Battery performance at low temperatures analyzed in new study

Battery performance in low temperatures is an ongoing challenge that is being looked into by a number of researchers around the world. Aqueous batteries (in a liquid solution) do better than non-aqueous batteries in terms of rate capability (a measure of energy discharged per unit of time) at low temperatures and scientists at China University of Hong Kong propose optimal design elements of aqueous electrolytes for use in low temperature aqueous batteries.

Researchers reviewed the physicochemical properties of aqueous electrolytes (that determine their performance in batteries) based on several metrics: phase diagrams, ion diffusion rates, and the kinetics of the redox reactions.

The main challenges for low temperature aqueous batteries are that the electrolytes freeze, the ions diffuse slowly, and the redox kinetics (electron transfer processes) are consequently sluggish. These parameters are closely related to the physicochemical properties of the low temperature aqueous electrolytes used in batteries.

To improve battery performance under cold conditions, therefore, requires understanding how the electrolytes respond to cold (–50 oC to –95 oC). Says study author and associate professor Yi-Chun Lu, “To obtain high performance low temperature aqueous batteries (LT-ABs), it is important to investigate the temperature dependent physicochemical properties of aqueous electrolytes to guide the design of low temperature aqueous electrolytes (LT-AEs).”

Evaluating Aqueous Electrolytes

The researchers compared various LT-AEs used in energy storage technologies, including aqueous Li+/Na+/K+/H+/Zn2+-batteries, supercapacitors, and flow batteries. The study collated information from many other reports regarding the performance of diverse LT-AEs, for example an antifreezing hydrogel electrolyte for an aqueous Zn/MnO2 battery; and an ethylene glycol (EG)-H2O based hybrid electrolyte for a Zn metal battery.

They systematically examined equilibrium and non-equilibrium phase diagrams for these reported LT-AEs in order to understand their antifreezing mechanisms. The phase diagrams showed how the electrolyte phase change across changing temperatures. The study also examined conductivity in LT-AEs with respect to temperature, electrolyte concentrations, and charge carriers.

Study author Lu predicted that “ideal antifreezing aqueous electrolytes should not only exhibit low freezing temperature Tm but also possess strong supercooling ability,” i.e. the liquid electrolyte medium remains liquid even below freezing temperature, thus enabling ion transport at ultra-low temperature.

The study authors found that, indeed, the LT-AEs that enable batteries to operate at ultralow temperatures mostly demonstrate low freezing points and strong supercooling abilities. Further, Lu proposes that “the strong supercooling ability can be realized by improving the minimum crystallization time τ and increasing the ratio value of glass transition temperature and freezing temperature (Tg/Tm) of electrolytes.”

The charge conductivity of the reported LT-AEs for use in batteries could be improved by lowering the amount of energy required for ion transfer to occur, adjusting the concentration of electrolytes, and choosing certain charge carriers that promote fast redox reaction rates. Says Lu “Lowering the diffusion activation energy, optimizing electrolyte concentration, choosing charge carriers with low hydrated radius, and designing concerted diffusion mechanism[s] would be effective strategies to improve the ionic conductivity of LT-AEs.”

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