Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have discovered a new mechanism to speed up the charging of lithium-ion batteries for electric vehicles. By exposing the cathode to a beam of concentrated light — for example, the white light from a xenon lamp — the battery charging time can be lowered by a factor of two or more. Charging for an electric car that is on empty currently takes about eight hours. Lithium-ion batteries work in a dark state, with the electrodes housed in a case. Argonne's photo-assisted technology would use a transparent container that allows concentrated light to illuminate the battery electrodes during charging. Artistic rendering of Argonne's photo-excitation technology for fast recharging of lithium-ion batteries. Exposing cathodes to light decreases charge time by a factor of two in lithium-ion batteries. Courtesy of Argonne National Laboratory. To probe the charge process, the research team made small lithium-ion cells (“coin cells”) with transparent quartz windows. They tested these cells with and without white light shining through the window onto the cathode. “We hypothesized that, during charging, white light would interact favorably with the typical cathode material, and that proved to be the case in our cell tests,” researcher Christopher Johnson said. This favorable reaction was due in part to the interplay of light with the cathode material, a semiconducting material, lithium manganese oxide (LiMn2O4, or LMO) that is known to interact with light. While absorbing the photons in the light during charging, the element manganese in the lithium manganese changes its charge state from trivalent to tetravalent (from Mn3+ to Mn4+). The induction of a microsecond long-lived charge separated state, consisting of Mn4+ (hole) plus electron, results in more oxidized metal centers, and ejected lithium ions are created under light and with voltage bias. The team found that lithium ions eject faster from the cathode than they would without the photon-excitation process and that the faster reaction resulted in faster charging without degrading battery performance or cycle life. “Our cell tests showed a factor of two decrease in charging time with the light turned on,” Johnson said. “This finding is the first of its kind whereby light and battery technologies are merged.” The discovery that exposure of LMO to light lowers charge transport resistance could lead to new, fast recharging battery technologies for consumer applications and battery-only electric vehicles. The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-12863-6).