Auswahl der wissenschaftlichen Literatur zum Thema „Superconcentrated electrolyte“

Based on the unique ubiquity of similar solvate structures found in solvate crystals and superconcentrated electrolytes, we performed a systematic study of four reported solvate crystals which consist of different lithium salts (i.e., LiMPSA, LiTFSI, LiDFOB, and LiBOB) solvated by acetonitrile (MeCN) based on first principles calculations. Based on the calculations, these solvate crystals are predicted to be electronic insulators and are expected to be similar to their insulating liquid counterpart (e.g., 4 M superconcentrated LiTFSI-MeCN electrolyte), which has been confirmed to be a promising electrolyte in lithium batteries. Although the MeCN molecule is highly unstable during the reduction process, it is found that the salt-MeCN solvate molecules (e.g., LiTFSI-(MeCN)2, LiDFOB-(MeCN)2) and their charged counterparts (anions and cations) are both thermodynamically and electrochemically stable, which can be confirmed by Raman vibrational modes through the unique characteristic variation in C≡N bond stretching of MeCN molecules. Therefore, in addition to the development of new solvents or lithium salts, we suggest it is possible to utilize the formation of superconcentrated electrolytes with improved electrochemical stability based on existing known compounds to facilitate the development of novel electrolyte design in advanced lithium batteries.

We here report a superconcentrated potassium acetate (KAC) solution (75[Formula: see text]wt.%, K : H2O [Formula: see text] 1 : 1.8, called as “water-in-salt”) as an aqueous electrolyte to improve the working voltage and increasing capacitance in enhancing the energy density of the active carbon-based aqueous supercapacitor. The stability potential window of the superconcentrated electrolyte realizes an AC//AC symmetric supercapacitor with operating voltage of 2.0 V and excellent cyclic performance. Meanwhile, the energy density of such supercapacitor achieves about twice as high as that of the supercapacitor using normal concentration of electrolyte.

Leveraging the most recent success in expanding the electrochemical stability window of aqueous electrolytes, in this work we create a unique Li-ion/sulfur chemistry of both high energy density and safety. We show that in the superconcentrated aqueous electrolyte, lithiation of sulfur experiences phase change from a high-order polysulfide to low-order polysulfides through solid–liquid two-phase reaction pathway, where the liquid polysulfide phase in the sulfide electrode is thermodynamically phase-separated from the superconcentrated aqueous electrolyte. The sulfur with solid–liquid two-phase exhibits a reversible capacity of 1,327 mAh/(g of S), along with fast reaction kinetics and negligible polysulfide dissolution. By coupling a sulfur anode with different Li-ion cathode materials, the aqueous Li-ion/sulfur full cell delivers record-high energy densities up to 200 Wh/(kg of total electrode mass) for >1,000 cycles at ∼100% coulombic efficiency. These performances already approach that of commercial lithium-ion batteries (LIBs) using a nonaqueous electrolyte, along with intrinsic safety not possessed by the latter. The excellent performance of this aqueous battery chemistry significantly promotes the practical possibility of aqueous LIBs in large-format applications.

We explore a superconcentrated electrolyte comprising N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, 1,2 dimethoxyethane and 3.2 mol kg⁻¹ LiFSI. It offers an alternative ion-transport mechanism, improved fluidity and ultra-stable Li metal battery performance.

Accurate analysis of the mechanism of interphase formation and its composition is imperative for the development of stable and efficient lithium/sodium-ion batteries. One of the most practically scalable ways to optimize interphase chemistry and morphology for reversible charge transport is electrolyte engineering and the application of formation protocol with specific current/voltage conditions. In this sense, the analysis of structure and dynamics within the electrode/electrolyte interface, i.e., the so-called electric-double layer, is essential to predicting the possible chemistry and mechanism of interphase formation. The latest was recently advanced by understanding the effect of electrolyte chemistry, e.g., the chemical structure of solvent and salt, their ratio, the presence of additives etc.1–4 Although, the role of electrode material is often neglected which creates an impression that both metallic and semiconductive electrodes form an identical electric-double layer within the same electrolyte, therefore, the mechanism of interphase formation should be very similar regardless of the chemical nature of the electrode. Here, we used examples of sodium-salt-containing ionic liquids and carbonate electrolytes to demonstrate that structural changes at the electrified interface are greatly affected by the dielectric nature of electrode material. The key observation is attributed to the different abilities of electrified electrodes to form van der Waals interactions with the electrolyte species. This affects the concentration of metal-anion complexes in relation to organic solvent near the electrode, therefore, different interphase chemistry will be formed (inorganic-rich vs. organic-rich), and different formation protocols must be used. For example, Fig. 1 shows the summary of this phenomenon for ionic liquid electrolytes with 50 mol% sodium bis(fluorosulfonyl)imide. Besides, metallic and semiconductive electrodes demonstrated different affinity toward polar organic solvent, i.e., ethylene carbonate, in 1.0 M NaPF6 in EC:DMC (1:1 by volume) electrolyte. This presentation will discuss the details of these fundings and introduce future applications. Fig. 1 | Schematic relationships between solid-electrolyte interphase (SEI) composition, applied current of preconditioning cycling, and dielectric nature of anode material. References Zheng, X. et al. Toward High Temperature Sodium Metal Batteries via Regulating the Electrolyte/Electrode Interfacial Chemistries. ACS Energy Lett. 7, 2032–2042 (2022). Rakov, D. A. et al. Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes. Nat. Mater. 19, 1096–1101 (2020). Rakov, D. A. et al. Polar Organic Cations at Electrified Metal with Superconcentrated Ionic Liquid Electrolyte and Implications for Sodium Metal Batteries. ACS Mater. Lett. 4, 1984–1990 (2022). Rakov, D. et al. Stable and Efficient Lithium Metal Anode Cycling through Understanding the Effects of Electrolyte Composition and Electrode Preconditioning. Chem. Mater. (2021). doi:10.1021/acs.chemmater.1c02981 Figure 1

Depuis 2015, le développement des électrolytes aqueux superconcentrés, dénommés « Water-in-salt electrolytes » (WiSE), a suscité un regain d’intérêt pour les batteries Li-ion (LIB) aqueuses. Les WiSE proposent une alternative aux électrolytes organiques commerciaux qui posent des problèmes de sécurité et de durabilité, tout en résolvant les faibles performances des électrolytes aqueux dilués limitées par l’étroitesse de la fenêtre électrochimique (1 .23 V). En effet, la superconcentration influe sur les propriétés physico-chimiques et la réactivité interfaciale. La formation d’une interphase solide/électrolyte inorganique (SEI) riche en fluorure de lithium (LiF) ouvre la voie à l’utilisation d’électrode négative à bas potentiel et donc à l’augmentation de la densité d’énergie de ces batteries. Cette thèse étudie la viabilité des électrolytes WiSE dans les LIB. Grâce à la mise en place d’une étude systématique, l’impact de la superconcentration sur les performances des batteries en fonction des conditions d’opération montre que la SEI formée ne prévient pas de la réduction de l’eau, appelée réaction d’évolution de l’hydrogène (HER) ni pendant le cyclage ni pendant les périodes de repos, i.e. l’autodécharge. L’évaluation des vitesses de consommation de l’eau souligne les limites des propriétés protectrices de la SEI malgré la passivation de l’interface. Par ailleurs, la détermination des énergies d’activation de la HER directe, ayant lieu pendant le cyclage, et du phénomène d’autodécharge suggère que l’autodécharge est gouvernée par la HER. Enfin, l’évaluation de la solubilité de LiF dans les WiSE, des observations au microscope environnemental à balayage électronique et des mesures de chromatographie en phase gaz suggèrent que l’instabilité de la SEI est d’avantage reliée à des défauts microstructuraux qui ne peuvent pas être comblés dû à l’absence d’auto-passivation de l’interface. Une étape de pré-imprégnation dans un électrolyte organique réduit la consommation d’eau, confirmant la nécessité de propriétés d’autoréparation de la SEI
The development of superconcentrated aqueous electrolytes, namely Water-in-salt electrolytes (WiSE), from 2015 onwards has renewed the interest for aqueous-based Li-ion battery (LIB). Indeed, they were proposed to overcome issues related to safety and sustainability of common carbonate-based organic solvent while solving the poor performances of diluted aqueous electrolyte due to the narrow electrochemical stability window (ESW) of water (1.23 V). Such achievements are largely attributed to modification of the electrolyte structure upon increase in concentration that changes the physico-chemical properties and the interfacial reactivity. An inorganic LiF-based solid electrolyte interphase (SEI) was reported to be formed, opening the path for the use of low potential negative electrodes, further increasing the energy density of these batteries. This work aims to provide answers regarding the viability of WiSE in LIB. By conducting a systematic study of the impact of superconcentration on battery performances as function of the operating conditions, we demonstrate that the SEI is not able to prevent water reduction following the hydrogen evolution reaction (HER), neither during cycling nor during resting period, i.e. self-discharge. Indeed, the rates for water consumption calculated during cycling and resting period are found within the same order of magnitude, highlighting the SEI limitation to prevent water reduction although the surface is passivated. Determining the activation energies for HER during cycling and self-discharge, we suggest that self-discharge is more likely driven by water reduction than Li+ deintercalation. Eventually, LiF solubility measurements, gas chromatography tests and environmental scanning electron microscopy suggest that SEI instability is related to structural defects that cannot be self-passivated in WiSE. A presoaking step in organic electrolyte of an artificial Li/LiF layer reduces water consumption and thus confirms the need for the SEI to self-repair