Anchoring and Catalytic Performance of Co@C2N Monolayer for Rechargeable Li-SexSy Batteries: A First-Principles Calculations

Abstract

Se_xS_y composite cathode materials, which offer superior theoretical capacity compared to pure selenium and improved electrochemical properties relative to pure sulfur, have aroused considerable interest in recent decades on account of their applications in electric vehicles and energy storage grids. In the current work, the feasibility of a Co@C₂N monolayer as a promising host candidate for the cathode material of Li-Se_xS_y batteries has been evaluated using first-principles calculations, and particular efforts have been devoted to underscoring the anchoring mechanism and catalytic performance of the Co@C₂N monolayer. The pronounced synergistic effects of Co-S and Li-N bonds lead to increased anchoring performance for Li₂Se_xS_y/Se_xS_y clusters on the surface of Co@C₂N monolayer, which effectively inhibit the shuttle effect. The charge density difference and Mulliken charge analysis underscores a substantial charge transfer from the Li₂Se_xS_y and Se_xS_y clusters to the Co@C₂N monolayer, which indicates a noticeable chemical interaction between them. Further electronic property calculations show that the Co@C₂N monolayer can improve the electrical conductivity of cathode materials for Li-Se_xS_y batteries by maintaining semi-metallic characteristics after anchoring of Li₂Se_xS_y/Se_xS_y clusters. Additionally, the catalytic performance of the Co@C₂N monolayer is evaluated in terms of the reduction pathway of Se₈ and the decomposition energy barrier of the Li₂SeS cluster, which highlights the catalytic role of the Co@C₂N monolayer in the formation and decomposition of the Li₂SeS cluster during the cycle processes. Overall, the Co@C₂N monolayer emerges as a promising host material and catalyst for Li-Se_xS_y batteries with remarkable anchoring and catalytic performance.

1. Introduction

Escalating global energy scarcity has spurred a concerted effort to innovate new energy technologies with the aim of addressing the energy crisis and satisfying the demand for eco-friendly energy solutions [1,2]. Lithium-ion batteries have emerged as vital players in fulfilling the surging needs of portable devices, electric vehicles, and energy storage grids, owing to their long cycle life, small size, high energy density, and minimal environmental pollution. However, the energy output provided by the renewable inserted lithium-ion batteries, even at peak development, falls short of increasing market demand [3,4,5,6,7]. Therefore, it is imperative to develop a new generation of cost-effective lithium-ion batteries to provide higher energy density. Notably, the chalcogens, such as oxygen, sulfur, and selenium, are abundant in nature and boast remarkable energy densities, which positions them as promising candidates for future energy storage applications. However, Li-O batteries encounter a series of challenges, including poor cycling performance, electrolyte decomposition, excessive lithium for anode, and large voltage polarization [8,9], which impede the widespread application of Li-O batteries. Remarkably, the rechargeable Li-S and Li-Se batteries emerge as the most promising contenders. Li-S batteries garner considerable attention not only because of their high specific capacity (1672 mAh·g⁻¹) and energy density (2600 Wh·kg⁻¹) [10] but also for their affordability, abundant reserves, low toxicity, and environmental friendliness. Nevertheless, with the further study of Li-S batteries, there are drawbacks to Li-S batteries, such as low conductivity of sulfur, the discharge intermediate products, the large volume change during the charging/discharging process, and the shuttle effect of high-order polysulfides, which diminish the sulfur utilization rate and Coulombic efficiency [11,12,13,14,15]. Selenium, akin to sulfur in chemical properties, emerges as a prospective cathode material for volume-sensitive applications. Importantly, Li-Se batteries exhibit superior electrical conductivity (1 × 10⁻³ S·m⁻¹) and enhanced electrochemical activity compared to Li-S batteries (5 × 10⁻²⁸ S·m⁻¹) [16]. Furthermore, Li-Se batteries boast obvious advantages, such as heightened active material utilization rate, robust rate capacity [17], and favorable electrochemical compatibility with conventional liquid electrolytes [18,19,20].

Amidst the fervent exploration of cathode materials for Li-S and Li-Se batteries, a kind of novel rechargeable Li-Se_xS_y batteries has emerged. Se_xS_y cathode materials exhibit a superior theoretical capacity compared to their isolated selenium counterpart and enhanced conductivity compared to pure sulfur cathodes [21]. Moreover, Sun et al. [22] demonstrated the performance of cathode materials with varying Se/S ratios in diverse ether-based electrolytes, highlighting the significant enhancement of Se₂S₆ materials in cycling performance. Therefore, the Se_xS_y composite emerges as a promising cathode material with prolonged cycle life and heightened power density. Despite the notable improvement in the cycle stability facilitated by the Se_xS_y composite, the initial Coulombic efficiency of the Se_xS_y/C composite remains low, which may be attributed to the side-reactions of sulfide/selenide with the electrolytes. Moreover, the weak interaction between the Se_xS_y composite and the carbon host material leads to the ineffective confinement of selenium and sulfur materials. Therefore, developing a novel host material for anchoring polyselenide/polysulphide intermediates becomes a matter of great urgency in Li-Se_xS_y batteries.

To address the challenges associated with volume expansion and shuttle effect in Li-Se_xS_y batteries, a prevalent strategy, namely developing different forms of carbon electrodes by using nano-sized carbon materials [23,24], is adopted. Carbon graphitization not only helps to trap selenium and sulfur active materials but also facilitates electrochemical activity of carbon through sp² hybridization. Moreover, a composite material formed by combining carbon with other atoms can also improve the electrochemical performance of Li-Se_xS_y batteries while accommodating their volume expansion during discharge. The typical example is the X (X = N, O, P) atoms doping carbon materials, which can not only suppress the shuttle effect but also enhance the electrical conductivity [25,26,27,28,29]. Very recently, a novel carbon nitride material, layered C₂N material with vesicular configuration and high nitrogen concentration, has garnered considerable attention in the field of energy storage on account of its expansive surface area and excellent structural, electrical, and mechanical properties.

By first-principles calculations, the anchoring mechanism of polyselenides on a C₂N monolayer [30] was systemically investigated, revealing a robust binding interaction between the C₂N monolayer and polyselenides. Similarly, a subsequent work explored by Lin et al. [31] further evaluated the feasibility of a transition metal-doped C₂N monolayer as a cathode material for Li-S batteries, demonstrating the highest adsorption energy and the lowest decomposition energy. Importantly, a recent theoretical work [32] substantiated that a Co@C₂N monolayer can mitigate the shuttle effect of high-order polyselenides, accelerate Li₂Se conversion, and improve the cycling performance of Li-Se batteries. Considering the similarity of Li-Se_xS_y batteries to Li-S or Li-Se batteries, the Co@C₂N monolayer is also expected to be a promising cathode material. Nevertheless, the design and application of Co@C₂N monolayers in Li-Se_xS_y batteries are still at a very early stage, and few works concerning on the anchoring and catalytic performance of Co@C₂N monolayers have been reported so far. Therefore, in the current work, the feasibility of a Co@C₂N monolayer as a cathode material for Li-Se_xS_y batteries is evaluated through first-principles calculations, and particular attention is paid to the exploration of the anchoring effect and catalytic performance. The binding energies, charge transfer mechanism, and electronic properties of Li₂Se_xS_y/Se_xS_y clusters anchored on a Co@C₂N monolayer are investigated. The present work could provide valuable insights into the potential development of Co@C₂N monolayers as cathode materials for high-performance Li-Se_xS_y batteries.

2. Results and Discussion

2.1. Pristine C₂N, Co@C₂N Monolayer, and Li₂Se_xS_y (Se_xS_y) Cluster Structures

Figure 1a depicts the optimized configuration of the C₂N monolayer, which comprises 12 carbon and 6 nitrogen atoms, and the lattice parameters (a = b = 8.332 Å) are consistent with the theoretical and experimental values (8.300 Å) [32,33].

For the favorable position of the Co atom within the C₂N monolayer, the Co@C₂N monolayer (Figure 1b) adopted in the previous work [34], where Co atom bonds with adjacent N atoms, is selected as the candidate cathode material of the Li-Se_xS_y batteries. Figure 1c–e illustrate the electronic band structures of a pristine C₂N monolayer and the Co@C₂N monolayer. Obviously, the pristine C₂N monolayer manifests a direct band gap semiconductor with a band gap of 1.76 eV, and the embedded Co atom reduces the band gap of the C₂N monolayer, imparting semi-metallic characteristics to the Co@C₂N monolayer. During the discharge process, Li⁺ undergoes reactions with sulfur and selenium to form a series of selenium-containing and sulfur-containing compounds, as presented in Table S1 of the Supporting Information, and Figure 2 depicts the most stable configurations of Li₂Se_xS_y and Se_xS_y (x + y = 2, 4, 6, 8) clusters, in which the sulfur atom prefers to bond with sulfur atoms and lithium atoms are inclined to bond with sulfur and selenium atoms.

2.2. Geometries, Binding Energies of Li₂Se_xS_y (Se_xS_y) on Co@C₂N Monolayer

To investigate the feasibility of utilizing the Co@C₂N monolayer as a cathode material for Li-Se_xS_y batteries, the most suitable adsorption sites for Li₂Se_xS_y and Se_xS_y (x + y = 2, 4, 6, 8) clusters on Co@C₂N monolayer are firstly explored by binding energies (E_b) [35] using the following definition:

(1)$E_{\mathrm{b}}=E_{\mathrm{C}\mathrm{o}@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}-(E_{\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}+E_{\mathrm{C}\mathrm{o}@{\mathrm{C}}_2\mathrm{N}})$

where the $E_{\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$, $E_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}}$, $E_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$ represent the total energies of Li₂Se_xS_y or Se_xS_y clusters, the Co@C₂N monolayer, and the total energies of Li₂Se_xS_y and Se_xS_y molecules adsorbed on the Co@C₂N monolayer, respectively. The E_bs for the Li₂Se_xS_y and Se_xS_y (x + y = 2, 4, 6, 8) clusters at different anchoring sites on the surface of the Co@C₂N monolayer are depicted in Figure 3a, and the corresponding adsorbed configurations associated with binding energies are presented in Table S2 of the Supporting Information. Obviously, negative E_bs are observed for Co@C₂N-Li₂Se_xS_y (Se_xS_y) (x + y = 2, 4, 6, 8), indicating the robust anchoring capability for Li₂Se_xS_y and Se_xS_y clusters on the surface of the Co@C₂N monolayer. More importantly, a positive correlation is observed for the E_b and the concentration of S atoms, i.e., an increase in the number of S atoms correlates with heightened E_b values for the Co@C₂N-Li₂Se_xS_y/Se_xS_y, indicating the enhanced anchoring effect. As the lithium reaction progresses from the Li₂Se_xS_y (x + y = 8) to Li₂SeS, an overall increase is observed for the E_bs for most stable Co@C₂N-Li₂Se_xS_y (x + y = 2, 4, 6, 8) configurations, which underscores the robust and strong adsorption strength of the Co@C₂N monolayer for Li₂Se_xS_y clusters. Figure 3b illustrates corresponding E_bs for the most stable configurations of Li₂Se_xS_y and Se_xS_y clusters anchored on the Co@C₂N monolayer, which span from −1.30 eV of Li₂Se₇S to −2.42 eV for Li₂SeS cluster. Moreover, the E_bs for Li₂Se_xS_y clusters anchored on the Co@C₂N monolayer increases with the lithiation and decreases with the reduction in the concentration of S atoms, suggesting the contributory role of S atoms in bolstering the binding strength between Li₂Se_xS_y clusters and the Co@C₂N monolayer and consequently suppressing the shuttle effect.

Figure 4 shows the most stable structures of Li₂Se_xS_y and Se_xS_y clusters adsorbed on the surface of the Co@C₂N monolayer. Noticeably, Li₂Se_xS_y (x + y = 6, 8) and Se_xS_y clusters align parallel to the Co@C₂N monolayer, whereas the Li₂Se_xS_y (x + y = 2, 4) clusters tend to orient perpendicular to the Co@C₂N monolayer. This finding is similar to the situations of M@C₂N-Li₂S_n (M = Mn, Fe, Co, Ni, Cu) [31]. As depicted in Table S3 of the Supporting Information, the average distance (d_S-Co@C₂N) between S atoms and the Co@C₂N monolayer diminishes with the decreasing concentrations of S and Se in Li₂Se_xS_y clusters, which reaches the minimum distance of 2.228 Å for Co@C₂N-Li₂SeS. Specifically, for the Li₂Se_xS_y clusters with the same composition (x + y = 4), the average distances from S atoms to the Co@C₂N monolayer decrease during the increase in the S concentration, which is beneficial for forming strong Co-S bonds. In contrast, for the Li₂Se_xS_y clusters, where x + y = 6 and 8, significant variations in the d_S-Co@C₂N values are discovered. Such differences can be attributed to the complexities arising from the increased concentration of S atoms and their varying distances to the Co@C₂N monolayer. In addition, with the decrease in Se and S atoms, Li atoms migrate towards the Co@C₂N monolayer, thereby decreasing the average Li-N bond length from Co@C₂N-Li₂Se_xS_y (x + y = 8) to Co@C₂N-Li₂SeS, aligning with the binding energies analysis. The results underscore the capability of the Co@C₂N monolayer to enhance the anchoring and cycling stability of Li-Se_xS_y batteries.

2.3. Charge Transfer Mechanism and Electronic Properties

In order to evaluate the anchoring mechanism of Li₂Se_xS_y and Se_xS_y clusters anchored on the surface of the Co@C₂N monolayer, the charge density difference [36] and Mulliken charge analysis [37,38] are explored for the Co@C₂N-Li₂Se_xS_y/Se_xS_y. The charge density difference is expressed as follows:

(2)$\Delta \rho ={\rho }_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}-{\rho }_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}}-{\rho }_{\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$

where ${\rho }_{\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$, ${\rho }_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}}$, and ${\rho }_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y/\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$ represent the charge density differences of Li₂Se_xS_y and Se_xS_y clusters, the independent Co@C₂N monolayer, and Li₂Se_xS_y and Se_xS_y clusters anchored on the Co@C₂N monolayer, respectively. As depicted by charge density difference analysis in Figure 5, significant charge transfer occurs between Li₂Se_xS_y/Se_xS_y clusters and the Co@C₂N monolayer, i.e., the Li and Co atoms lose electrons and the charges accumulate near the C and N atoms.

Notably, the charges accumulation is evident near the Li-N and Co-S bonds, whereas a reduction in the charge density is observed near the Li-S bond. These findings suggest a strengthening of the Li-N and Co-S bonds, alongside a weakening of the Li-S bonds. Furthermore, the charge densities between Li₂Se_xS_y and the Co@C₂N monolayer increase with the decrease in S and Se contents. Therefore, Li₂Se_xS_y clusters exhibit robust binding to Co@C₂N monolayer by strong Li-N and Co-S bonds, thus effectively suppressing the shuttle effect of Li₂Se_xS_y and Se_xS_y clusters. Furthermore, the amount of charge transfer between Li₂Se_xS_y/Se_xS_y clusters and the Co@C₂N monolayer is analyzed through Mulliken charge calculations, as depicted in Figure 6. Obviously, positive Mulliken charges are discovered for the Li₂Se_xS_y/Se_xS_y clusters, indicating that the charge transfers from these clusters to the Co@C₂N monolayer. The specific charge transfer values of Co@C₂N-Li₂Se_xS_y/Se_xS_y clusters range from 0.31 e to 1.15 e. This electron transfer enhances the chemical interaction between Li₂Se_xS_y/Se_xS_y clusters and the Co@C₂N monolayer, resulting in the elongation of Li-Se and Li-S bonds in Li₂Se_xS_y clusters. Notably, a positive correlation is observed between charge transfer and binding energies. For example, Co@C₂N-Li₂Se_xS_y exhibits lower charge transfer and poorer anchoring performance in comparison with the counterparts for the Co@C₂N-Se_xS_y.

Figure 7 illustrates the electron localization function (ELF) [39,40] of the Li-N and Co-S bonds in Co@C₂N-Li₂Se_xS_y. In general, the ELF values are located in the range of 0 to 1, and those within 0.50~0.75 (0.75~1.00) are related to the metallic (covalent) bond. Conversely, an ELF value between 0 and 0.5 signifies an ionic bond characterized by robust bonding energy. The ELF analysis reveals the formation of stable bonds between Li₂Se_xS_y/Se_xS_y clusters and the Co@C₂N monolayer, i.e., the Co@C₂N monolayer can effectively immobilize Li₂Se_xS_y and Se_xS_y clusters through strong Co-S and Li-N bonds.

The ideal cathode material for Li-Se_xS_y batteries should possess excellent electrical conductivity, which is beneficial to provide additional electrons to enhance the redox kinetics of Li₂Se_xS_y and Se_xS_y clusters during the cycle process. As depicted in Figure S1 of the Supporting Information, the density of states (DOS) of Co@C₂N-Li₂Se_xS_y/Se_xS_y is evaluated. Notably, some overlaps for the peaks corresponding to the Co atom and Li₂Se_xS_y/Se_xS_y clusters are observed in the DOS, indicating strong orbital hybridization, consequently resulting in a robust Co-S bond. Upon anchoring the Li₂Se_xS_y/Se_xS_y clusters on the surface of the Co@C₂N monolayer, significant peaks of Li₂Se_xS_y are observed near the Fermi level. Meanwhile, with the anchoring of Li₂Se_xS_y and Se_xS_y clusters on the surface of the Co@C₂N monolayer, the Co 3d orbitals hybridize with Li₂Se_xS_y and Se_xS_y clusters orbitals near the Fermi level, and Co@C₂N-Li₂Se_xS_y/Se_xS_y systems exhibit metallic properties. Such finding underscores the promising prospect of Co@C₂N monolayers as a candidate for Li-Se_xS_y batteries. Further analysis on the DOS near the Fermi level reveals the dominant contribution of Li₂Se_xS_y clusters, whereas the corresponding contribution of the Co atom primarily concentrates in the low-energy region. With commendable electrical conductivity, the Co@C₂N monolayer is beneficial for accelerating the reversible conversion reaction of Li₂Se_xS_y clusters, thus holding great significance for expediting the electrochemical reaction of Li-Se_xS_y batteries.

2.4. Average Open Circuit Voltage of Co@C₂N-Li₂Se_xS_y

The average open circuit voltage (V) [41,42,43], as a key parameter for evaluating the performance of Li-Se_xS_y batteries, is directly estimated from the energy change during the cycling process, which is defined using the following equation:

(3)$V=-{\displaystyle \frac{E_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_{x_1}\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}-E_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_{x_2}\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}-(x_1-x_2)E_{\mathrm{L}\mathrm{i}}}{\mathrm{e}(x_1-x_2)}}$

where $E_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_{x_1}\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$, $E_{\mathrm{C}o@{\mathrm{C}}_2\mathrm{N}+\mathrm{L}{\mathrm{i}}_{x_2}\mathrm{S}{\mathrm{e}}_x{\mathrm{S}}_y}$, and $E_{\mathrm{L}\mathrm{i}}$ represent the total energies of Co@C₂N + Li_x1Se_xS_y, Co@C₂N + Li_x2Se_xS_y, and metallic lithium crystals, respectively. x₁ and x₂ represent the number of Li atoms in the Co@C₂N-Li_x1Se_xS_y and Co@C₂N-Li_x2Se_xS_y structures, and e denotes the electric charge. The calculated average open circuit voltages of the Co@C₂N-Li₂Se_xS_y system are depicted in Figure 8.

Generally, the high values of average open circuit voltages are achieved through extracting one Li from the Co@C₂N-Li₂Se_xS_y systems. Notably, with the increase selenium and sulfur concentrations, the maximum average open circuit voltage of Co@C₂N-Li₂Se_xS_y (x + y = 6) reaches 2.61 V. However, with the further rising concentration of selenium and sulfur, the average open circuit voltage decreases to 1.90 V for Co@C₂N-Li₂Se_xS_y (x + y = 8).

2.5. Energy Profiles and the Decomposition of Li₂SeS on the Surface of Co@C₂N Monolayer

As reported in previous studies [31,32,34,35], the transition metals deposited on the C₂N monolayer served as the pivotal adsorption and catalytic centers, showcasing significant catalytic effect in Li-Se batteries. To unravel the reversible conversion mechanism of Li₂Se_xS_y (x + y = 8) clusters on the surface of the Co@C₂N monolayer, the energy distribution of the Se₈ reduction pathway on the surface of the Co@C₂N monolayer is evaluated, as depicted in Figure 9. In the discharge process, the initial step involved the reduction of two Li atoms and Se_xS_y to form Li₂Se_xS_y (x + y = 8) clusters, followed by a sequence of reduction and disproportionation reactions, gradually yielding Li₂Se_xS_y (x + y = 6), Li₂Se_xS_y (x + y = 4), and Li₂SeS clusters. Upon the spontaneous exothermic transformation from Co@C₂N-Se_xS_y (x + y = 8) to Co@C₂N-Li₂Se_xS_y (x + y = 8), Co@C₂N-Li₂Se_xS_y (x + y = 6) clusters are engendered, exhibiting an average moderate exothermicity of 0.60 eV. However, in the reaction processes of Co@C₂N-Li₂Se_xS_y (x + y = 4) and Co@C₂N-Li₂SeS, the transformation processes become endothermic, aligning with the situations [34,36] for Li-Se batteries.

To elucidate the catalytic effect of Co@C₂N monolayer on the decomposition kinetics of Li₂Se_xS_y clusters, the decomposition of Li₂SeS on Co@C₂N and C₂N were further evaluated as a representative example, as depicted in Figure 10. The decomposition of Li₂SeS produces a single Li⁺ and LiSeS⁻ species by the reaction of Li₂SeS→LiSeS⁻ + Li⁺ with an activation barrier of 7.34 eV, which is 0.36 eV lower than the counterpart observed without Co doping. Such a finding suggests that the presence of the Co atom in the Co@C₂N monolayer reduces the decomposition energy of Li₂SeS, which in turn leads to a higher utilization rate of active Se_xS_y species. Overall, the Co@C₂N monolayer is beneficial for both facilitating the phase transition of Li₂SeS and promoting the redox reaction.

3. Computational Methodology

All first-principles calculations were carried out using the Cambridge serial total energy package (CASTEP) module within the Materials studio (2019) software [44]. The exchange-correlation interactions were handled using the Perdew–Burke–Ernzerhof (PBE) functional within the framework of the generalized gradient approximation (GGA) [45,46]. However, considering the limitations of the GGA approach in accurately describing the strong exchange-correlation effects in the 3d orbitals of transition metals [35,47], a Hubbard parameter correction (U_Co = 4.0 eV) [48] was implemented to account for the self-interaction of electrons in Co 3d orbitals. Ultrasoft pseudopotentials [49] were employed to describe the interaction between valence electrons and ionic nuclei, and empirical corrections within the Grimme’s scheme [50] were utilized to describe the Van der Waals interactions. Structure optimization and electronic property calculations were performed using a cutoff energy of 450 eV, and a 20 Å vacuum region was used to avoid layered interactions caused by adjacent Co@C₂N monolayers. Brillouin zone sampling employed Monkhorst-Pack grids set at 3 × 3 × 1 and 5 × 5 × 1 for structure optimization and electronic structure calculations, respectively, associating with the maximum force and energy convergence criteria of 0.05 eV/Å and 2 × 10⁻⁵ eV/atom, respectively.

4. Conclusions

In the work, the structure, anchoring mechanism, and catalytic performance of a Co@C₂N monolayer as a substrate for a Se_xS_y composite cathode material were investigated with the help of first-principles calculations. The computational outcomes show that the pronounced synergistic effect of Co-S and Li-N bonds leads to increased anchoring performance for Li₂Se_xS_y/Se_xS_y clusters, which in turn effectively mitigates the shuttle effect of high-order Li₂Se_xS_y (x + y = 4, 6, 8) clusters. The charge density difference Mulliken charge analysis underscores a substantial charge transfer from the Li₂Se_xS_y and Se_xS_y clusters to the Co@C₂N monolayer, indicating a noticeable chemical interaction between them. Consequently, the Li-S and Li-Se bond lengths in Li₂Se_xS_y clusters decrease with the anchoring process. Further density of states analysis reveals that the semi-metallic characteristics of the Co@C₂N monolayer persist even after the adsorption of Li₂Se_xS_y and Se_xS_y clusters, thereby facilitating the redox reaction. In addition, the catalytic performance of the Co@C₂N monolayer as a candidate cathode material for Li-Se_xS_y batteries is evaluated by the decomposition of Li₂SeS clusters, highlighting the role of the Co@C₂N monolayer in promoting the formation and decomposition of Li₂SeS during the discharge and charging processes. Overall, the Co@C₂N monolayer emerges as a promising candidate material and catalyst for Li-Se_xS_y batteries with remarkable anchoring and catalytic performance. Our present work provides some theoretical insights into designing promising cathode materials for achieving faster, longer-lasting, and higher-energy-density Li-Se_xS_y batteries.

Author Contributions

X.L.: Conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation; Y.Z.: writing—review and editing, visualization, supervision, project administration, funding acquisition; C.L. and S.T.: methodology, software, validation, formal analysis, investigation, resources, data curation. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article material; further inquiries for the raw calculation data can be directed to the corresponding author.

Acknowledgments

The authors appreciate the valuable comments and suggestions from the reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. (a,b) The optimized structures of intrinsic C2N and Co@C2N monolayers. The magenta, blue, and purple circles denote the Co, N, and C atoms, respectively. (c–e) The electronic band structures of (c) intrinsic C2N and (d) spin-up and (e) spin-down states of Co@C2N monolayers along the high symmetrical Γ-Μ-Κ-Γ path. Reproduced with permission from the Journal “Nanoscale”/Royal Society of Chemistry, ref. [32].

Figure 2. The most stable structures of Li2SexSy and SexSy (x + y = 2, 4, 6, 8) clusters.

Figure 3. (a) Binding energies of SexSy and Li2SexSy (x + y = 2, 4, 6, 8) clusters anchored on the surface of the Co@C2N monolayer at different adsorption positions. (b) Binding energies of the most stable SexSy and Li2SexSy (x + y = 2, 4, 6, 8) clusters on the surface of the Co@C2N monolayer.

Figure 4. The most stable configurations of SexSy and Li2SexSy clusters anchored on the Co@C2N monolayer. All considered structures are shown in Table S2 of the Supporting Information.

Figure 5. Electron density difference of Li2SexSy and SexSy clusters anchored on the surface of the Co@C2N monolayer. The charge density for the isovalue contour is 0.03 e Å−3. The cyan and yellow colors refer to the charge accumulation and depletion, respectively.

Figure 6. Mulliken charge transfer between Li2SexSy/SexSy clusters and the Co@C2N monolayer.

Figure 7. Electron localization function (ELF) plots of Li2SexSy/SexSy clusters adsorbed on the Co@C2N monolayer.

Figure 8. The calculated average open circuit voltages (V) of Li2SexSy clusters adsorbed on the Co@C2N monolayer.

Figure 9. Energy profiles for the reduction of Li2SexSy/SexSy clusters on the surface of the Co@C2N monolayer. The magenta, blue, purple, orange, and yellow circles denote the Co, N, C, Se and S atoms, respectively.

Figure 10. Energy barriers of the decomposition of Li2SeS on intrinsic C2N and the Co@C2N monolayer. The IS, TS, and FS represent the initial state, transition state, and final state along the decomposition pathway of Li2SeS cluster.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29225264/s1, Table S1: Structure and total energy of Li₂Se_xS_y/Se_xS_y clusters; Table S2: Adsorbed structure and binding energy (E_b) of Li₂Se_xS_y and Se_xS_y (x + y = 2, 4, 6, 8) on the Co@C₂N monolayer at different positions; Table S3: The average distances from the S atom to the Co@C₂N monolayer and the average distances between Li atom and adjacent N atom of Co@C₂N monolayer; Figure S1: The calculated density of states for Co@C₂N-Li₂Se_xS_y (Se_xS_y) structures.

References

1. Lin, D.; Liu, Y.; Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat. Nanotechnol.; 2017; 12, pp. 194-206. [DOI: https://dx.doi.org/10.1038/nnano.2017.16] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28265117]

2. Cheng, X.B.; Zhang, R.; Zhao, C.Z. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev.; 2017; 117, pp. 10403-10473. [DOI: https://dx.doi.org/10.1021/acs.chemrev.7b00115]

3. Choubey, P.K.; Chung, K.; Kim, M.; Lee, J.; Srivastava, R.R. Advance review on the exploitation of the prominent energy-storage element Lithium. Part II: From sea water and spent lithium ion batteries (LIBs). Miner. Eng.; 2017; 110, pp. 104-121. [DOI: https://dx.doi.org/10.1016/j.mineng.2017.04.008]

4. He, B.; Liu, D.; Cheng, Z.; Miao, Z.; Rao, Z.; Zhang, H.; Yuan, L.; Li, Z.; Huang, Y. Enabling Selenium-Rich Se_xS_y Cathodes to Work in Carbonate-Based Electrolytes. Adv. Energy Mater.; 2022; 12, 2102832. [DOI: https://dx.doi.org/10.1002/aenm.202102832]

5. Nirmale, T.; Kale, B.; Varma, A. A review on cellulose and lignin based binders and electrodes: Small steps towards a sustainable lithium ion battery. Int. J. Biol. Macromol.; 2017; 103, pp. 1032-1043. [DOI: https://dx.doi.org/10.1016/j.ijbiomac.2017.05.155]

6. Zeng, X.; Li, J.; Liu, L. Solving spent lithium-ion battery problems in China: Opportunities and challenges. Renew. Sustain. Energy Rev.; 2015; 52, pp. 1759-1767. [DOI: https://dx.doi.org/10.1016/j.rser.2015.08.014]

7. Li, W.; Ma, Y.S.; Li, P.; Jing, X.Y.; Jiang, K.; Wang, D.H. Synergistic Effect between S and Se Enhancing the Electrochemical Behavior of Se_xS_y in Aqueous Zn Metal Batteries. Adv. Funct. Mater.; 2021; 31, 2101237. [DOI: https://dx.doi.org/10.1002/adfm.202101237]

8. McCloskey, B.D.; Speidel, A.; Scheffler, R.; Miller, D.C.; Viswanathan, V.; Hummelshøj, J.S.; Nørskov, J.K.; Luntz, A.C. Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li–O₂ Batteries. J. Phys. Chem. Lett.; 2012; 3, pp. 997-1001. [DOI: https://dx.doi.org/10.1021/jz300243r] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26286562]

9. Zhao, Y.; Xu, L.; Mai, L.; Zhang, Q. Hierarchical mesoporous perovskite La_0.5Sr_0.5CoO_2.91 nanowires with ultrahigh capacity for Li-air batteries. Proc. Natl. Acad. Sci. USA; 2012; 109, pp. 19569-19574. [DOI: https://dx.doi.org/10.1073/pnas.1210315109]

10. Zeng, L.; Zeng, W.; Jiang, Y.; Wei, X.; Li, W.; Yang, C.; Zhu, Y.; Yu, Y. A Flexible Porous Carbon Nanofi bers-Selenium Cathode with Superior Electrochemical Performance for Both Li-Se and Na-Se Batteries. Adv. Energy Mater.; 2015; 5, 1401377. [DOI: https://dx.doi.org/10.1002/aenm.201401377]

11. Zhou, L.; Zhang, K.; Hu, Z.; Tao, Z.; Mai, L.; Kang, Y.; Chou, S.; Chen, J. Recent Developments on and Prospects for Electrode Materials with Hierarchical Structures for Lithium-Ion Batteries. Adv. Energy Mater.; 2018; 8, 1701415. [DOI: https://dx.doi.org/10.1002/aenm.201701415]

12. Sun, J.; Du, Z.; Liu, Y.; Ai, W.; Wang, K.; Wang, T.; Du, H.; Liu, L.; Huang, W. State-of-the-Art and Future Challenges in High Energy Lithium–Selenium Batteries. Adv. Mater.; 2021; 33, 2003845. [DOI: https://dx.doi.org/10.1002/adma.202003845]

13. Eftekhari, A.; Kim, D.W. Cathode materials for lithium–sulfur batteries: A practical perspective. J. Mater. Chem. A; 2017; 5, pp. 17734-17776. [DOI: https://dx.doi.org/10.1039/C7TA00799J]

14. Liu, X.; Huang, J.Q.; Zhang, Q.; Mai, L.Q. Nanostructured Metal Oxides and Sulfides for Lithium–Sulfur Batteries. Adv. Mater.; 2017; 29, 1601759. [DOI: https://dx.doi.org/10.1002/adma.201601759] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28160327]

15. Thackeray, M.M.; Wolverton, C.; Isaacs, E.D. Electrical energy storage for transportation—Approaching the limits of, and going beyond, lithium-ion batteries. Energy Environ. Sci.; 2012; 5, pp. 7854-7863. [DOI: https://dx.doi.org/10.1039/c2ee21892e]

16. Lu, P.; Liu, F.; Zhou, F.; Qin, J.; Shi, H.; Wu, Z. Lignin derived hierarchical porous carbon with extremely suppressed polyselenide shuttling for high-capacity and long-cycle-life lithium-selenium batteries. J. Energy Chem.; 2021; 55, pp. 476-483. [DOI: https://dx.doi.org/10.1016/j.jechem.2020.07.022]

17. Luo, C.; Xu, Y.; Zhu, Y.; Liu, Y.; Zheng, S.; Liu, Y.; Langrock, A.; Wang, C. Selenium@mesoporous carbon composite with superior lithium and sodium storage capacity. ACS Nano; 2013; 7, pp. 8003-8010. [DOI: https://dx.doi.org/10.1021/nn403108w]

18. Peng, H.; Huang, J.; Zhang, Q. A review of flexible lithium-sulfur and analogous alkali metal-chalcogen rechargeable batteries. Chem. Soc. Rev.; 2017; 46, pp. 5237-5288. [DOI: https://dx.doi.org/10.1039/C7CS00139H]

19. Xu, G.; Liu, J.; Amine, R.; Chen, Z.; Amine, K. Selenium and selenium-sulfur chemistry for rechargeable lithium batteries: Interplay of cathode structures, electrolytes, and interfaces. ACS Energy Lett.; 2017; 2, pp. 605-614. [DOI: https://dx.doi.org/10.1021/acsenergylett.6b00642]

20. Yang, C.; Yin, Y.; Guo, Y. Elemental selenium for electrochemical energy storage. J. Phys. Chem. Lett.; 2015; 6, pp. 256-266. [DOI: https://dx.doi.org/10.1021/jz502405h]

21. Abouimrane, A.; Dambournet, D.; Chapman, K.W.; Chupas, P.J.; Weng, W.; Amine, K. A new Class of lithium and sodium rechargeable batteries based on selenium and selenium-sulfur as a positive electrode. J. Am. Chem. Soc.; 2012; 134, pp. 4505-4508. [DOI: https://dx.doi.org/10.1021/ja211766q] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/22364225]

22. Sun, F.; Cheng, H.; Chen, J.; Zheng, N.; Li, Y.; Shi, J. Heteroatomic Se_nS_8–n Molecules Confined in Nitrogen-Doped Mesoporous Carbons as Reversible Cathode Materials for High-Performance Lithium Batteries. ACS Nano; 2016; 10, pp. 8289-8298. [DOI: https://dx.doi.org/10.1021/acsnano.6b02315] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27522865]

23. Chen, H.; Wang, C.; Dong, W.; Lu, W.; Du, Z.; Chen, L. Monodispersed Sulfur Nanoparticles for Lithium–Sulfur Batteries with Theoretical Performance. Nano Lett.; 2015; 15, pp. 798-802. [DOI: https://dx.doi.org/10.1021/nl504963e]

24. Fei, L.; Li, X.; Bi, W.; Zhuo, Z.; Wei, W.; Sun, L.; Lu, W.; Wu, X.; Xie, K.; Wu, C. et al. Graphene/sulfur hybrid nanosheets from a space-confined “sauna” reaction for high performance lithium-sulfur batteries. Adv. Mater.; 2015; 27, pp. 5936-5942. [DOI: https://dx.doi.org/10.1002/adma.201502668]

25. Zhang, J.; Fan, L.; Zhu, Y.; Xu, Y.; Liang, J.; Wei, D.; Qian, Y. Selenium/interconnected porous hollow carbon bubbles composites as the cathodes of Li-Se batteries with high performance. Nanoscale; 2014; 6, pp. 12952-12957. [DOI: https://dx.doi.org/10.1039/C4NR03705G]

26. Liu, L.; Wei, Y.; Zhang, C.; Zhang, C.; Li, X.; Wang, J.; Ling, L.; Qiao, W.; Long, D. Enhanced electrochemical performances of mesoporous carbon microsphere/selenium composites by controlling the pore structure and nitrogen doping. Electrochim. Acta; 2015; 153, pp. 140-148. [DOI: https://dx.doi.org/10.1016/j.electacta.2014.11.199]

27. Shiraz, M.H.A.; Zhu, H.; Liu, Y.; Sun, X.; Liu, J. Activation-free synthesis of microporous carbon from polyvinylidene fluoride as host materials for lithium-selenium batteries. J. Power Sources; 2019; 438, 227059. [DOI: https://dx.doi.org/10.1016/j.jpowsour.2019.227059]

28. Jiang, Y.; Ma, X.; Feng, J.; Xiong, S. Selenium in nitrogen-doped microporous carbon spheres for high-performance lithium-selenium batteries. J. Mater. Chem. A; 2015; 3, pp. 4539-4546. [DOI: https://dx.doi.org/10.1039/C4TA06624C]

29. Han, K.; Liu, Z.; Shen, J.; Lin, Y.; Dai, F.; Ye, H. A free-standing and ultralong-life lithium-selenium battery cathode enabled by 3D mesoporous carbon/graphene hierarchical architecture. Adv. Funct. Mater.; 2015; 25, pp. 455-463. [DOI: https://dx.doi.org/10.1002/adfm.201402815]

30. Deshpande, S.S.; Deshpande, M.D.; Alhameedi, K.; Ahuja, R.; Hussai, T. Carbon nitride monolayers as efficient immobilizers toward lithium selenides: Potential applications in lithium-selenium batteries. ACS Appl. Energy Mater.; 2021; 4, pp. 3891-3904. [DOI: https://dx.doi.org/10.1021/acsaem.1c00283]

31. Lin, H.; Jin, R.; Wang, A.; Zhu, S.; Li, H. Transition metal embedded C₂N with efficient polysulfide immobilization and catalytic oxidation for advanced lithium-sulfur batteries: A first principles study. Ceram. Int.; 2019; 45, pp. 17996-18002. [DOI: https://dx.doi.org/10.1016/j.ceramint.2019.06.018]

32. Tang, S.; Liu, C.; Sun, W.; Zhang, X.; Shen, D.; Dong, W.; Yang, S. Understanding the anchoring and catalytic effect of Co@C₂N monolayer in Lithium-selenium Batteries: A first-principles study. Nanoscale; 2021; 13, pp. 16316-16323. [DOI: https://dx.doi.org/10.1039/D1NR03406E]

33. Mahmood, J.; Lee, E.K.; Jung, M.; Shin, D.; Jeon, I.; Jung, S.; Choi, H.; Seo, J.; Bae, S.; Sohn, S. et al. Nitrogenated holey two-dimensional structures. Nat. Commun.; 2015; 6, pp. 6486-6492. [DOI: https://dx.doi.org/10.1038/ncomms7486] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25744355]

34. Du, J.; Xia, C.; Xiong, W.; Zhao, X.; Wang, T.; Jia, Y. Tuning the electronic structures and magnetism of two-dimensional porous C₂N via transition metal embedding. Phys. Chem. Chem. Phys.; 2016; 18, pp. 22678-22686. [DOI: https://dx.doi.org/10.1039/C6CP03210A] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27476579]

35. Tang, S.; Liu, W.; Yang, Z.; Liu, C.; Bai, S.; Zhang, J.; Luo, D. Unveiling the anchoring and catalytic effect of Co@C₃N₃ monolayer as a high-performance selenium host material in lithium–selenium batteries: A first-principles study. Phys. Chem. Chem. Phys.; 2023; 25, pp. 21054-21064. [DOI: https://dx.doi.org/10.1039/D3CP01902K]

36. Liu, W.; Yang, Z.; Bai, S.; Ai, P.; Wang, H.; Zheng, T.; Zhang, C.; Tang, S. Theoretical insights into the anchoring and catalytic effect of bilayer C₃N₃ for lithium-selenium batteries. Diam. Relat. Mater.; 2024; 143, 110880. [DOI: https://dx.doi.org/10.1016/j.diamond.2024.110880]

37. Segall, M.D.; Pickard, C.J.; Shah, R.; Payne, M.C. Population analysis in plane wave electronic structure calculations. Mol. Phys.; 1996; 89, pp. 571-577. [DOI: https://dx.doi.org/10.1080/002689796173912]

38. Segall, M.D.; Shah, R.; Pickard, C.J.; Payne, M.C. Population analysis of plane-wave electronic structure calculations of bulk materials. Phys. Rev. B; 1996; 54, pp. 16317-16320. [DOI: https://dx.doi.org/10.1103/PhysRevB.54.16317]

39. Becke, A.D.; Edgecombe, K.E. A simple measure of electron localization in atomic and molecular systems. J. Chem. Phys.; 1990; 92, pp. 5397-5403. [DOI: https://dx.doi.org/10.1063/1.458517]

40. Silvi, B.; Savin, A. Classification of chemical bonds based on topological analysis of electron localization functions. Nature; 1994; 371, pp. 683-686. [DOI: https://dx.doi.org/10.1038/371683a0]

41. Pan, Y.; Guan, W.M. Prediction of New Phase and Electrochemical Properties of Li₂S₂ for the Application of Li-S Batteries. Inorg. Chem.; 2018; 57, pp. 6617-6623. [DOI: https://dx.doi.org/10.1021/acs.inorgchem.8b00747] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/29756769]

42. Xu, Q.; Ban, C.; Dillon, A.C.; Wei, S.; Zhao, Y. First-Principles Study of Lithium Borocarbide as a Cathode Material for Rechargeable Li ion Batteries. J. Phys. Chem. Lett.; 2011; 2, pp. 1129-1132. [DOI: https://dx.doi.org/10.1021/jz200440m] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26295314]

43. Yang, Z.; Liu, W.; Bai, S.; Ai, P.; Wang, H.; Zheng, T.; Li, Q.; Tang, S. Anchoring and catalytic insights into bilayer C₄N₃ material for lithium–selenium batteries: A first-principles study. Phys. Chem. Chem. Phys.; 2024; 26, pp. 2291-2303. [DOI: https://dx.doi.org/10.1039/D3CP05075K] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38165716]

44. Clark, S.J.; Segall, M.D.; Pickard, C.J.; Hasnip, P.J.; Probert, M.I.J.; Refson, K.; Payne, M.C. First principles methods using CASTEP. Z. Kristallogr. Cryst. Mater.; 2005; 220, pp. 567-570. [DOI: https://dx.doi.org/10.1524/zkri.220.5.567.65075]

45. Perdew, J.P.; Burke, K.; Ernzerhof, M. Perdew, Burke, and Ernzerhof Reply. Phys. Rev. Lett.; 1998; 80, 891. [DOI: https://dx.doi.org/10.1103/PhysRevLett.80.891]

46. Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett.; 1996; 77, 3865. [DOI: https://dx.doi.org/10.1103/PhysRevLett.77.3865]

47. Perdew, J.P.; Wang, Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. B; 1992; 45, pp. 13244-13249. [DOI: https://dx.doi.org/10.1103/PhysRevB.45.13244]

48. Li, Y.; Sun, W.; Liang, J.; Sun, H.; Marco, I.D.; Ni, L.; Tang, S.; Zhang, J. Understanding the electrochemical properties of A₂MSiO₄ (A = Li and Na; M = Fe, Mn, Co and Ni) and the Na doping effect on Li₂MSiO₄ from first-principles calculations. J. Mater. Chem. A; 2016; 4, pp. 17455-17463. [DOI: https://dx.doi.org/10.1039/C6TA05313K]

49. Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B; 1990; 41, pp. 7892-7895. [DOI: https://dx.doi.org/10.1103/PhysRevB.41.7892]

50. Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem.; 2006; 27, pp. 1787-1799. [DOI: https://dx.doi.org/10.1002/jcc.20495]