Asymmetric Coordinated Single-Atom Catalysts Offering Zero-Order Sulfur Redox Kinetics for High Performance Li–S Batteries

Abstract

Accelerating the sluggish sulfur redox kinetics through electrocatalysis has been regarded as one of the key factors to achieve Li–S batteries of cell-level energy densities exceeding 600 Wh kg⁻¹. Though single-atom catalysts (SACs), typically with symmetric M-N₄ coordination structures have demonstrated attractive electrocatalytic performance in Li–S batteries, herein we discovered that an asymmetric-coordinated metal center distinctly shifts sulfur redox reaction (SRR) kinetics—from first-order (concentration-dependent) behavior in the symmetric-coordinated SACs—to zero-order (surface-saturated) kinetics, highlighting fundamentally altered reaction pathways, leading to a concurrent polysulfide conversion. Experimental and theoretical studies on the Ni atom-based SACs showed that symmetry breaking raises the Ni d-band center, enabling a monodentate-to-bidentate Li₂S₄ adsorption transition, which strengthens polysulfide adsorption and shifts the rate-limiting step from sluggish solid-solid transformation (Li₂S₂ → Li₂S) to a more favorable liquid–solid conversion (Li₂S₄ → Li₂S₂), effectively lowering the overall energy barrier of the SRR process. Consequently, Li–S cells employing Ni-NPG, a SACs with asymmetric Ni-N₃P₁ coordination, achieved a specific capacity of 877 mAh g⁻¹ at 4 C. Even under a high sulfur loading of 6 mg cm⁻², the cell retained 92% of its capacity after 200 cycles at 0.2 C, outperforming conventional SACs with symmetric coordination structures.