Discovering Underpotentially Deposited Crystalline Sodium Metal Nanoparticles on the Surface of Hard Carbon Anodes

Abstract Sodium-ion batteries are attractive candidates for low-cost and scalable energy storage in which hard carbon (HC) anodes are used. However, sodium (Na) metal plating in HC anodes at voltages above the plating potential remains a critical problem. The location and spatial distribution of crystalline sodium (c-Na) metal nanoparticles, the clusters associated with Na storage in HCs, are important for understanding this underpotential plating behavior but remain unclear. Here, we implement a cryogenic electron microscopy (cryo-EM) and electron tomography (cryo-ET) workflow that directly localizes c-Na metal clusters in sodiated HC samples: cryo-ET maps nanoscale three-dimensional (3D) distributions, while cryo-EM resolves local structure and chemistry. Our workflow reveals that c-Na metal clusters reside predominantly on the surfaces or near surfaces of individual HC particles. This finding challenges existing interpretations in the literature that locate the c-Na metal clusters within closed nanopores in the bulk of the HC, presumably because prior characterization using nonimaging techniques and two-dimensional (2D) projection images associated with traditional transmission electron microscopy (TEM) lacks the spatial 3D information necessary to resolve the full spatial distribution. Our discovery is an important step toward explaining the underpotential plating phenomenon of Na metal in HC anodes, suggesting that particle-level surface and near-surface interfaces are relevant targets to mitigate plating alongside bulk-pore, electrode, and electrolyte/interphase design.