Seen close-up, the flow of ions between battery electrodes is actually a series of erratic, atomic-scale hops. Experiments in a laser lab at SLAC National Accelerator Laboratory showed that when shaken with a jolt of voltage, most ions briefly hopped backward to their previous positions before resuming their usual erratic journeys — the first indication that they remembered, in a sense, where they had just been.

Credit: Greg Stewart/SLAC National. Accelerator Laboratory