Our digital lifestyles are powered by energy storage by rechargeable battery technology, which also helps renewable energy integration into the power grid. However, battery performance in freezing temperatures remains a concern, spurring research into increasing battery low-temperature performance. At low temperatures, aqueous batteries (in a liquid solution) perform better than non-aqueous batteries in terms of rate capability (the amount of energy discharged per unit of time).
Engineers from the China University of Hong Kong have proposed optimal design components for aqueous electrolytes for use in low-temperature aqueous batteries in a new study published in the journal Nano Research Energy. The study examines the physicochemical features of aqueous electrolytes (which govern their battery performance) using numerous metrics, including phase diagrams, ion diffusion rates, and redox reaction kinetics.
The electrolytes freeze, the ions diffuse slowly, and the redox kinetics (electron transfer processes) are thus sluggish in low-temperature aqueous batteries. The physicochemical characteristics of the low-temperature aqueous electrolytes used in batteries are strongly connected to these factors.
Understanding how the electrolytes respond to cold (–50 oC to –95 oC / –58 oF to –139 oF) is required to increase battery performance under cold temperatures. “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),” says study author and associate professor Yi-Chun Lu.
Evaluating Aqueous Electrolytes
The researchers examined aqueous Li+/Na+/K+/H+/Zn2+-batteries, supercapacitors, and flow batteries, as well as other LT-AEs employed in energy storage technologies. The study compiled data from a number of earlier papers on the performance of various LT-AEs, such as an antifreezing hydrogel electrolyte for an aqueous Zn/MnO2 battery and a hybrid electrolyte based on ethylene glycol (EG) and water for a Zn metal battery.
To further understand the antifreezing processes of these reported LT-AEs, they looked at equilibrium and non-equilibrium phase diagrams. The phase diagrams demonstrated how the electrolyte phase changes as temperature changes. The researchers looked into LT-AE conductivity in relation to temperature, electrolyte concentrations, and charge carriers.
“Ideal antifreezing aqueous electrolytes should not only exhibit low freezing temperature Tm but also possess strong supercooling ability,” according to study author Lu, which means the liquid electrolyte medium remains liquid even below freezing temperature, allowing ion transport at extremely low temperatures.
Indeed, the LT-AEs that enable batteries to operate at ultralow temperatures have low freezing points and strong supercooling characteristics, according to the study's authors.
"The strong supercooling ability can be realized by improving the minimum crystallization time t and increasing the ratio value of glass transition temperature and freezing temperature (Tg/Tm) of electrolytes,” Lu adds.
Lowering the amount of energy required for ion transfer, changing the quantity of electrolytes, and selecting charge carriers that promote rapid redox reaction rates might increase the charge conductivity of the reported LT-AEs for application in batteries. 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,” Lu says.
The authors plan to expand their research into the physicochemical features of electrolytes that lead to better aqueous battery performance at low temperatures in the future. “We would like to develop high-performance low-temperature aqueous batteries (LT-ABs) by designing aqueous electrolytes possessing low freezing temperature, strong supercooling ability, high ionic conductivity, and fast interfacial redox kinetic,” explains Lu.