Design Guidelines for Anode Modifications to Eliminate Soft Short Circuits in Anode-Free Na Batteries Under High...

Soft short circuits, an underrecognized failure mode, are especially critical in anode-free sodium systems, where limited Na inventory makes cells highly vulnerable to irreversible Na loss. However, clear interphase-design principles for suppressing soft shorting remain lacking because its interfacial origin and governing descriptors are still poorly understood. Herein, we propose a mechanistically guided interphase design strategy for soft-short-free Na metal anodes under practical conditions. An ultrathin (20 nm) poly(1H,1H,7H-dodecafluoroheptyl acrylate) (pDFHA) layer was conformally deposited on an Al current collector via a solvent-free vapor-phase polymerization process. Its minimal thickness and intrinsic ionic conductivity enabled favorable Na⁺ migration kinetics by shortening interfacial transport distance. Upon initial Na plating, interfacial conversion generated a ∼4 nm NaF-rich inorganic domain, forming an integrated organic-inorganic hybrid interphase. Finite element analysis showed that the laterally uniform interphase homogenized Na⁺ flux and mitigated localized current amplification, thereby markedly reducing soft short initiation. Meanwhile, the hybrid architecture combined mechanical strength with the ability to accommodate substantial volume fluctuation. As a result, symmetric cells with pDFHA-modified electrodes cycled stably for over 2000 h without soft shorting. Anode-free full cells paired with Na₃V₂(PO₄)₃ cathodes delivered 90.1 mAh g^-1 at 1C and retained 90.8% capacity after 200 cycles, demonstrating practical robustness.