Strong d-π Orbital Coupling of Co-C4 Atomic Sites on Graphdiyne Boosts Potassium-Sulfur Battery Electrocatalysis.

Potassium-sulfur (K-S) batteries are severely limited by the sluggish kinetics of the solid-phase conversion of K2S3/K2S2 to K2S, the rate-determining and performance-governing step, which urgently requires a cathode with facilitated sulfur accommodation and improved catalytic efficiency. To this end, we leverage the orbital-coupling approach and herein report a strong d-π coupling catalytic configuration of single-atom Co anchored between two alkynyls of graphdiyne (Co-GDY). The d-π orbital coupling of the Co-C4 moiety fully redistributes electrons two-dimensionally across the GDY, and as a result, drastically accelerates the solid-phase K2S3/K2S2 to K2S conversion and enhances the adsorption of sulfur species. Applied as the cathode, the S/Co-GDY delivered a record-high rate performance of 496.0 mAh g-1 at 5 A g-1 in K-S batteries. In situ and ex situ characterizations coupling density functional theory (DFT) calculations rationalize how the strong d-π orbital coupling of Co-C4 configuration promotes the reversible solid-state transformation kinetics of potassium polysulfide for high-performance K-S batteries.

Toward Complete Transformation of Sodium Polysulfides by Regulating the Second-Shell Coordinating Environment of Atomically Dispersed Fe.

Room temperature sodium-sulfur (RT Na-S) batteries are highly competitive as potential energy storage devices. Nevertheless, their actually achieved reversible capacities are far below the theoretical value due to incomplete transformation of polysulfides. Herein, atomically dispersed Fe-N/S active center by regulating the second-shell coordinating environment of Fe single atom is proposed. The Fe-N4 S2 coordination structure with enhanced local electronic concentration around the Fermi level is revealed via synchrotron radiation X-ray absorption spectroscopy (XAS) and theoretical calculations, which can not only significantly promote the transformation kinetics of polysulfides, but induce uniform Na deposition for dendrite-free Na anode. As a result, the obtained S cathode delivers a high initial reversible capacity of 1590 mAh g-1 , nearly the theoretical value. This work opens up a new avenue to facilitate the complete transformation of polysulfides for RT Na-S batteries.

Room-Temperature Sodium-Sulfur Batteries: Rules for Catalyst Selection and Electrode Design.

Seeking an optimal catalyst to accelerate conversion reaction kinetics of room-temperature sodium-sulfur (RT Na-S) batteries is crucial for improving their electrochemical performance and promoting the practical applications. Herein, theoretical calculations of interfacial interactions of catalysts and polysulfides in terms of the surface adsorption state, interfacial ions migration, and electronic concentration around the Fermi level are systematically proposed as guiding principles of catalyst selection for RT Na-S batteries. As a case, MoN catalyst is accurately selected from transition metal nitrides with different d orbital electrons, and for experiment, it is introduced into the carbon nanofibers as a dual-functioning host (MoN@CNFs). The MoN@CNFs can effectively anchor polysulfides and accelerate their conversion reaction. In addition, for the sodium anode, the MoN@CNFs can also induce uniform deposition of Na and inhibit dendrite growth, which are supported by in situ characterizations and finite element simulation technique. As a result, the as-prepared RT Na-S battery displays high reversible capacity of 990 mAh g-1 at 0.2 A g-1 after 100 cycles and long lifespan over 1500 cycles at 2 A g-1 . Even with high S loading of 5 mg cm-2 , the RT Na-S battery still exhibits a high areal capacity of 2.5 mAh cm-2 .

An Open-Ended Ni3 S2 -Co9 S8 Heterostructures Nanocage Anode with Enhanced Reaction Kinetics for Superior Potassium-Ion Batteries.

Sulfides are perceived as promising anode materials for potassium-ion batteries (PIBs) due to their high theoretical specific capacity and structural diversity. Nonetheless, the poor structural stability and sluggish kinetics of sulfides lead to unsatisfactory electrochemical performance. Herein, Ni3 S2 -Co9 S8 heterostructures with an open-ended nanocage structure wrapped by reduced graphene oxide (Ni-Co-S@rGO cages) are well designed as the anode for PIBs via a selective etching and one-step sulfuration approach. The hollow Ni-Co-S@rGO nanocages, with large surface area, abundant heterointerfaces, and unique open-ended nanocage structure, can reduce the K+ diffusion length and promote reaction kinetics. When used as the anode for PIBs, the Ni-Co-S@rGO exhibits high reversible capacity and low capacity degradation (0.0089% per cycle over 2000 cycles at 10 A g-1 ). A potassium-ion full battery with a Ni-Co-S@rGO anode and Prussian blue cathode can display a superior reversible capacity of 400 mAh g-1 after 300 cycles at 2 A g-1 . The unique structural advantages and electrochemical reaction mechanisms of the Ni-Co-S@rGO are revealed by finite-element-simulation in situ characterizations. The universal synthesis technology of bimetallic sulfide anodes for advanced PIBs may provide vital guidance to design high-performance energy-storage materials.