,2-tetrafluoroethyl 2,2,3,3,-tetrafluoropropyl ether (HFE), is introduced into concentrated LiFSA/TMP electrolytes. Upon dilution, the viscosity drastically decreases to 11.0 mPa s and the ionic conductivity slightly increases to 0.87 mS cm–1. More importantly, both of the nonflammable character and the unusual passivation ability are retained even after dilution. A spectroscopic analysis shows that the diluted LiFSA/TMP:HFE has a local coordination state similar to that in the concentrated LiFSA/TMP, which leads to the formation of a FSA anion-derived inorganic surface film. This work suggests the importance of the peculiar local coordination state in designing safe battery electrolytes with better passivation ability.
A 62 m K-ion aqueous electrolyte Ko, Seongjae; Yamada, Yuki; Yamada, Atsuo
Electrochemistry communications,
July 2020, 2020-07-00, 2020-07-01, Volume:
116
Journal Article
Peer reviewed
Open access
Display omitted
•A K-ion aqueous electrolyte with the highest alkali-ion concentration is discovered.•The N(SO2F)2 anion is resistant to hydrolysis in K-ion systems.•The K-ion electrolyte shows both ...high ionic conductivity and wide potential window.
Concentrated aqueous electrolytes have been widely explored for high-voltage aqueous batteries. To achieve higher voltage, even higher alkali-ion molalities are being pursuit up to 55.5 mol kg−1 (m) that corresponds to monohydrate, but the sacrificially lowered ionic conductivity is problematic. Here we report a 61.7 m K-ion aqueous electrolyte composed of KN(SO2F)2 (KFSI) and KSO3CF3 (KOTf). The pioneered beyond-monohydrate realm is unique to K-ion systems, in which FSI− anion is stable to hydrolysis. The K-ion electrolyte has two-order higher ionic conductivity (12 mS cm−1 at 30 °C) than a comparable 55.5 m Li-ion system (0.1 mS cm−1) while exhibiting a 2.7 V potential window on Pt being among the widest in all alkali-ion aqueous electrolytes. This work suggests the superior overall performance of aqueous K-ion batteries over other alkali-ion batteries.
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