Design Guidelines for Anode Modifications to Eliminate Soft Short Circuits in Anode‐Free Na Batteries Under High Current Density With Large‐Capacity Plating, Demonstrated via Initiated Chemical Vapor Deposition (iCVD)

Abstract

ABSTRACT 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 Na 3 V 2 (PO 4 ) 3 cathodes delivered 90.1 mAh g −1 at 1C and retained 90.8% capacity after 200 cycles, demonstrating practical robustness.