Ionic Liquid Electrolytes

Conventional lithium-ion electrolytes are lithium salts, typically LiPF6, dissolved in a blend of volatile alkyl carbonates. This combination gives good ionic conductivity and low viscosity, which is why it has dominated the industry for three decades. It is also the root cause of most of the industry’s safety problems. Carbonate solvents are flammable, and LiPF6 decomposes at elevated temperature to release PF5, a Lewis acid that catalyses further carbonate breakdown and reacts with trace moisture to form hydrofluoric acid. That HF attacks the cathode, strips the SEI, and accelerates the runaway cascade that every pack thermal engineer designs around.

Ionic liquids offer a different starting point. They are salts that are molten at, or close to, room temperature, built from bulky, asymmetric organic cations (imidazolium, pyrrolidinium, piperidinium) paired with fluorinated anions such as TFSI or FSI. Because the entire electrolyte is composed of ions rather than a molecular solvent carrying dissolved ions, it has negligible vapour pressure and does not sustain a flame. That single property is the reason ionic liquids keep reappearing in the literature on high-temperature, high-voltage, and safety-critical cells: eliminate the volatile solvent and you eliminate the fuel for thermal runaway, even if you can’t eliminate every failure mode.

The trade-off is transport. Strong Coulombic attraction between the bulky ions gives neat ionic liquids a viscosity one to two orders of magnitude higher than carbonate solvents, and because both the IL cation and anion are mobile under an applied field, they compete with the working ion (Li+ or Na+) for charge transport. Cation transference numbers in neat ionic liquids are often well below 0.5, which drives concentration polarisation at anything beyond modest C-rates. Below around −20°C, viscosity climbs sharply enough to risk lithium plating rather than intercalation. These are not minor caveats; they are the reason ionic liquids have taken thirty years to move from bench chemistry to niche commercial deployment, and they define most of the engineering effort covered on this page.

That engineering effort falls into a few recognisable families, each trading transport for safety and mechanical stability in a different way:

  • Neat ionic liquid electrolytes, used where thermal stability matters more than rate capability, sometimes with co-solvents or additives to manage the transference number problem.
  • Gel and polymer hybrid electrolytes, which embed an IL/salt mixture in a host polymer (PVDF, PVDF-HFP, PMMA) to trade some conductivity for mechanical integrity.
  • Polymerised ionic liquids (PILs), where the cation or anion is covalently fixed to a polymer backbone, producing a solid, single-ion conductor with a transference number approaching unity.
  • Composite solid electrolytes, where an ionic liquid acts as a wetting agent between ceramic or MOF particles and the electrode, solving the interfacial contact problem that plagues all-solid-state designs.

Each of these has a distinct role, and each shows up in different parts of the battery, from neat IL wetting agents at a solid-state cathode interface to PIL membranes replacing separators outright. The pages that follow this introduction work through the chemistry, the interfacial behaviour at both electrodes, and where each configuration currently sits on the path to commercial deployment.

References

  1. Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567–5580.
  2. Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621–629.
  3. MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Kar, M.; Passerini, S.; Pringle, J. M.; Ohno, H.; Watanabe, M.; Yan, F.; Zheng, W.; Zhang, S.; Zhang, J. Ionic liquids and their solid-state analogues as materials for energy generation and storage. Nat. Rev. Mater. 2016, 1, 15005. (Also the source for the Walden 1914 historical reference discussed above; the 1914 original predates DOIs and was not independently verifiable.)
  4. Watanabe, M.; Thomas, M. L.; Zhang, S.; Ueno, K.; Yasuda, T.; Dokko, K. Application of ionic liquids to energy storage and conversion materials and devices. Chem. Rev. 2017, 117, 7190–7239.
  5. Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem. Rev. 2004, 104, 4303–4417.

See also: Solid Polymer Electrolyte, High-Concentration Electrolytes, Electrolyte Decomposition, Solid Electrolyte Interphase, Thermal Runaway