Lithium-ion batteries are popular in a wide variety of energy storage applications because of their high energy density and other desirable properties (such as their ability to maintain effectiveness over a large number of charge cycles). They are widely used in many consumer electronics and in hybrid or electric cars. In order for a lithium-ion (Li-ion) battery to function, lithium ions must be able to migrate from the battery's anode, where oxidation of lithium metal occurs, to the cathode, where the reduction of lithium ions to lithium metal takes place. This is ordinarily facilitated by a liquid electrolyte, which allows ion transport between the electrodes without being an electron conductor. Current electrolytes in commercial Li-ion batteries are typically polar organic solvents with a dissolved lithium salt. [1] These solvents have a number of inherent limitations and drawbacks. There is active research on a variety of approaches to eliminate or mitigate these problems; one such approach is the replacement of conventional battery electrolytes with special solvents known as room temperature ionic liquids.
Most ionic compounds (i.e., salts) are solids at room temperature because of the strong electrostatic attraction between positive and negative ions. Room temperature ionic liquids (RTILs) are a relatively recently developed class of salts, having been first synthesized early in the 20th century but only having been commercially available since 1999, after which research on them increased substantially. [2,3] RTILs have melting points below 100°C by definition, and may have melting points as low as -90°C. [3,4] These low melting points are achieved because of RTIL cations (and sometimes anions) include bulky, asymmetrical organic groups which interfere with the salt's ability to form stable crystal lattices. [2,5] Quaternary ammonium cations with alkyl substituents, such as 1-alkyl-3-methylimidazoliums, are common RTIL cations. Anions vary widely and may include halides, tetrafluoroborate, and many others. [1-3]
Conventional electrolytes for Li-ion batteries consist of an organic solvent (typically ethylene carbonate combined with a linear carbonate, such as dimethyl carbonate) with a dissolved lithium salt (such as lithium hexafluorophosphate). [4,6] These solvents can pose some safety concerns; perhaps the most widely reported of these is the flammability and volatility of the solvents. Under the right circumstances, the electrolyte in an Li-ion battery can ignite or even explode. [7]
Alkyl carbonates, particularly the linear carbonates necessary to keep battery electrolytes liquid at room temperature, are flammable and can vaporize at temperatures that are attainable during battery operation. [4,7] This can allow pressure to build up in an overheating cell and, in the event of a physical failure of the battery, lead to the aforementioned fire or explosion. [1,7,8] These hazards have caused safety problems for Li-ion batteries in some consumer electronics and have complicated the deployment of Li-ion batteries for some electric and hybrid cars. [7,8]
Generally speaking, RTILs exhibit negligible vapor pressure, similar to solid salts. [1,3] Unlike most organic solvents, RTILs do not vaporize unless heated to the point of thermal decomposition, typically 200-300°C or more[3,6]. As a result, RTIL-based cells lack the explosion or pressure risks of Li-ion cells using conventional electrolytes. [1,3] Furthermore, typical RTILs will not sustain combustion, and so do not pose fire hazards. [1] Combining ionic liquids in mixtures with conventional carbonate solvents can also result in electrolytes with lower vapor pressure and flammability than the carbonate solvents alone. [6]
Current Li-ion battery electrolytes also suffer from a limited operational temperature range, a problem that RTILs may be able to help solve. At temperatures above ~ 60°C, the electrolyte solution used in most commercial Li-ion batteries can deteriorate. [1] The dissolved lithium salts can undergo chemical reactions with the solvents at these elevated temperatures, negatively affecting performance. Because these reactions are irreversible, function is not fully restored even when the temperature is reduced, and the reaction products can pose additional safety issues. Performance also suffers below -20°C as this is approaching the freezing point of the normal electrolyte solutions. [1]
Ionic liquids can easily address the high temperature limit, as most are stable to temperatures greater than 200°C. Proper selection of ionic liquid can yield solvents with freezing points as low as -90°C, giving RTILs a much broader potential range of operating temperatures than current electrolytes. [3]
In an Li-ion battery, unwanted electrochemical reactions can occur at the electrodes, both between solvent molecules and between the solvent and the electrode material. This is addressed in typical cells by choosing electrolyte mixtures that react to form passive, insoluble thin films (called the solid electrolyte interface, or SEI) at the electrode surface when the battery is first charged. [6] These films, typically lithium carbonate or lithium fluoride, passivate the electrode surface against further reactions while still allowing the passage of lithium ions. Most pure ionic liquids, unfortunately, do not form electrochemical reaction products capable of creating a passivation layer, but this problem can be addressed by adding small amounts of carbonates and/or lithium salts to the ionic liquid to provide material for forming the SEI. [4.6] This can lead to a trade-off between ensuring an adequate SEI and preserving the advantageous properties of the RTIL. Ensuring that any new RTIL-based electrolyte can form a stable, ion-conductive SEI will be crucial to its success as a battery technology.
Another challenge for implementation of RTILs as Li-ion battery electrolytes is their somewhat limited lithium ion transport properties. Ionic liquids generally have overall ion conductivities on the order of tens of mS/cm, which is comparable to traditional Li-ion battery electrolytes. [2] Ion conductivity in an ionic liquid, however, can also be due in part to migration of the RTIL ions themselves; for optimum battery operation, it is desirable to maximize conductivity of lithium ions specifically. Lithium ion conductivity in RTIL electrolyte-based cells can be limited by the high viscosity of typical RTILs, as well as by the capability of the RTIL ions to act as alternative charge carriers. [6] For a typical RTIL electrolyte, only around 20-50% of the overall ion conductivity may be due to lithium ion transport, though this varies with the particular ionic liquid and any additives that may be present. [9] Limited lithium ion transport may have negative implications for cyclability and power delivery of RTIL-based batteries, but research into these issues, and potential solutions, is ongoing.
Lithium-ion batteries are likely to remain commercially important for a vast array of energy storage applications for a long time to come, making improvements to their safety issues and operational limitations valuable. As a diverse group of conductive solvents largely free from fire and explosion hazards, room temperature ionic liquids may play a role in providing safer, more robust energy storage for the future.
© Christian Lawler. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. S. Lee et al., "Ionic Liquids as Electrolytes for Li Ion Batteries," J. Ind. Eng. Chem., 10, 1086 (2004).
[2] M. Galinski, A. Lewandowski, and I. Stepniak, "Ionic Liquids as Electrolytes," Electrochim. Acta 51, 5567 (2006).
[3] H. D. B. Jenkins, "Ionic Liquids - An Overview," Sci. Prog. 94, 265 (2011).
[4] M. Holzapfel et al, "Stabilisation of Lithiated Graphite in an Electrolyte Based on Ionic Liquids: an Electrochemical and Scanning Electron Microscopy Study," Carbon 43, 1488 (2005).
[5] P. M. Dean, J. M. Pringle, and D. R. MacFarlane, "Structural Analysis of Low Melting Organic Salts: Perspectives on Ionic Liquids," Phys. Chem. Chem. Phys., 12, 9144 (2010).
[6] A. Lewandowski and A. Swiderska-Mocek, "Ionic Liquids as Electrolytes for Li-ion Batteries - An Overview of Electrochemical Studies," J. Power Sources, 194, 601 (2009).
[7] Q. Wang et al., "Thermal Runaway Caused Fire and Explosion of Lithium Ion Battery," J. Power Sources 208, 210 (2012).
[8] E. Quartarone and P. Mustarelli, "Electrolytes For Solid-State Lithium Rechargeable Batteries: Recent Advances and Perspectives," Chem. Soc. Rev., 40, 2525 (2011).
[9] J. Shin, W. A. Henderson, and S. Passerini, "Ionic Liquids to the Rescue? Overcoming the Ionic Conductivity Limitations of Polymer Electrolytes," Electrochem. Commun. 5, 1016 (2003).