Charging an electric car today takes at least half an hour, whereas refueling a gasoline car takes just a few minutes. A team of scientists from the University of Maryland led by Professor Chunsheng Wang is looking for a way to cut this time to 10 minutes - roughly how long it takes to drink a cup of coffee. They described this in an article in the journal Science.
The US Department of Energy (DOE) has set a goal: to charge a battery to 80% in less than 15 minutes. Wang's laboratory aims to surpass this benchmark. "By the time you finish your coffee, the car will be charged," says researcher Chang-Xin Zhao.
Earlier attempts to speed up charging amounted to changing the structure of the electrodes - the parts through which current flows. But this reduced the battery's capacity, that is, its energy reserve. Wang's group decided to bypass the trade-off by turning to chemistry.
The scientists discovered a limitation in the electrolyte - the liquid medium through which lithium ions move during charging and discharging. During charging, ions travel from the positive electrode to the negative one, dragging solvent molecules along with them. If charging is slow, the solvent has time to return into the pores of the positive electrode. During fast charging it does not keep up, the electrode dries out, and the process slows sharply. This phenomenon is called electro-osmotic drag; it has long been known in fuel cells, but in lithium-ion batteries it had not been taken into account until now.
The researchers modified the electrolyte by adding fluorine-containing chemical groups to the solvent molecules. This weakened the attraction between the solvent and lithium ions and helped the liquid return more easily to the positive electrode. As a result, the risk of drying out decreased even during fast charging.
The idea was tested on a small battery - a so-called pouch cell. Fully charging it without losing other characteristics is difficult, but the results met the DOE goals: 80% in 12.8 minutes and 90% in 14.7 minutes.
Now Wang wants to cut the time to 10 minutes by combining the fluorinated solvent with another development - an adaptive electrolyte that separates and protects both electrodes from the stresses of fast charging. In addition, the team is working on increasing battery shelf life in the idle state and expanding the operating temperature range. "We want to create a battery that meets all requirements without compromising anything," says Wang.