Elucidating Synergistic Mechanisms of Adsorption and Electrocatalysis of Polysulfides on Double-Transition Metal MXenes for Na-S Batteries

Tuning the Surface Stability and Li/Na Storage of MXenes by Controlling the Surface Termination Coverage

2D transition metal carbides and/or nitrides, MXenes, are a class of widely studied materials with great potential for energy storage applications. The control of surface chemistry is an effective approach for preparing novel MXenes and modifying their electrochemical properties. However, an in-depth and systematic atomic-scale study of the effect of surface termination on MXene stability and electrochemical performance is scarce and thus is highly desired. Here, through high-throughput first-principles calculations, 28 stable chalcogen-functionalized M2CTz (M = V, Nb, and Ta, T = S, Se, and Te) under different chemical environments are identified. The reduction of termination coverage improves electrical conductivity but weakens in-plane stiffness. Intriguingly, based on charge transfer mechanism, the diffusion barrier of lithium/sodium atoms on the M2CTz exhibits a volcano-like relationship with termination coverage, and the ion diffusion channel formed in half termination coverage greatly accelerates lithium ion diffusion and returns to or exceeds sodium ion diffusion rate at full termination coverage. V2CSe2/Nb2CSz not only displays the large lithium/sodium capacity (592/409-466 mAhg-1) but also exhibits low barrier energy and open-circuit voltage, suggesting a promising candidate anode material for lithium/sodium-ion batteries. These findings provide insights into the design and fabrication of MXenes and tuning the electrochemical performance of MXenes by controlling termination coverage.

Theoretical insights into single-atom catalysts for improved charging and discharging kinetics of Na-S and Na-Se batteries

Dissolution of poly-sulfide/selenides (p-S/Ses) intermediates into electrolytes, commonly known as the shuttle effect, has posed a significant challenge in the development of more efficient and reliable Na-S/Se batteries. Single-atom catalysts (SACs) play a crucial role in mitigating the shuttling of Na-pS/Ses and in promoting Na2S/Se redox processes at the cathode. In this work, single transition metal atoms Co, Fe, Ir, Ni, Pd, Pt, and Rh supported in nitrogen-deficient graphitic carbon nitride (rg-C3N4) are investigated to explore the charging and discharging kinetics of Na-S and Na-Se batteries using Density Functional Theory calculations. We find that SAs adsorbed on reduced g-C3N4 monolayers are substantially more effective in trapping higher-order Na2Xn than pristine g-C3N4 surfaces. Moreover, our ab initio molecular dynamics calculations indicate that the structure of X8 (X = S, Se) remains almost intact when adsorbed on Fe, Co, Ir, Ni, Pt, and Rh SACs, suggesting that there is no significant S or Se poisoning in these cases. Additionally, SACs reduce the free energies of the rate-determining step during discharge and present a lower decomposition barrier of Na2X during charging of Na-X electrode. The underlying mechanisms behind this fast kinetics are thoroughly examined using charge transfer, bonding strength, and d-band center analysis. Our work demonstrates an effective strategy for designing single-atom catalysts and offers solutions to the performance constraints caused by the shuttle effect in sodium-sulfur and sodium-selenium batteries.

The Role of Transition Metal Versus Coordination Mode in Single-Atom Catalyst for Electrocatalytic Sulfur Reduction Reaction

Electrocatalytic sulfur reduction reaction (SRR) is emerging as an effective strategy to combat the polysulfide shuttling effect, which remains a critical factor impeding the practical application of the Li-S battery. Single-atom catalyst (SAC), one of the most studied catalytic materials, has shown considerable potential in addressing the polysulfide shuttling effect in a Li-S battery. However, the role played by transition metal vs coordination mode in electrocatalytic SRR is trial-and-error, and the general understanding that guides the synthesis of the specific SAC with desired property remains elusive. Herein, we use first-principles calculations and machine learning to screen a comprehensive data set of graphene-based SACs with different transition metals, heteroatom doping, and coordination modes. The results reveal that the type of transition metal plays the decisive role in SAC for electrocatalytic SRR, rather than the coordination mode. Specifically, the 3d transition metals exhibit admirable electrocatalytic SRR activity for all of the coordination modes. Compared with the reported N3C1 and N4 coordinated graphene-based SACs covering 3d, 4d, and 5d transition metals, the proposed para-MnO2C2 and para-FeN2C2 possess significant advantages on the electrocatalytic SRR, including a considerably low overpotential down to 1 mV and reduced Li2S decomposition energy barrier, both suggesting an accelerated conversion process among the polysulfides. This study may clarify some understanding of the role played by transition metal vs coordination mode for SAC materials with specific structure and desired catalytic properties toward electrocatalytic SRR and beyond.

Aprotic Sulfur-Metal Batteries: Lithium and Beyond

Metal-sulfur batteries constitute an extraordinary research playground that ranges from fundamental science to applied technologies. However, besides the widely explored Li-S system, a remarkable lack of understanding hinders advancements and performance in all other metal-sulfur systems. In fact, similarities and differences make all generalizations highly inconsistent, thus unavoidably suggesting the need for extensive research explorations for each formulation. Here we review critically the most remarkable open challenges that still hinder the full development of metal-S battery formulations, starting from the lithium benchmark and addressing Na, K, Mg, and Ca metal systems. Our aim is to draw an updated picture of the recent efforts in the field and to shed light on the most promising innovation paths that can pave the way to breakthroughs in the fundamental comprehension of these systems or in battery performance.

Investigation of the anchoring and electrocatalytic properties of pristine and doped borophosphene for Na-S batteries

Designing an anchoring layer on the sulfur electrode has been considered one of the effective approaches to promoting the real application of room-temperature sodium-sulfur (RT-Na-S) batteries. In this work, based on the first-principles calculation method, the potential of pristine and doped borophosphene (BP) as anchoring materials for Na-S batteries has been investigated. The calculated adsorption energies of sodium polysulfides (NaPSs) adsorbed on pristine and doped substrates are higher than those of NaPSs adsorbed with the electrolytes (DOL&DME), indicating that the shuttle effect could be well alleviated. Meanwhile, the projected density of states (PDOS) suggests that the metallic characteristics of the adsorption systems are still well preserved, which is in favor of improving the electronic conductivity. More importantly, excellent electrocatalytic properties of the substrates are exhibited by reducing the catalytic decomposition energy barriers of Na₂S, in which 0.27/0.79/1.02 eV is found on the pristine/N-doped/C-doped BP, indicating that the electrochemical processes could be improved smoothly. Therefore, it could be expected that pristine and doped BP are excellent anchoring materials for sodium-sulfur batteries.

Synergistically boosting the anchoring effect and catalytic activity of MXenes as bifunctional electrocatalysts for sodium-sulfur batteries by single-atom catalyst engineering

MXene based sulfur hosts have attracted enormous attention in room temperature sodium-sulfur (RT Na-S) batteries due to their strong affinity towards soluble sodium polysulfides (NaPSs). However, their electrocatalytic performance needs further improvement. Here, a series of single non-noble transition metal (TM = Fe, Co, Ni, and Cu) atoms anchored on Ti₂CS₂ (TM@Ti₂CS₂) were proposed as bifunctional sulfur hosts for Na-S batteries. The results testify that the introduction of TMs dramatically enhanced the chemical interaction between sulfur-containing species and Ti₂CS₂, which is attributed to the co-formation of TM-S and Na-S covalent bonds. Importantly, compared with pristine Ti₂CS₂, the sulfur reduction reaction (SRR) is thermodynamically more favorable on TM@Ti₂CS₂. In addition, the incorporation of Fe, Co, and Ni atoms is also conducive to promoting the dissociation of Na₂S. The density of states (DOS) results suggest that TM@Ti₂CS₂ maintains metallic conductivity during the whole charge and discharge process. Overall, constructing single atom catalysts is an effective strategy to further improve the electrochemical performance of MXene based sulfur hosts for Na-S batteries.

MXene: fundamentals to applications in electrochemical energy storage

A new, sizable family of 2D transition metal carbonitrides, carbides, and nitrides known as MXenes has attracted a lot of attention in recent years. This is because MXenes exhibit a variety of intriguing physical, chemical, mechanical, and electrochemical characteristics that are closely linked to the wide variety of their surface terminations and elemental compositions. Particularly, MXenes are readily converted into composites with materials including oxides, polymers, and CNTs, which makes it possible to modify their characteristics for a variety of uses. MXenes and MXene-based composites have demonstrated tremendous promise in environmental applications due to their excellent reducibility, conductivity, and biocompatibility, in addition to their well-known rise to prominence as electrode materials in the energy storage sector. The remarkable characteristics of 2D MXene, including high conductivity, high specific surface area, and enhanced hydrophilicity, account for the increasing prominence of its use in storage devices. In this review, we highlight the most recent developments in the use of MXenes and MXene-based composites for electrochemical energy storage while summarizing their synthesis and characteristics. Key attention is paid to applications in supercapacitors, batteries, and their flexible components. Future research challenges and perspectives are also described.

Covalent surface modification of bifunctional two-dimensional metal carbide MXenes as sulfur hosts for sodium-sulfur batteries

Room temperature sodium-sulfur (RT Na-S) batteries show extraordinary potential in large-scale energy storage. MXenes have been demonstrated to be promising sulfur hosts for Na-S batteries, and their surface functional groups play a pivotal role in their performance. However, the effect of different surface functional groups of MXenes on their anchoring effect and catalytic performance has not been systematically investigated. Herein, density functional theory (DFT) calculations were employed to explore the various electrochemical performances of a series of Ti₂CT_x (T = O, S, N, F, Cl, and Br) MXenes as sulfur hosts for Na-S batteries. We find that surface functional groups significantly affect the structural properties of MXenes as well as their electrochemical performance. Ti₂CO₂, Ti₂CS₂, and Ti₂CN₂ exhibit prominent affinity toward soluble sodium polysulfides. Moreover, they display excellent catalytic activity toward the sulfur reduction reaction and the decomposition reaction of Na₂S. Finally, during the whole discharge process, Ti₂CO₂, Ti₂CS₂, and Ti₂CN₂ always maintain their metallic conductivity, which could improve the rate capability of Na-S batteries. Overall, Ti₂CO₂, Ti₂CS₂, and Ti₂CN₂ are proposed as promising bifunctional sulfur hosts for Na-S batteries, and our results may also provide insights for modulating the performance of MXenes in other applications.