Knowledge of dynamic and non-equilibrium interfacial processes is essential to the development of high-performance batteries. Yet, elucidation of the dynamics of interfacial states at the nanoscale during battery operation is limited by either the spatial resolution of current methods or their inability to monitor metastable states. Now, writing in Science Advances, Yuzhang Li and colleagues report a cryo-electron microscopy (cryo-EM) approach that enables the nanoscale imaging of dynamic interfaces within lithium metal batteries.

In practice, however, the concept proved challenging to realize, as it requires a cell that is thin enough to be rapidly frozen and that exhibits electrochemical behaviour representative of that in a real cell. The answer to this conundrum was the ‘tweezer cell’ — an electrochemical cell assembled at the tip of a tweezer. The two tips of the tweezer are electrically isolated, with one tip in contact with lithium metal (as the counter electrode), the other tip in contact with a copper transmission electron microscopy grid (as the working electrode) and a battery separator containing the electrolyte sandwiched between them. During lithium deposition, the whole cell can be plunged into liquid propane to freeze the dynamic state. The transmission electron microscopy grid is subsequently removed and transferred into the electron microscope for imaging. “eCryo-EM snapshots at selected time intervals can then be stitched together — a bit like a flipbook — to hint at the dynamic processes happening at key interfaces between battery materials,” explains Li.