Cation‐Diffusive Carbon Interlayers Stabilize Na Metal and Double the Current in Na‐S Redox‐Flow Batteries for Grid‐Scale Energy Storage

ABSTRACT The sodium‐sulfur nonaqueous redox‐flow batteries (Na‐S NARFBs) using earth‐abundant elements are highly attractive due to the low material cost and improved energy density for grid‐scale energy storage. However, the low current performance, poor Na ⁰ /Na ⁺ redox kinetics, and Na dendrite growth pose severe challenges. We introduce cation‐diffusive layers (CDLs): thin and Na ⁺ affinitive interlayers at the Na anode that direct Na ⁺ transport and stabilize Na deposition. Benchmarking three archetypal materials—carbon paper (CP), glass microfiber paper (GF), and foam—across Na‐Na and Na‐Cu, and Na‐S cells identifies CP as the optimum. CP reduces symmetric cell overpotential by more than 70%, achieves 98% Na plating‐stripping efficiency, and doubles the Na‐S cell current density from 0.5 to 1.0 mA cm ⁻² without sacrificing capacity or efficiency. Ex situ electrochemical and SEM/XPS analysis, combined with molecular dynamics (MD) studies, reveal that electron‐rich carbon fibers disperse supporting salt aggregates, enrich near‐surface Na ⁺ density, and create ion transport pathways for fast Na ⁰ /Na ⁺ exchange while mitigating membrane degradation. Because of the ion‐centric mechanism, CDLs can be generalized to other metal‐anode designs. Further, this work establishes CDL design rules—cationic affinity and appropriate micro/nanostructure—as a simple, scalable route to high‐current, durable metal‐anode flow batteries.