Undercoordination engineering of chromium single-atom catalyst with optimized d-p hybridization for lithium-sulfur batteries

Sluggish sulfur redox kinetics remain a critical bottleneck in the advancement of high-performance lithium-sulfur batteries (LSBs). Single-atom catalysts (SACs) offer a promising solution to this limitation, particularly when their coordination structures are carefully engineered. Here, we develop a chromium-based SAC featuring a unique undercoordinated CrN₃ configuration to boost sulfur electrochemistry. Compared with conventional CrN₄, the CrN₃ motif lowers 3d orbital occupancy and meanwhile activates the in-plane hybridizations with S 3p orbitals upon interaction with polysulfides, contributing to moderate adsorption strength and reduced energy barriers for bidirectional sulfur conversions. Additionally, the integration of the 2D porous framework ensures abundant electrochemically active surfaces and efficiently exposed active sites. As a result, CrN₃-based cells demonstrate fast and durable sulfur redox reactions, enabling an ultralow capacity decay of 0.0075 % per cycle over 1000 cycles and a high-rate capability of 651.9 mAh g^-1 at 5 C. The CrN₃ catalyst retains robust catalytic efficiency under demanding conditions, delivering a high areal capacity of 5.53 mAh cm^-2 at high sulfur loading and lean electrolyte. This work establishes a compelling paradigm of SAC coordination engineering for designing advanced sulfur electrocatalysts for next-generation LSBs.