Solid-state batteries store and release charge by pushing ions back and forth between two electrodes. From our usual point of view, the ions flow through the solid electrolyte of the battery like a soft stream.
But when seen at the atomic scale, this smooth flow is an illusion: individual ions randomly jump from one open space to another in the spacious atomic lattice of the electrolyte, pushing toward the electrode by a constant voltage. These spikes are difficult to predict, and difficult to cause and detect.
Now, in the first study of its kind, researchers have given the jumping ions a jolt of voltage by hitting them with a pulse of laser light. To their surprise, most of the ions briefly changed direction and returned to their previous positions before resuming their normal, more random travels. This was the first indication that ions somehow remember where they've just been.
A research team from the Department of Energy's SLAC National Accelerator Laboratory, Stanford University, the University of Oxford and the University of Newcastle described what they found, in the January 24 issue ofNature.
Electronic cornstarch
"You can think of the ions as behaving like a mixture of cornstarch and water,", — said Andriy D. Poletaev, an Oxford doctoral student who helped conduct the experiment when he was a postdoctoral fellow at SLAC. If we gently push this cornstarch mixture, it will flow like a liquid; but if we pierce it, it becomes solid. The ions in the battery are like electronic cornstarch. They resist the strong jolt from the shock of the laser light by moving backwards.
“Fuzzy memory” ions, as Poletaev puts it, lasts only a few billionths of a second. But knowledge of its existence will help scientists predict for the first time what traveling ions will do next – an important moment for the discovery and development of new materials.
Electrolyte designed for speed
For their experiments at the SLAC Laser Laboratory, the researchers used thin, transparent crystals of a solid electrolyte from a group of materials called beta-alumina. These materials were the first highly conductive electrolytes discovered. They contain tiny channels through which hopping ions can move quickly, and have the advantage of being safer than liquid electrolytes. Beta alumina is used in solid-state batteries, sodium-sulfur batteries, and electrochemical cells.
As the ions jumped through the beta-alumina channels, the researchers hit them with pulses of laser light lasting only trillionths of a second, then measured the light, which returned from the electrolyte.
By varying the time between the laser pulse and the measurement, they were able to determine exactly how the ion's speed and preferred direction changed a few trillionths of a second after the laser pulse.
Strange and Unusual< /h4>
"A lot of strange and unusual things happen during the ion jump", — said SLAC and Stanford professor Aaron Lindenberg, a researcher at the Stanford Institute for Materials Science and Energy (SIMES) who led the study.
"When we apply a force that shakes the electrolyte, the ion does not respond immediately, as in most materials", — he said. "Ion can sit for a while, jump up suddenly, and then sit there again for quite a long time. You might have to wait a while and then suddenly there is a giant shift. So there is an element of randomness in this process that complicates these experiments.
Until now, the researchers say, the way ions travel has been considered a classic “random walk'': they jostle, bump and swallow like a drunken man staggering down the sidewalk, but eventually reach a destination in a way that may seem intentional to the observer. . Or think of a skunk releasing a foul-smelling spray into a room full of people; the molecules in the spray randomly push and collide, but reach your nose too quickly.
When it comes to bouncing ions, "this picture turns out to be wrong at the atomic scale", — said Poletaev, "but it is not the fault of the people who came to such a conclusion. It's just that researchers have been studying ion transport with macroscopic instruments for so long that they couldn't observe what we saw in this study.
The atomic-scale discoveries made here, he said, “will help bridge the gap.'' between the atomic motions that we can simulate in a computer and the macroscopic properties of the material that made our research so difficult.