Cu Nanocluster Size Effect Inducing the Transformation of Polysulfides to Cu<sub>2</sub>S/CuS for Durable Sodium Storage
Canpei Wang, Mengting Zheng, Tiefeng Liu, Liguang Wang, Jun Lü
Abstract
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 Cu 2 S and CuS to enable reversible sulfur redox chemistry. Operando analysis demonstrates that Cu foil-derived nanoclusters form single-crystalline Cu 2 S via strong electrostatic coupling with long-chain PSs, effectively suppressing shuttling. As cycling progresses, Cu nanocluster refinement lowers the energy for Cu 2 S-to-CuS conversion, creating a kinetically favorable dual-phase structure (Cu 2 S 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 Cu 9 S 5, Cu 2 S, NiS 2, and CoS 2 . Our work bridges nanoscale metal dynamics with macroscopic battery performance, offering atomic-level insights into sulfur electrochemistry for practical energy storage.