Mg‐Gradient Polyacrylonitrile Nanofiber Layer Enabling Li Coble Creep and Stress Relaxation in Anode‐Free All‐Solid‐State Batteries

Anode‐free all‐solid‐state batteries (AF‐ASSBs) offer high energy density and safety but suffer from low Coulombic efficiency (CE) and poor cycle stability due to uneven Li deposition and interfacial contact loss. Here, we present an Mg‐gradient polyacrylonitrile nanofiber protective interlayer (PAN@Mg) for AF‐ASSBs. The nanoporous PAN structure and the mixed ionic‐electronic conductivity of lithiated Mg facilitate Li Coble creep, guiding Li toward low‐stress regions. The PAN@Mg layer, with Mg‐rich top and Mg‐deficient bottom regions, was separated during Li deposition. This unique configuration, supported by the superior elasticity of PAN nanofibers, alleviates stress‐induced solid electrolyte (SE) cracking and maintains interfacial integrity. Furthermore, the separation of Li and SE prevents their direct contact, suppressing side reactions and Li dendrite growth. The cell employing SUS‐PAN@Mg achieves stable cycling with an average CE of 99.94% and a 46.8% improvement in cycle retention over SUS@Mg after 300 cycles. Moreover, the ultra‐thin (~2 µm) PAN@Mg layer can enhance volumetric energy density up to 925 Wh L −1 .