Methods and reports for improving battery performance in low temperature environments

　　Storing energy through rechargeable battery technology has since enabled our digital lifestyles to be fully powered, and on the one hand, renewable energy sources can be integrated into the grid. However, battery function in cold conditions remains a challenge, prompting research to improve the low-temperature performance of batteries.

　　SES Power has successfully launched lithium iron phosphate batteries that can be used at -60 degrees Celsius and can be discharged at a rate of -40 degrees Celsius. They have been used in cold and high temperature areas, such as outdoor power tools, electric sleds, electric Snow removal vehicles, outdoor monitoring, power grid monitoring, etc. But we are still concerned with different routes for improving the low temperature performance of lithium batteries.

　　The study found that aqueous batteries (in liquid solutions) were better than non-aqueous batteries in terms of discharge rate (a measure of energy released per unit time) at low temperatures.

　　New research by engineers at the University of Hong Kong, recently published in the journal Nano Research Energy, proposes optimal design elements for aqueous electrolytes for low-temperature aqueous batteries. The study examined the physicochemical properties of aqueous electrolytes (determining their performance in batteries) based on several metrics: phase diagrams, ionic diffusivity, and the kinetics of redox reactions.

　　The main challenge of low-temperature aqueous batteries is that the electrolyte freezes and ions diffuse slowly, resulting in sluggish redox kinetics (electron transfer process). These parameters are closely related to the physicochemical properties of the low-temperature water-based electrolytes used in batteries.

　　Therefore, to improve the performance of batteries in cold conditions, it is necessary to understand the response of electrolytes to cold (-50 oC to -95 oC/-58 oF to -139 oF). "In order 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)," said study author and associate professor Yi-Chun Lu. "

　　The figure shows the design strategy of water electrolyte, including antifreeze thermodynamics, ion diffusion kinetics, and interfacial redox kinetics.

　　The researchers compared various LT-AEs for energy storage technologies, including Li+/Na+/K+/H+/Zn2+-batteries, supercapacitors, and flow battery technologies. This study collates information from many other reports on the performance of various LT-AEs, such as antifreeze hydrogel electrolytes for Zn/MnO2 water batteries; and ethylene glycol (EG)-H2O hybrid electrolytes for Zn metal batteries.

　　They systematically studied the equilibrium and nonequilibrium phase diagrams of these reported LT-AEs to understand their antifreeze mechanisms. The phase diagram shows the changes in the electrolyte phase at different temperatures. The study also examined the electrical conductivity of LT-AEs as a function of temperature, electrolyte concentration, and charge carriers.

　　Study author Lu predicts that "an ideal antifreeze water electrolyte should not only exhibit a low freezing temperature Tm, but also possess a strong overload capability," that is, the liquid electrolyte medium remains liquid even at sub-freezing temperatures, enabling ultra-low temperature ion transport.

　　The study authors found that most of the LT-AEs that enabled the battery to operate at ultra-low temperatures exhibited low freezing points and strong supercooling capabilities. Furthermore, "strong overload capability can be achieved by increasing the minimum crystallization time t and increasing the ratio of the glass transition temperature and freezing temperature (Tg/Tm) of the electrolyte".

　　The charge conductivity of the reported LT-AEs for batteries can be improved by reducing the energy required for ion transfer to occur, adjusting the concentration of the electrolyte, and selecting certain charge carriers that promote fast redox reaction rates.

　　"Lowering the diffusion activation energy, optimizing the electrolyte concentration, selecting charge carriers with low hydration radii, and designing a cooperative diffusion mechanism would be effective strategies to improve the ionic conductivity of LT-AEs," Lu said.

　　In the future, the authors hope to further investigate the physicochemical properties of electrolytes that can help improve the performance of water batteries at low temperatures. "We hope to develop high-performance low-temperature water batteries (LT-ABs) by designing water-based electrolytes with low freezing temperature, strong supercooling capability, high ionic conductivity, and fast interfacial redox kinetics," Lu said.