Lithium-ion batteries dominate portable electronics and electric vehicles, but the cost and scarcity of lithium drive interest in sodium-ion alternatives. Room-temperature sodium-sulfur (RT Na-S) batteries are particularly attractive due to their high theoretical energy density (up to 1274 Wh kg⁻¹). However, challenges remain, primarily the complex and uncontrollable formation of long-chain and short-chain sodium polysulfides (NaPSs) during cycling. Long-chain NaPSs are metastable intermediates, soluble in common electrolytes, leading to active material loss, interfacial degradation, and capacity fade. To overcome these limitations, advanced catalysts are needed to regulate reaction pathways and enhance product selectivity. Product selectivity, well-established in organic catalysis, is less explored in battery technology. This study focuses on single-atom catalysts (SACs) to improve sulfur redox kinetics and stability by manipulating electron transfer (ET) processes to favor the formation of stable, short-chain NaPSs and suppress the formation of the less desirable long-chain NaPSs. The goal is to improve the performance of RT Na-S batteries by promoting the formation of desired products and mitigating the 'shuttle effect' and parasitic reactions.

Extensive research has focused on improving RT Na-S batteries by incorporating various catalysts into the cathode. However, a comprehensive understanding of the relationship between catalyst electronic structure, electron transfer capabilities, reaction pathways, and product selectivity remains limited. Previous studies have highlighted the importance of controlling the sulfur redox reactions by confining sulfur or modifying the electrolyte, but the use of single-atom catalysts to directly manipulate the electron transfer dynamics to improve product selectivity remains relatively unexplored. The existing literature lacks a systematic study on how different single-atom metal catalysts affect the pathway selectivity in Na-S batteries, making the current research crucial for advancing the field.

This study used a combination of experimental and computational methods. First, a machine learning (ML) model was developed to predict potential single-atom catalysts (SACs) for Na-S batteries based on their ability to facilitate electron transfer and control product selectivity. The model considered the relationship between adsorption energy, bond length, and electron transfer rate, guided by the Sabatier principle and Marcus theory. Six SACs (Mn, Fe, Co, Sn, Ni, Cu) supported on porous nitrogen-doped carbon nanospheres (M1-PNC) were synthesized via in-situ deposition followed by sulfur infusion (S@M1-PNC). The materials were characterized using various techniques including scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), atom probe tomography (APT), X-ray absorption fine structure (XAFS) spectroscopy, thermogravimetric analysis (TGA), and in-situ/ex-situ characterization methods. Electrochemical performance was evaluated using coin cells in a standard electrolyte. The electrochemical performance of S@M1-PNC as cathode materials was investigated by assembling coin cells. Cyclic voltammetry (CV) was utilized to assess the electrochemical characteristics of the fabricated cathodes, and galvanostatic charge-discharge cycling was implemented to determine their overall performance. Rate capability tests and long-term cycling stability experiments were conducted to understand the influence of SACs on electrochemical reaction kinetics and stability of the batteries. In-situ synchrotron powder X-ray diffraction (XRD), ex-situ X-ray absorption spectroscopy (XAS), and in-situ transmission electron microscopy (TEM) were used to study the reaction pathways and mechanism. Density functional theory (DFT) calculations were performed to investigate the adsorption energies and electron transfer processes at the single-atom catalyst sites.

Machine learning predictions identified manganese (Mn) as a promising single-atom catalyst. Experimental results confirmed that the S@Mn1-PNC cathode exhibited superior electrochemical performance compared to other SACs (Fe, Co, Sn, Ni, Cu). The Mn1 catalyst showed high pathway selectivity toward short-chain NaPSs (Na2S2 and Na2S), significantly suppressing the formation of long-chain NaPSs. In-situ XRD and ex-situ XAS revealed that the Mn1 catalyst facilitates the direct conversion of sulfur to short-chain NaPSs during discharge and promotes the reversible conversion of Na2S during charge, avoiding the accumulation of insoluble Na2S. In-situ TEM demonstrated that the Mn1 catalyst accelerates Na-ion diffusion during both discharge and charge processes. The S@Mn1-PNC cathode exhibited excellent cycling stability (84% capacity retention after 120 cycles at 0.2 A g⁻¹), high-rate capability (776 mAh g⁻¹ at 1 A g⁻¹), and long-term cycling performance (344.1 mAh g⁻¹ at 2 A g⁻¹ after 3000 cycles). Ex-situ XANES analysis confirmed the significant electron transfer capabilities of Mn1, facilitating both the discharge and charge processes. DFT calculations supported the experimental findings showing stronger adsorption of NaPSs on Mn1 sites compared to other SACs. The superior performance of the Mn1 catalyst is attributed to its unique electronic structure, which facilitates fast electron transfer, leading to high product selectivity and enhanced reaction kinetics.

The findings demonstrate the effectiveness of single-atom catalysts in enhancing the performance of RT Na-S batteries by precisely controlling the reaction pathways and improving product selectivity. The Mn1 catalyst's superior electron transfer capabilities are crucial for both the discharge and charge processes, leading to improved reaction kinetics and enhanced reversibility. The synergistic effect of the catalyst in improving both the discharge and charge processes is a key contribution of this research. The in-situ characterization techniques provided direct evidence for the reaction mechanism, supporting the conclusions drawn from the electrochemical measurements. This work shows a clear link between catalyst design and battery performance via detailed mechanistic studies, which guides the rational design of advanced single atom catalysts for RT Na-S batteries and similar energy storage systems.

This study successfully demonstrates the effectiveness of a single-atom charging strategy using Mn1-PNC catalysts to enhance the performance of RT Na-S batteries. The Mn1 catalyst displays exceptional product selectivity towards short-chain NaPSs, significantly improving cycling stability, rate performance, and long-term cycling life. The mechanistic studies elucidated the role of electron transfer in accelerating the redox kinetics. This research provides valuable insights into the design of highly efficient single-atom catalysts for energy storage applications, paving the way for future advancements in RT Na-S batteries. Future work could focus on exploring other single-atom catalysts and optimizing the catalyst support materials to further improve battery performance.

The study focuses on a specific electrolyte system (1 M NaClO₄ in EC/PC with 5 wt% FEC). The performance of the Mn1 catalyst in other electrolyte systems may vary. The scalability of the synthesis method for industrial applications needs further investigation. While the DFT calculations provide valuable insights into the electronic structure and adsorption behavior, further theoretical studies could explore the effect of the catalyst support on the reaction pathways.