Energy storage with rechargeable battery technologies powers our digital lifestyles and supports the integration of renewables into the power grid. However, operating batteries in cold conditions is an ongoing challenge, motivating researchers to improve battery performance at low temperatures. Aqueous batteries (in a liquid solution) outperform non-aqueous batteries in throughput capability (a measure of energy discharged per unit time) at low temperatures.
New research from engineers at the Chinese University of Hong Kong published April 17 in Nanometer Research Energy offers optimal design elements of aqueous electrolytes for use in low temperature aqueous batteries. The research reviews the physicochemical properties of aqueous electrolytes based on several metrics: phase diagrams, ion diffusion rates, and kinetics of 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 process) are consequently slow. These parameters are closely related to the physico-chemical properties of low temperature aqueous electrolytes used in batteries.
To improve battery performance in cold conditions, it is therefore necessary to understand how electrolytes react to cold (–50 ohC to -95 ohVS). Study author and associate professor Yi-Chun Lu states that “To obtain high-performance low-temperature aqueous batteries (LT-ABs), it is important to study the temperature-dependent physicochemical properties of electrolytes to guide the design of low-temperature aqueous electrolytes (LT-AB).-AE).”
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 gathered information from many other reports regarding the performance of various LT-AEs, for example an antifreeze hydrogel electrolyte for an aqueous Zn/MnO.2 battery; and an ethylene glycol (EG)-H2O-based hybrid electrolyte for a Zn metal battery.
The study systematically examined equilibrium and non-equilibrium phase diagrams for these reported LT-AEs to understand their antifreeze mechanisms. Phase diagrams showed how the phase of the electrolyte changes with changing temperatures. The study also examined the conductivity of LT-AEs as a function of temperature, electrolyte concentrations, and charge carriers.
Study author Lu predicted that “ideal antifreeze aqueous electrolytes should not only exhibit a low freezing temperature Jm but also possess a high supercooling capacity”, i.e. the liquid electrolyte medium must remain liquid even below the freezing temperature, thus allowing the transport of ions at very low temperatures.
The study authors found that indeed, LT-AEs that enable batteries to operate at ultra-low temperatures primarily exhibit low freezing points and strong supercooling capabilities. Further, Lu proposes that “the strong supercooling ability can be achieved by improving the minimum crystallization time τ and increasing the value of the ratio between the glass transition temperature and the freezing temperature (Jg/Jm) of electrolytes.”
The charge conductivity of LT-AEs reported for use in batteries could be improved by reducing the amount of energy required for ion transfer, adjusting the concentration of electrolytes, and choosing certain charge carriers that promote higher rates. rapid redox reactions. Lu says that “lowering diffusion activation energy, optimizing electrolyte concentration, choosing low hydrated radius charge carriers, 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 help improve the performance of aqueous batteries at low temperatures. “We would like to develop high-performance low-temperature aqueous batteries (LT-AB) by designing aqueous electrolytes possessing low freezing temperature, strong supercooling ability, high ionic conductivity, and fast interfacial redox kinetics,” says Lu.
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Yi-Chun Lu et al, Design Strategies for Low Temperature Aqueous Electrolytes, Nanometer Research Energy (2022). DOI: 10.26599/NRE.2022.9120003
Provided by Tsinghua University Press
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