An Aqueous Electrolyte for Fast Energy Storage at -70oC
Authors: Viola Wesley , Andrew Trisha L. 
Published: December 2020

Sustainable, safe energy storage will be key to fully utilizing renewable power sources and ending the dependence on fossil fuels. One significant limitation of electrochemical energy storage, including batteries and supercapacitors, is performance loss or complete failure at low temperatures. This shortcoming impacts the efficiency of mature technologies such as electric vehicles in cold climates. In more extreme environments, such as in arctic regions and at high altitudes, the development of power-demanding technologies is even more severely limited. Clearly, a means of low temperature energy storage is needed.

For commercial lithium-ion batteries, the lower limit of operation is typically -20oC; supercapacitors are more robust and can perform as low as -40oC. At temperatures lower than this, failure occurs at two sites: the electrolyte and the electrode-electrolyte interface. Freezing and/or precipitation of salt species in the electrolyte will typically destroy its ability to carry an ionic current, ceasing redox operation in supercapacitors or batteries. Secondly, the charge transfer resistance at the electrode-electrolyte interface is highly temperature dependent and is another source of low temperature failure, particularly for intercalation type electrodes. Due to these considerations, the design of low temperature energy storage focuses primarily on the electrolyte and the electrode interfaces. The simplest approach is to add a low-freezing solvent (such as dioxolane) to existing organic electrolytes; this solves the issue of electrolyte freezing but not necessarily the increased charge transfer resistance at low temperatures. Other efforts also include developing low-freezing ionic liquid mixtures matched to activated carbons with specially engineered pore distributions to accommodate bulky ions. Such materials enable energy storage down to -70oC, though they are exotic chemicals that have a correspondingly high cost.

Perhaps an unlikely candidate for low temperature energy storage are aqueous electrolytes. Despite the growing interest for this safe and low-cost alternative to organic electrolytes, little consideration has been made for these materials in the context of low temperature energy storage, maybe due to the high freezing point of pure water. Here, we find that a mixture of the commodity salt lithium chloride (LiCl) and water forms an excellent low temperature electrolyte, supporting energy storage in supercapacitors as low as -70oC. To the best of our knowledge, the lowest reported performance of an aqueous based energy storage device is -40oC. The electrolyte used here is a eutectic mixture of LiCl (24.8 wt%) in water with a massively depressed freezing point of -74 oC, allowing for high conductivity (~1.83 mS/cm) even at -70oC. Efficient charging of an off-the-shelf activated carbon (Kuraray) at low temperatures demonstrates the practical nature of the electrolyte, possibly owed to the excellent charge-transfer supporting characteristics of water as a solvent. Not only does this aqueous electrolyte support exceptional low-temperature charging kinetics, its stability window is widened from 1.95 to 2.96 V from room temperature to -70oC, making it feasible when compared to organic electrolytes.

Taken together, these features highlight the unexpected performance of aqueous electrolytes for low temperature applications. Given the low cost of this electrolyte, it shows great potential for energy storage in arctic regions as well as in high-altitude gliders where energy management must be performed at temperatures as low as -60oC. More broadly speaking, the efficiency (and safety and cost) of electric vehicles could also be vastly improved in cold climates using such a eutectic LiCl electrolyte.

Figure 1