Till Ortmann

Interfacial contact limitations challenge reversible, high-rate sodium deposition in “anode-free” sodium solid-state batteries and motivate interlayer-based interface engineering strategies. Here, we systematically investigate how microstructural properties of non-graphitic carbon interlayers, i.e., the structural arrangement of graphene domains, particle size, and network porosity, govern the early stages of sodium nucleation and growth in NaSICON-based half-cells (≤ 750 μAh·cm−2). Comparing coarse- and fine-particle hard carbon and carbon black interlayers (23–1200 nm), prepared by tape-casting, shows that finer, more compactable carbons improve solid electrolyte contact and promote uniform lateral sodium growth while suppressing dendrites. Focused ion-beam scanning electron microscopy and energy-dispersive X-ray spectroscopy confirm that sodium electrodeposition occurs predominantly at the interlayer|SE interface, with partial sodium insertion into the carbon framework. Mechanical interlocking between the compliant carbon interlayer and a rough solid electrolyte surface enhances adhesion, stabilizes interfacial contact, prevents delamination of the carbon-coated current collector, and promotes laterally extended sodium growth. These findings elucidate how tailored interlayer microstructure and interfacial mechanics govern sodium electrodeposition behavior. Carbonaceous interlayers mediate interfacial contact and homogenize current distribution at the current collector|solid electrolyte interface, thereby defining design principles for solid-state sodium batteries during electrodeposition.

“Anode‐free” solid‐state battery concepts are explored extensively as they promise a higher energy density with less material consumption and simple anode processing. Here, the homogeneous and uniform electrochemical deposition of alkali metal at the interface between current collector and solid electrolyte plays the central role to form a metal anode within the first cycle. While the cathodic deposition of lithium has been studied intensively, knowledge on sodium deposition is scarce. In this work, dense and uniform sodium layers of several microns thickness are deposited at the Cu|Na_3.4Zr₂Si_2.4P_0.6O₁₂ interface with high reproducibility. At current densities of ≈1 mA∙cm⁻², relatively uniform coverage is achieved underneath the current collector, as shown by electrochemical impedance spectroscopy and 3D confocal microscopy. In contrast, only slight variations of the coverage are observed at different stack pressures. Early stages of the sodium metal growth are analyzed by in situ transmission electron microscopy revealing oriented growth of sodium. The results demonstrate that reservoir‐free (“anode‐free”) sodium‐based batteries are feasible and may stimulate further research efforts in sodium‐based solid‐state batteries.

In recent years, all‐solid‐state batteries (ASSBs) with metal anodes have witnessed significant developments due to their high energy and power density as well as their excellent safety record. While intergranular dendritic lithium growth in inorganic solid electrolytes (SEs) has been extensively studied for lithium ASSBs, comparable knowledge is missing for sodium‐based ASSBs. Therefore, polycrystalline Na‐β″‐alumina is employed as a SE model material to investigate the microstructural influence on sodium filament growth during deposition of sodium metal at the anode. The research focuses on the relationship between the microstructure, in particular grain boundary (GB) type and orientation, sodium filament growth, and sodium ion transport, utilizing in situ transmission electron microscopy (TEM) measurements in combination with crystal orientation analysis. The effect of the anisotropic sodium ion transport at/across GBs depending on the orientation of the sodium ion transport planes and the applied electric field on the current distribution and the position of sodium filament growth is explored. The in situ TEM analysis is validated by large field of view post‐mortem secondary ion mass spectrometer (SIMS) analysis, in which sodium filament growth within voids and along grain boundaries is observed, contributing to the sodium network formation potentially leading to failure of batteries.