Cu Nanocluster Size Effect Inducing the Transformation of Polysulfides to Cu₂S/CuS for Durable Sodium Storage

The limited conversion efficiency of polysulfides (PSs) in sodium-sulfur batteries remains a critical bottleneck to achieving optimized sulfur utilization and stable cycling. While copper-based materials present promise in anchoring PSs, the dynamic evolution of Cu nanostructures during cycling and their size-dependent interaction with PSs are poorly understood. Herein, we reveal a size-governed electrochemical mechanism in which Cu nanoclusters (<1 nm) dynamically regulate the phase transition between Cu2S and CuS to enable reversible sulfur redox chemistry. Operando analysis demonstrates that Cu foil-derived nanoclusters form single-crystalline Cu2S via strong electrostatic coupling with long-chain PSs, effectively suppressing shuttling. As cycling progresses, Cu nanocluster refinement lowers the energy for Cu2S-to-CuS conversion, creating a kinetically favorable dual-phase structure (Cu2S outer/CuS inner) that accelerates solid-phase sulfur regeneration. Simultaneously, the Cu nanoclusters and CuS synergistically catalyze PSs-to-S conversion, achieving near-theoretical sulfur utilization. The S-loaded carbon on the Cu foil (S@C-Cu) cathode delivers ultrastable cycling (1164.9 mAh g-1 after 3000 cycles at 5 A g-1) and high areal capacity (3.68 mAh cm-2 in pouch cell). This size-effect-driven phase evolution is generalizable to Cu9S5, Cu2S, NiS2, and CoS2. Our work bridges nanoscale metal dynamics with macroscopic battery performance, offering atomic-level insights into sulfur electrochemistry for practical energy storage.