Supersalty Water Boosts Battery Safety

Flammable solvents currently used in lithium-ion batteries, such as ethylene carbonate, can turn the devices into chemical volcanoes if they fail. Aqueous electrolytes are a safe alternative, but water breaks down at the voltages required by electric cars and other power hogs. That may change thanks to a team led by Liumin Suo and Chunsheng Wang from the University of Maryland, College Park, and Kang Xu from the Army Research Laboratory. To boost the stability of water in a battery, the team cranked up the concentration of the aqueous electrolyte (Science 2015, DOI: 10.1126/science.aab1595). Typical batteries use electrolytes with salt concentrations of about 6 M or less, Xu and Wang tell C&EN. They instead used lithium bis(trifluoromethane sulfonyl)imide solutions at better than 20 M. The salty solution roughly doubled water’s useful voltage range in batteries. In these concentrated electrolytes, the salt’s anions reduce before water starts breaking down electrochemically. A lithium fluoride film then forms at the anode, the team speculates, protecting against the decomposition of water at elevated voltages. This is the first report of such a protective barrier in a battery using aqueous chemistry, the team says.