Lithium metal batteries could store more charge in a space than lithium-ion, so there is a race to make them for next-generation electronics and cars.

One of the obstacles is the silent struggle between two components of the battery. The electrolyte (the liquid between the electrodes) corrodes the anode’s surface and covers it with a thin layer known as the solid-electrolyteinterphase or SEI.

Researchers believe that SEI formation is inevitable. However, they want to manage and stabilize this growth in order to optimize the battery’s performance. They haven’t been able to see what the SEI looks like after it has been saturated with electrolyte.

Researchers from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have now taken the first high-resolution images of the layer in its plump, squishy natural state. Cryogenic electron microscopy , a groundbreaking technology that reveals as little as atoms, enabled this breakthrough.

According to the researchers, the results suggest that the right electrolyte may reduce swelling and improve battery performance. This could give scientists a new tool for tweaking and improving battery design. Researchers also have a new tool to study batteries in everyday work environments.

Their work was described in a paper published by Science January 6, 2022.

Zewen Zhang, a Stanford PhD candidate who conducted the experiments in collaboration with SLAC and Stanford professors Yi Cui & Wah Chiu, stated that there are no other technologies that can view the interface between the electrode & the electrolyte at such high resolution. “We wanted to show that we can image the interface at previously inaccessible scales, and see the pure, native state these materials in batteries.

Cui said, “We believe this swelling to be almost universal.” Although its effects are not well understood by the battery research community, we discovered that they have a significant effect on battery performance.

An ‘exciting’ tool for energy research

This is the latest in a string of remarkable results that have been published over the past five year. They show that cryo-EM, originally developed for biology, offers “thrilling possibilities” in energy research. The team also wrote a separate review in July in Accounts.

CryoEM is a type of electron microscopy that uses electrons instead of light to see the world of the very tiny. Scientists can view the cells that perform life’s functions by flash-freezing samples to a transparent, glassy state. Three scientists received the 2017 Nobel Prize for Chemistry for their innovative contributions to cryo-EM.

Cui and Chiu were inspired by success stories in biological cryoEM. They teamed up to investigate whether cryoEM could be as useful a tool to study energy-related materials, as it is for studying living systems.

One of those SEI layers that can be found on a battery electrode was one of their first considerations. In 2017, they published the first atomic-scale images and images of finger-like growths in lithium wire. These can penetrate the barrier between the battery’s two halves, causing short circuits or fires.

To make these images, they had to remove the battery components from the electrolyte so that the SEI could dry into a shrunken condition. It was impossible to imagine what it would look like inside a functioning battery.

Blotter paper comes to your rescue

Researchers devised a method to freeze thin electrolyte liquid films that contained small lithium metal wires. This allowed for the capture of SEI in its native environment.

They first inserted a metal grid that was used to hold cryo-EM samples in a coin cell. They removed the grid and found that thin electrolytes adhered to the tiny holes in the grid. This was held in place by the surface tension for just enough time to complete the rest of the steps.

But the films were too thick to allow the electron beam through and produce sharp images. Chiu suggested that blotter papers be used to absorb the liquid. To freeze the tiny films, the blotted grid was immersed in liquid nitrogen. This preserved the SEI perfectly. The films were protected from air exposure by the closed system.