HFC electrolyte delivers energy-dense lithium battery that keeps running at −50 °C - Engineering - Energy & Green Tech

Credit: Nankai University

A research team in China has developed an electrolyte using monofluorinated hydrofluorocarbon (HFC) solvents capable of achieving energy densities higher than 700 Wh kg⁻¹ at room temperature and about 400 Wh kg⁻¹ at −50 °C, a significant improvement over current technologies. Their work, recently published in Nature, has potential applications in electric vehicles, aerospace, and grid storage for operation in extreme climates.

Swapping out electrolyte solvents

The electrolyte material in electrochemical energy storage devices, such as lithium batteries, helps to carry charge between the cathode and anode. This is facilitated by solvents that help to dissolve lithium salts. Electrolyte solvents in batteries have traditionally used oxygen- and nitrogen-based ligands. However, these materials hinder charge transfer at the electrode–electrolyte interface, especially under fast-charging or low-temperature conditions due to strong binding.

Scientists have attempted to modify oxygen- and nitrogen-based solvents, but this often resulted in increased solvent viscosity or reduced performance at low temperatures. HFCs have been proposed as a substitution, although some studies showed poor salt solubility and instability with lithium metal. But, the research team involved in the new study believed HFCs could dissolve lithium salts better with some modifications.

"If the exclusive HFCs–F and Li⁺ coordination can be delicately designed by strengthening the Lewis basicity of F atoms to enable substantial Li-salt dissolution, the predictable low binding energies will promote the interfacial kinetics of energy-dense batteries," they write.

Higher energy density, lower temperatures

To achieve these goals, the team synthesized and characterized six different HFC solvents, and then tested their electrochemical performance in coin and pouch cells across a wide temperature range. The newly synthesized solvents were found to dissolve lithium salts at >2 mol/L. One solvent, in particular, exceeded expectations, working well even at a temperature of −50 °C.

"Among them, 1,3-difluoropropane (DFP)-based Li-ion electrolyte is endowed with all merits for energy-dense and low-temperature batteries, including low viscosity (0.95 cp), high oxidation stability (>4.9 V) and ionic conductivity of 0.29 mS cm⁻¹ at −70 °C. By incorporating F atoms in the first solvation shell, the weak F–Li⁺ coordination facilitates the Li plating/stripping process with Coulombic efficiency (CE) up to 99.7% and exchanges current density one magnitude larger than O–Li⁺ coordination at −50 °C.

"The electrolytes further enable the operation of lithium-metal pouch cells under an electrolyte amount of less than 0.5 g Ah⁻¹, achieving energy densities greater than 700 Wh kg⁻¹ at room temperature and about 400 Wh kg⁻¹ at −50 °C," the study authors write.

Current high-performance cells, such as those used in electric vehicles, typically reach around 250–270 Wh/kg at room temperature, meaning the new electrolyte offers a marked improvement and ability to use batteries in extreme climates. The team notes that further improvements could improve temperature range and stability even more.

The study authors write, "By further modulating the carbon and fluorine numbers, high-boiling-point (>100 °C) HFCs with well-maintained Li-metal compatibility can be designed. The F-coordination chemistry puts forward a promising pathway to break the power and energy density ceiling of batteries."

by Krystal Kasal, Phys.org

edited by Gaby Clark, reviewed by Robert Egan 

Source: HFC electrolyte delivers energy-dense lithium battery that keeps running at −50 °C