Superior Anchoring of Sodium Polysulfides to the Polar C2N 2D Material: A Potential Electrode Enhancer in Sodium-Sulfur Batteries

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.

Single-atom catalysts based on C2N for sulfur cathodes in Na-S batteries: a first-principles study

Several major roadblocks, including the "shuttle effect" caused by the dissolved higher-order sodium polysulfides (NaPSs), extremely poor conductivity of sulfur cathodes, and sluggish conversion kinetics of charging-discharging reactions, have hindered the commercialization of sodium-sulfur batteries (NaSBs). In our study, representative C2N-based single-atom catalysts (SACs), TM@C2N (TM = Fe, Ni and V), are proposed to improve the comprehensive performance of NaSBs. Based on first-principles calculations, we first discuss in detail the anchoring behavior of all adsorption systems, TM@C2N/(S8 and NaPSs). The results indicate that compared to pristine C2N, TM@C2N substrates exhibit a stronger capability to capture S8/NaPSs clusters through physical/chemical binding, with V@C2N showing the most outstanding capability ranging from -2.37 to -5.03 eV. The density of states analysis reveals that metallic properties can be well maintained before and after adsorption of polysulfides. More importantly, TM@C2N configurations can greatly reduce the energy barriers of charging and discharging reactions, thereby accelerating the conversion efficiency of NaSBs. It is worth mentioning that V@C2N has lower charge-discharge energy barriers and Na ion migration rates, since the embedded TM atom weakens the strong binding of Na+ in the N6 cavity of C2N. The intrinsic mechanism analysis reveals that the interaction between the d orbitals of V and the p orbitals of S leads to the weakening of Na-S bonds, which can not only effectively inhibit the shuttle effect, but also promote the dissociation of Na2S. Overall, this work not only offers excellent catalytic materials, but also provides vital guidance for designing SACs in NaSBs.

Colossal figure of merit and compelling HER catalytic activity of holey graphyne

Herein, we have conducted a comprehensive study to uncover the thermal transport properties and hydrogen evolution reaction catalytic activity of recently synthesized holey graphyne. Our findings disclose that holey graphyne has a direct bandgap of 1.00 eV using the HSE06 exchange-correlation functional. The absence of imaginary phonon frequencies in the phonon dispersion ensures its dynamic stability. The formation energy of holey graphyne turns out to be - 8.46 eV/atom, comparable to graphene (- 9.22 eV/atom) and h-BN (- 8.80 eV/atom). At 300 K, the Seebeck coefficient is as high as 700 μV/K at a carrier concentration of 1 × 10¹⁰ cm^-2. The predicted room temperature lattice thermal conductivity (κ_l) of 29.3 W/mK is substantially lower than graphene (3000 W/mK) and fourfold smaller than C₃N (128 W/mK). At around 335 nm thickness, the room temperature κ_l suppresses by 25%. The calculated p-type figure of merit (ZT) reaches a maximum of 1.50 at 300 K, higher than that of holey graphene (ZT = 1.13), γ-graphyne (ZT = 0.48), and pristine graphene (ZT = 0.55 × 10^-3). It further scales up to 3.36 at 600 K. Such colossal ZT values make holey graphyne an appealing p-type thermoelectric material. Besides that, holey graphyne is a potential HER catalyst with a low overpotential of 0.20 eV, which further reduces to 0.03 eV at 2% compressive strain.

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.

Efficient Detection of Nerve Agents through Carbon Nitride Quantum Dots: A DFT Approach

V-series nerve agents are very lethal to health and cause the inactivation of acetylcholinesterase which leads to neuromuscular paralysis and, finally, death. Therefore, rapid detection and elimination of V-series nerve agents are very important. Herein, we have carried out a theoretical investigation of carbon nitride quantum dots (C₂N) as an electrochemical sensor for the detection of V-series nerve agents, including VX, VS, VE, VG, and VM. Adsorption of V-series nerve agents on C₂N quantum dots is explored at M05-2X/6-31++G(d,p) level of theory. The level of theory chosen is quite adequate in systems describing non-bonding interactions. The adsorption behavior of nerve agents is characterized by interaction energy, non-covalent interaction (NCI), Bader's quantum theory of atoms in molecules (QTAIM), frontier molecular orbital (FMO), electron density difference (EDD), and charge transfer analysis. The computed adsorption energies of the studied complexes are in the range of -12.93 to -17.81 kcal/mol, which indicates the nerve agents are physiosorbed onto C₂N surface through non-covalent interactions. The non-covalent interactions between V-series and C₂N are confirmed through NCI and QTAIM analysis. EDD analysis is carried out to understand electron density shifting, which is further validated by natural bond orbital (NBO) analysis. FMO analysis is used to estimate the changes in energy gap of C₂N on complexation through HOMO-LUMO energies. These findings suggest that C₂N surface is highly selective toward VX, and it might be a promising candidate for the detection of V-series nerve agents.

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.

Exploring the Superior Anchoring Performance of the Two-Dimensional Nanosheets B2C4P2 and B3C2P3 for Lithium-Sulfur Batteries

Potential anchoring materials in lithium-sulfur batteries help overcome the shuttle effect and achieve long-term cycling stability and high-rate efficiency. The present study investigates the two-dimensional nanosheets B₂C₄P₂ and B₃C₂P₃ by employing density functional theory calculations for their promise as anchoring materials. The nanosheets B₂C₄P₂ and B₃C₂P₃ bind polysulfides with adsorption energies in the range from -2.22 to -0.75 and -2.43 to -0.74 eV, respectively. A significant charge transfer occurs from the polysulfides, varying from -0.74 to -0.02e and -0.55 to -0.02e for B₂C₄P₂ and B₃C₂P₃, respectively. Upon anchoring the polysulfides, the band gap of B₃C₂P₃ reduces, leading to enhanced electrical conductivity of the sulfur cathode. Finally, the calculated barrier energies of B₂C₄P₂ and B₃C₂P₃ for Li₂S indicate fast diffusion of Li when recharged. These enthralling characteristics propose that the nanosheets B₂C₄P₂ and B₃C₂P₃ could reduce the shuttle effect in Li-S batteries and significantly improve their cycle performance, suggesting their promise as anchoring materials.

Adsorption mechanisms of different toxic molecular gases on intrinsic C2N and Ti-C2N-V monolayer: a DFT study

Recently, the excessive emission of chemical toxic gases such as nitrogen trifluoride (NF₃), ammonia (NH₃), phosgene (COCL₂), and benzene (C₆H₆) has caused serious environmental problems. Adsorption of these chemical toxic gas molecules is a promising method to reduce environmental pollution. In this work, density functional theory (DFT) calculations are used to investigate the adsorption properties of these chemical toxic molecules on intrinsic C₂N and Ti-C₂N_-V monolayer. The results show that NF₃, NH₃, C₆H₆, and COCL₂ can all be adsorbed to the intrinsic C₂N monolayer with weak adsorption energy, while the adsorption properties of these gas molecules were greatly improved after doping Ti atom. The adsorption energy of NH₃, C₆H₆, COCL₂, and NF₃ increased from - 0.585, - 0.432, - 0.633, and - 0.362 eV to - 2.214, - 1.699, - 1.822, and - 0.799 eV, respectively, which increased by 2 ~ 4 times compared with that before doping. Besides, the results of the electron distribution, work function, the total density of states (TDOS), and the partial density of states (PDOS) analysis indicate that the doped Ti atom can be used as a bridge to connect the adsorbed molecules with the C₂N_-V monolayer, strengthen their interaction, and significantly improve the adsorption capacity. Therefore, Ti-doped C₂N_-V (Ti-C₂N_-V) monolayer is a promising adsorbent for the enrichment and utilization of harmful gases.

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

Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (Na₂S_n) in electrolytes, and the slower kinetics for the decomposition of solid Na₂S, make sodium-sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, Mo₂TiC₂T₂, (T = O and S) are studied as an anchoring material (AM) to immobilize higher-order polysulfides and to expedite the otherwise slower kinetics of insoluble short-chain polysulfides. Density functional theory (DFT) calculations are carried out to justify and compare the effectiveness of Mo₂TiC₂S₂ and Mo₂TiC₂O₂ as AMs by analyzing their interactions with S₈/Na₂S_n (n = 1, 2, 4, 6, and 8). Mo₂TiC₂S₂ provides moderate adsorption strength compared to Mo₂TiC₂O₂, therefore, it is expected to effectively inhibit Na₂S_n dissolution and shuttling without causing decomposition of Na₂S_n. The calculated Gibbs free energies of the rate-determining step for sulfur reduction reactions (SRR) are found to be significantly lower (0.791 eV for S and 0.628 eV for O functionalization) than that in vacuum (1.442 eV), suggesting that the SRR is more thermodynamically favorable on Mo₂TiC₂T₂ during discharge. Additionally, both Mo₂TiC₂S₂ and Mo₂TiC₂O₂ demonstrated effective electrocatalytic activity for the decomposition of Na₂S, with a substantial reduction in the energy barrier to 1.59 eV for Mo₂TiC₂S₂ and 1.67 eV for Mo₂TiC₂O₂. While Mo₂TiC₂O₂ had superior binding properties, structural distortion is observed in Na₂S_n, which may adversely affect cyclability. On the other hand, because of its moderate binding energy, enhanced electronic conductivity, and significantly faster oxidative decomposition kinetics of polysulfides, Mo₂TiC₂S₂ can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.

Two-Dimensional Titanium Carbide (Ti3C2Tx) MXenes to Inhibit the Shuttle Effect in Sodium Sulfur Batteries

Room-temperature sodium sulfur batteries (RT-NSBs) are among the promising candidates for large-scale energy storage applications because of the natural abundance of the electrode materials and impressive energy density. However, one...

Mechanistic Insights into Interactions of Polysulfides at VS2 Interfaces in Na-S Batteries: A DFT Study

Room temperature sodium-sulfur (Na-S) batteries, because of their high theoretical energy density and low cost, are considered as a promising candidate for next-generation energy storage devices. However, the practical utilization of the Na-S batteries is greatly hindered by various deleterious factors such as dissolution of sodium polysulfides (Na₂S_n) into the electrolyte commonly termed as "shuttle effect," sluggish decomposition of solid Na₂S, and poor electronic conductivity of sulfur. To overcome the challenges, we introduced single-layer vanadium disulfide (VS₂) as an anchoring material (AM) to immobilize higher-order polysulfides from the dissolution and also to accelerate the otherwise sluggish kinetics of insoluble short-chain polysulfides. We employ density functional theory (DFT) calculations to elucidate the Na₂S_n interactions at the VS₂ interfaces. We show that the adsorption strengths of various Na₂S_n species on the VS₂ basal plane are adequate (1.21-4.3 eV) to suppress the shuttle effect, and the structure of Na₂S_n are maintained without any decomposition, which is necessary to mitigate capacity fading. The calculated projected density of states (PDOS) reveals that the metallic character of the pristine VS₂ is retained even after Na₂S_n adsorption. The calculated Gibbs free energy of each elementary sulfur reduction reaction indicates a significant decrement in the free energy barrier due to the catalytic activity of the VS₂ surface. Furthermore, VS₂ is found to be an excellent catalyst to significantly reduce the oxidative decomposition barrier of Na₂S, which facilitates accelerated electrode kinetics and higher utilization of sulfur. Overall, VS₂ with strong adsorption behavior, enhanced electronic conductivity, and improved oxidative decomposition kinetics of polysulfides can be considered as an effective AM to prevent the shuttle effect and to improve the performance of Na-S batteries.

Metal-N4@Graphene as Multifunctional Anchoring Materials for Na-S Batteries: First-Principles Study

Developing highly efficient anchoring materials to suppress sodium polysulfides (NaPSs) shuttling is vital for the practical applications of sodium sulfur (Na-S) batteries. Herein, we systematically investigated pristine graphene and metal-N₄@graphene (metal = Fe, Co, and Mn) as host materials for sulfur cathode to adsorb NaPSs via first-principles theory calculations. The computing results reveal that Fe-N₄@graphene is a fairly promising anchoring material, in which the formed chemical bonds of Fe-S and N-Na ensure the stable adsorption of NaPSs. Furthermore, the doped transition metal iron could not only dramatically enhance the electronic conductivity and the adsorption strength of soluble NaPSs, but also significantly lower the decomposition energies of Na₂S and Na₂S₂ on the surface of Fe-N₄@graphene, which could effectively promote the full discharge of Na-S batteries. Our research provides a deep insight into the mechanism of anchoring and electrocatalytic effect of Fe-N₄@graphene in sulfur cathode, which would be beneficial for the development of high-performance Na-S batteries.

Efficient suppression of the shuttle effect in Na-S batteries with an As2S3 anchoring monolayer

Sodium-sulfur batteries (NaSBs) have emerged as a promising energy storage technology for large-scale stationary applications such as smart electrical grids due to their exceptionally high energy density and cost-effectiveness. However, one of the challenging problems impeding their practical applications is the sulfur shuttle effect by which the active redox intermediates are gradually dissolved in electrolytes. In this work, we have employed first-principles density functional theory (DFT) calculations to unravel the suppression of the shuttle effect in NaSBs with a two-dimensional (2D) As2S3 monolayer as the anchoring material. We show that semiconducting As2S3 is a suitable anchoring layer to inhibit the dissolution of the polysulfide intermediates in common electrolytes because of its stronger chemical binding with sodium polysulfides than with the electrolytes. The immense adsorption is attributed to the electron donation from the unfilled S-3p states of the polysulfides to As2S3. These mechanisms increase the carrier population and consequently improve the electrical conductivity of As2S3. Hence, the use of As2S3 can both reduce the shuttle effect and enhance the cathode electron conductivity to enable improved cycling stability and coulombic efficiency of the battery.