The Electrochemical Society hosted Prof. Jenny Pringle’s live webinar, “The Development of New Ionic Electrolytes for Energy Storage Devices” on April 21, 2021. Her answers to questions that followed the presentation are provided below.
NOTE: Registration is required to view the webinar.
Prof. Jenny Pringle works in the Institute for Frontier Materials at Deakin University, Australia, as a chief investigator in the ARC Centre of Excellence for Electromaterials Science (ACES) and the ARC Industrial Transformation Training Centre “StorEnergy.” She received her undergraduate degrees and PhD at The University of Edinburgh, Scotland, before moving to Monash University, Australia, in 2002. From 2008-2012, she held an ARC Queen Elizabeth II Fellowship investigating the use of ionic electrolytes for dye-sensitized solar cells. Pringle moved to Deakin University in 2013. There she leads research into the development of new ionic liquids and organic ionic plastic crystals for applications including thermal energy harvesting, gas separation membranes, and lithium and sodium batteries.
Any guidance on principles of design of new ILs? How do you search for new candidates?
It would be great to have clear design principles for ILs and for OIPCs, but unfortunately the field isn’t quite at that point. We believe that charge delocalization is important for lower Tm and improving electrochemical stability and transport. But the effect of functional groups (e.g., ether oxygens in the sidechain or on the ring), and even ion size and symmetry, can be substantial but hard to predict. In future, considerations such as recyclability and low toxicity are also going to be increasingly important.
Supercooling is a real challenge in characterizing IL-based electrolytes. Do you have any advice for how to perform experiments to ensure the thermodynamically-stable state rather than a meta-stable state is being studied?
Indeed, this can be a very frustrating problem! It’s easier to manage if you have some idea of what the Tm is, in which case you can anneal (isotherm in the DSC) just below that temperature to encourage crystallization. In the lab, contacting the material with a rough surface can encourage crystallization (scratching the inside of your round bottom flask is a common trick). My general advice is to avoid putting it straight into the freezer as any rapid cooling will more likely form a glass.
In some electrochemical applications of IL, viscosity is a key factor. Which mixture of Li FSI-and IL would have the lower viscosity and larger conductivity?
In the materials I discussed, the phase diagram of these salt mixtures has regions of solids, solid/liquid mixtures, as well as liquids. In that case, finding the eutectic composition will likely give you the most fluid and conductive composition. When you start with an IL, I don’t think there is any one composition always guaranteed to be the most conductive/fluid as it will depend on the Li speciation in the liquid. Also keep in mind that you might be more interested in high transference number than total ionic conductivity, and these might be optimum at different compositions.
For portable electronics, what do you think is more promising in terms of safety and performance: sodium or Li metal batteries?
In my opinion, for portable electronics, lithium is more promising. At least until supply issues become critical and we are forced into alternatives. However, for larger scale applications, where cost considerations become more important and weight less important, then sodium could be the better option.
For the ionic liquids with high salt content, have you explored the ion conduction mechanism?
That is indeed very interesting. No, not for the new materials that I talked about. However, others in our group have used MD simulations to explore the conduction mechanism of Na or Li ions in ILs and also how anions impact this. For example: https://pubs.acs.org/doi/abs/10.1021/acs.jpclett.9b02416
Has anyone studied [N111CN] using DCA anion? I’m curious about how the symmetry in both the cation- and anion-containing cyano groups might impact the solvation and transport dynamics of Li.
Yes, we made the DCA salt with that cation (DOI: 10.1039/d0ta03502e). It melts at 49 °C, so it is not a room temperature ionic liquid. However, it is quite likely that some composition in combination with Li DCA (or other Li salt) would form a liquid—that would be very interesting, but we have not investigated it yet.
Could you hazard any guesses as to which systems are most likely to be used in the next generation of batteries?
My guess would be that pyrrolidinium- and phosphonium-based ionic liquids will be widely applied first, as they are commercially available on a large scale, and that they will be used in lithium metal batteries. We can already make high performance IL-based Li metal pouch cells at BatTRI-Hub. From there, adaption of scaled up manufacturing procedures to analogous plastic-crystal based Li (and ultimately Na) devices will be more feasible.
Have you looked for any difference in performance of a plastic ionic crystal battery or electrochemical cell before and after melting to the liquid state? Does the performance remain the same after returning to the plastic state?
Yes, we have done some studies on this. The performance after melting and re-solidifying the OIPC electrolytes was generally found to be better than before, for two reasons: 1) Some OIPC electrolytes display higher ionic conductivity in the second heating scan (after melt) due to the presence of a fraction of metastable, amorphous phase; 2) Better contact between the electrode and electrolyte after melting and re-solidifying.