Carbonaceous Interlayers as Contact Mediators for Sodium Electrodeposition in “Anode-Free” Solid-State Batteries

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. ... mehrThese 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.