The researchers from Southern University of Science and Technology in Shenzhen, Harbin Institute of Technology and North-Western Polytechnical University in Xian created a new electrolyte solution for lithium batteries operating even under extremely low temperatures (down to–60 °C). This development may radically reduce the production costs of the next generation lithium batteries production making them safer for the environment.
Metallic lithium batteries are believed to be one of the highest potential development areas in the energy sector. As an anode, lithium has the record-high specific capacitance and the lowest electrochemical potential among all the metals; however, its high chemical reactivity makes the system unstable. During the charging process, dendrites are growing on lithium surface – thin acicular structures capable of piercing the separator and cause a short circuit. To improve the system’s stability, the so-called high-concentration electrolyte solutions are used, in which almost all the solvent’s molecules are bonded with lithium ions, and thanks to that firm protective film of non-organic compounds is formed on the anode.
The problem is that high concentration of salt makes such electrolytes heavy, expensive and not eco-friendly. Fluorine-containing salts, e.g., lithium bis(fluorine sulfonyl)imide (LiFSI), provide for stability, but make up to 90% of the solution cost and noticeably boost its density and toxicity. For a long time, specialists were not successful in their attempts to decrease the salt content without compromising the properties.
The Chinese researchers decided to check is that was possible to achieve using phenyl fluoride – a light and inexpensive dilutant known for its compatibility with lithium metal. However, when diluting the salt in the mix with extended ether 1,2-dimethoxyethane, the previously stable solution suddenly became aggressive to lithium: the metal started deteriorating already in a few hours, and the electrolyte solution darkened. The Analysis showed that given low content of salt in the solution “free” ether molecules remain, and they launch a chain of side reactions. Lithium interacts with phenyl fluoride forming phenyl-lithium – an active interim compound, which then destroys both the solution and the anode. Eventually, the battery completely loses its performance ability.
To stop this process, the scientists replaced dimethoxy-ethane with dimethyl-acetal – ether with more branched molecule. Its spatial structure impedes formation of active complexes with phenyl-lithium and prevents the start of the chain reaction. In such system phenyl fluoride preserves its chemical inertness, and a stable protective film of non-organic compounds (mainly, of lithium fluoride) is formed on the anode.
The new electrolyte solution (a mix of LiFSI, phenyl fluoride and dimethyl-acetal) demonstrated impressive results. Coulombic efficiency, i.e., the share of electric charge returned by the battery when discharging versus the charging amount reached 99.4% under 25 °C and 97.7% under minus 40 °C. The battery with sulfur-polymer cathode survived through 500 cycles losing only 17% of capacitance. It remained functional even under minus 60 °C, and the specific energy of the pilot cell made 334 W*h/kg – the level comparable with the best modern prototypes.
Thus, phenyl fluoride may perform as a cheap and light component of the electrolyte solution if you select the solvent correctly. The researchers believe that they made an important step on the way towards metallic lithium batteries capable of operating in different climates – from desert or tropical heat to Arctic freezing cold.
