Superconcentrated electrolytes

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Superconcentrated electrolytes, also known as water-in-salt or solvent-in-salt liquids, usually refer to chemical systems, which are liquid near room temperature and consist of a solvent-to-dissoved salt in a molar ratio near or smaller than ca. 4-8, i.e. where all solvent molecules are coordinated to cations, and no free solvent molecules remain. [1] Since ca. 2010 such liquid electrolytes found several applications, primarily for batteries. In the case of lithium metal batteries and lithium-ion batteries most commonly used anions for superconcentrated electrolytes are those, that are large, asymmetric and rotationally-vibrationally flexible, such as bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide. Noteworthy, lithium chloride and sodium perchlorate also form water-in-salt solutions. [2]

Contents

Advantages

Superconcentrated electrolytes demonstrate the following advantages: [3]

(1) Many show a good oxidative stability. [1] In particular, some can suppress oxidative corrosion of an Al current collector without a source of fluoride ion (such as hexafluorophosphate) and enable the use of 5 V lithium-ion battery cathode materials. [3]

(2) Some are resistant to electrochemical reduction. It is believed, that some sulfonimides (e.g., those with S-F and F-(H)C-N fragments, form a solid electrolyte interface similar to that formed by some organic carbonate solvents. [4] Properties #1 and #2 are responsible for very large (4-5 volt) voltage window, which is useful for advanced batteries.

(3) Related to #2 is the ability of some superconcentrated electrolytes to allow for reversible intercalation of Li+ ions into graphite in the absence of ethylene carbonate solvent, [5] therefore enabling a new class of safer lithium-ion batteries.

(4) Solvent vapor pressure is lower, thermal stability is higher, and flammability is absent, [1] which contributes to a better battery safety. [3]

(5) The concentration of charge-carrying ion is larger, which translates into smaller ion travelling distances. [3]

(6) In some cases, and contrary to expectations, faster rates of electrode reactions are observed, than in conventional low-salt-concentration electrolytes. [3]

(7) Polysulfide dissolution is sometimes suppressed, which enables cycling of such batteries as lithium-sulfur. [3]

(8) Some studies report, that Li+ transference number in such liquids is close to one, which means, that Li+ concentration gradient between anode and cathode does not develop during the battery's charge and discharge. [3]

(9) Electrodeposition of lithium metal from superconcentrated electrolytes is often nodular (without dendrites) and reversible. [3]

Disadvantages

At the same time, highly concentrated electrolytes are not without disadvantages: [3]

(1) Their ionic conductivity is generally lower than that of corresponding dilute (~1 M) electrolytes. [3]

(2) Their viscosity is higher than that of conventional electrolytes. [3]

(3) Their cost is usually higher, because manufacturing of some anions, such as sulfonimides, requires several low-yield synthetic steps. [3]

Origin of the unusual properties

The exact mechanism of high-voltage stability of superconcentrated electrolytes have not been established as of 2023. The two main proposed mechanisms are: [6]

(1) a decrease of water molecules' thermodynamic activity, when all water molecules are coordinated to cations, such as Li+.

(2) decomposition of an anion with the formation of a solid electrolyte interface.

Most recent studies suggest, that the anion decomposition mechanism (2) dominates in a majority of cases.

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References

  1. 1 2 3 Samanta, Prakas; Ghosh, Souvik; Kundu, Aniruddha; Samanta, Pranab; Chandra Murmu, Naresh; Kuila, Tapas (2023-03-01). "A strategic way of high-performance energy storage device development with environmentally viable "Water-in-salt" electrolytes". Journal of Energy Chemistry. 78: 350–373. doi:10.1016/j.jechem.2022.11.045. ISSN   2095-4956. S2CID   254321865.
  2. ↑ Sakita, Alan Massayuki Perdizio; Noce, Rodrigo Della (2022-08-17). "Low-cost water-in-salt electrolytes for electrochemical energy storage applications: a short review". Eclética Química Journal. 47 (2SI): 18–29. doi: 10.26850/1678-4618eqj.v47.2SI.2022.p18-29 . ISSN   1678-4618. S2CID   251791572.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 Yamada, Yuki; Yamada, Atsuo (2015). "Review—Superconcentrated Electrolytes for Lithium Batteries". Journal of the Electrochemical Society. 162 (14): A2406–A2423. doi: 10.1149/2.0041514jes . ISSN   0013-4651. S2CID   100598493.
  4. ↑ Superconcentrated electrolytes for a high-voltage lithium-ion battery. 2016. Nat Commun. 7/9. J.H. Wang, Y. Yamada, K. Sodeyama, C.H. Chiang, Y. Tateyama, A. Yamada. doi: 10.1038/ncomms12032. 2. Stable non-corrosive sulfonimide salt for 4-V-class lithium metal batteries. 2022. Nat Mater. 21/4, 455-62. L. Qiao, U. Oteo, M. Martinez-Ibañez, A. Santiago, R. Cid, E. Sanchez-Diez, et al. doi: 10.1038/s41563-021-01190-1.
  5. ↑ Huang, Yaohui; Wen, Bo; Jiang, Zhuoliang; Li, Fujun (2022-09-21). "Solvation chemistry of electrolytes for stable anodes of lithium metal batteries". Nano Research. 16 (6): 8072–8081. Bibcode:2023NaRes..16.8072H. doi:10.1007/s12274-022-4839-8. ISSN   1998-0124. S2CID   252445680.
  6. ↑ Tian, Xue; Zhu, Qizhen; Xu, Bin (2021-06-21). ""Water‐in‐Salt" Electrolytes for Supercapacitors: A Review". ChemSusChem. 14 (12): 2501–2515. doi:10.1002/cssc.202100230. ISSN   1864-5631. PMID   33830655. S2CID   233184600.
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