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.