Energy storage via rechargeable battery technology drives our digital lifestyle and supports the integration of renewable energy into the power grid. However, battery function in cold conditions remains a challenge, motivating research to improve low-temperature battery performance. Aqueous batteries (in a liquid solution) perform better than non-aqueous batteries in terms of performance (a measure of energy delivered per unit time) at low temperatures.
New research by engineers from China University of Hong Kong recently published in the journal nano Research Energy, proposes optimal design elements of aqueous electrolytes for use in low-temperature aqueous batteries. The research examines the physico-chemical properties of aqueous electrolytes (which determine their performance in batteries) based on several metrics: phase diagrams, ionic diffusion rates and the kinetics of redox reactions.
The main challenges for aqueous low-temperature 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 physico-chemical properties of the aqueous low-temperature electrolytes used in batteries.
Therefore, to improve battery performance in cold conditions, an understanding of how the electrolytes react to cold (-50 OC to -95 OC/−58 OF to -139 OF). Study author and associate professor Yi-Chun Lu says, “To obtain high-performance low-temperature aqueous batteries (LT-ABs), it is important to study the temperature-dependent physicochemical properties of aqueous electrolytes to guide the design of low-temperature aqueous electrolytes (LT -AEs).”
Evaluation of aqueous electrolytes
Researchers compared various LT-AEs used in energy storage technologies, including aqueous Li+/N / A+/K+/H+/Zn2+– Batteries, supercapacitors and flow batteries. The study collated information from many other reports on the performance of various LT-AEs, such as an antifreeze 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 to understand their antifreeze mechanisms. The phase diagrams showed how the electrolyte phase changes with changing temperatures. The study also examined conductivity in LT-AEs in relation to temperature, electrolyte concentration and charge carriers.
Study author Lu predicted that “ideal antifreeze electrolytes should not only have a low freezing temperature Tm but also possess a strong supercooling ability”, ie the liquid electrolyte medium remains liquid even below freezing temperature, thus enabling ion transport at ultra-low temperature.
The study authors found that the LT-AEs that enable batteries to operate at ultra-low temperatures actually mostly exhibit low freezing points and strong supercooling capabilities. Further, Lu suggests that “the strong supercooling ability can be realized by improving the minimum crystallization time t and increasing the ratio value of the glass transition temperature and the 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, adjusting the electrolyte concentration, and choosing specific charge carriers that promote fast redox reaction rates. Lu: “Lowering the diffusion activation energy, optimizing the electrolyte concentration, selecting charge carriers with small hydrated radius, and designing a concerted diffusion mechanism[s] would be effective strategies to improve the ionic conductivity of LT-AEs.”
In the future, the authors hope to further investigate the physicochemical properties of electrolytes that contribute to improved low-temperature performance of aqueous batteries. “We aim to develop high-performance low-temperature aqueous batteries (LT-ABs) by developing aqueous electrolytes with low freezing temperature, strong supercooling ability, high ionic conductivity, and fast interfacial redox kinetics,” says Lu.
Reference: “Design Strategies for Low-Temperature Aqueous Electrolytes” by Liwei Jiang, Dejian Dong, and Yi-Chun Lu, April 17, 2022, nano research energy.
The paper is authored by Liwei Jiang, Dejian Dong and Yi-Chun Lu.
This research was funded by the Research Grant Council of Hong Kong SAR, China.
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