Exploring the potential of MB₂ MBene family as promising anodes for Li-ion batteries

Abstract

In recent years, finding high-performance energy storage materials has become a major challenge for Li-ion batteries. B-based two-dimensional materials have become the focus of attention because of their abundant reserves and non-toxic characteristics. A series of two-dimensional transition metal borides (MBenes) are reported and their electrochemical properties as anode materials for Li-ion batteries are investigated by density functional theory (DFT) calculations. The surface of MB₂ possesses medium adsorption strength and diffusion energy barrier for Li atoms, which are conducive to the insertion and extraction of Li-ions during the charge/discharge process of Li-ion batteries. Herein, we explore the potential of MB₂ (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn) as the anode material for LIBs. Excitingly, the Li atom can be stably adsorbed on the surface of MB₂ (M = Sc, Ti, V, Nb, Mo, W) monolayers, and the theoretical capacity of the MB₂ monolayer is high (521.77–1610.20 mA h g⁻¹). The average open circuit voltage range is within 0.10–1.00 V (vs. Li/Li⁺). The relationship between the p-band center of the B atom and the adsorption energy of Li on the surface of MB₂ is also investigated. Furthermore, it is found that the charge transfer of Li atom and metallic center in the most stable position is strongly related to the corresponding value of diffusion energy barrier. These results confirm that MB₂ monolayers are promising 2D anode materials for Li-ion batteries, demonstrating the application prospects of B-based 2D materials.

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1. Introduction

The rapidly increasing global demand for advanced energy storage and conversion technologies has made lithium-ion batteries (LIBs) an important electrochemical energy storage candidate in the field of energy storage.^1–4 The performance of LIBs depends largely on the properties of the electrode materials, especially the anode materials, which mainly limit the performance of LIBs.^5–7 Consequently, the pursuit of high-performance anode materials has become a key challenge for the development of next-generation LIBs. Compared with three-dimensional materials, e.g., graphite, two-dimensional materials are widely used as anode materials for LIBs owing to their large surface area, excellent physical and chemical properties, and high charge carrier mobility.⁸ The successful extraction of 2D graphene has garnered significant research attention; unfortunately, high diffusion barrier and lithium nucleation over graphene limit its application in terms of durability.^9–11

Apart from graphene, 2D materials with novel compositions and properties are being actively pursued for advanced applications. One of the latest members of the 2D material family is MXene, which is produced from the topochemical deintercalation of the A layer from a laminate MAX phase with a general formula of M_n+1AX_n, where M refers to an early transition metal, A represents an A-group element (Al or Si), and X denotes C and/or N.^12,13 The specific 2D structure coupled with the metal sites endows MXenes with attractive performance for energy storage and catalytic applications.^14–16 So far, MXenes are limited to carbides, nitrides, or carbonitrides due to the constraints of MAX phase precursors. Recently, analogous 2D MBenes (transitional metal borides) have been predicted by theoretical calculations as excellent alternatives for a wide range of applications, such as metal-ion batteries¹⁷ and catalysts.¹⁸ In general, MBenes can refer to all 2D transition borides, such as FeB₂,¹⁹ MoB₂ (ref. 20) and TiB₁₂,²¹ showing a wide variety of structures. Among currently reported MBenes, MoB, and FeB monolayers have been theoretically verified to possess an omnidirectional small diffusion energy barrier and high storage capacity for Li,¹⁷ whereas VB, CrB and MnB monolayers have been demonstrated to possess high Young's modulus and unique in-plane anisotropy, low diffusion potential and low open-circuit voltage as anode materials for LIBs.²² Moreover, Mo₂B₂ and Fe₂B₂ possess metal-like electronic conductivity, small diffusion energy barrier and high Li storage capacity.¹⁷

Furthermore, monolayer MB₂ (M = Mn, Be, Mg, Mo, Fe and Ti) have been explored as promising anode candidates for high-energy-density LIBs due to their excellent electrical conductivity during the lithiation process.^23–27 Therefore, the proposal of MB₂ monolayers may offer a novel strategy for the rational design of 2D anode materials for LIBs. Hence, Zhang et al. have examined 34 MB₂ monolayers with different M elements, ranging from group IIA to IVA, to screen out stable materials for promising applications using particle swarm optimization (PSO) and density functional theory (DFT) computations. The screening process identified eight stable MB₂ monolayers with M = Be, Mg, Ti, Hf, V, Nb, Ta and Fe.²⁸ Therefore, the systematic research of MB₂ for secondary-ion batteries is still scarce, which leads to sluggish development of MB₂. Hence, more fundamental studies of vast MB₂ are required. Specifically, the covalent network of borides can facilitate the formation of monolayer structures, which are tempting for the large specific surface and high utilization.

Herein, we explore the potential of MB₂ in energy storage by investigating the potential of a class of MB₂ monolayers as anodes in LIBs using first-principle calculations. We mainly use 2D MB₂ as an electrode of LIBs to predict its Li-ion storage performance. Then, the kinetic stability and electrochemical properties of these single-layer materials as anode materials for LIBs are calculated. In order to understand the Li storage interaction mechanism of MB₂, we have also investigated the projective state density, p-band center and charge density difference. The current work aims to explore potential high-performance anode materials, providing a reference for the subsequent research on the application of 2D borides in Li-ion batteries.

2. Computational details

Fig. 1 shows top and side views of the relaxed crystal structure of MB₂ (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn) monolayer (2 × 2 × 1 supercell). Each single cell of MB₂ contains one M atom and two B atoms. The basic building block of MB₂ is composed of M and B atomic layers, where the arrangement of the upper B atomic layer is similar to the honeycomb distribution of C atoms in graphene and M atoms are placed directly below the center of the B–B honeycomb. The layer thickness of MB₂ are shown in Table S1.†

Fig. 1 (a) Top and (b) side views of MB₂ monolayers (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn), site 1 to 6 are different adsorption sites, where purple and green colors denote the transition metal and boron atoms, respectively.

After an extensive structural search using the CALYPSO structure prediction method,^29,30 two-dimensional monolayer MB₂ were obtained. All calculations, such as structural optimization, electronic properties and energy, within the DFT framework, were implemented in the Vienna ab initio Simulation Package (VASP).³¹ The exchange–correlation energy of the electrons was described within the generalized gradient approximation (GGA) framework using the Perdew–Burke–Ernzerhof (PBE) functional.³² The interactions between nuclear electrons and valence electrons were described using the projector-enhanced wave (PAW) method.³³ The cut-off energy was set to 520.00 eV. All structures were fully relaxed until the convergence tolerance for the energy and force on each atom were less than 1.0 × 10⁻⁷ eV and 0.01 eV Å⁻¹, respectively. All atomic structures and charge density distributions were visualized using the VESTA package.³⁴ The 5 × 5 × 1 k-point lattices were used in structural optimization calculations, and the 11 × 11 × 1 k-point lattices were applied in electronic structure analysis. Thermodynamic stability is assessed through ab initio molecular dynamics (AIMD) simulations conducted within the canonical ensemble (NVT) at 300 K, employing a Nose–Hoover thermostat with a 6 ps period, and utilizing a time step of 2 fs. The DFT-D3 method was employed to correct van der Waals interactions between the adsorption atom and layers since it can correctly handle remote dispersion interactions.³⁵ An inter-layer vacuum space of 20 Å was used to avoid the interactions between adjacent layers. To confirm the dynamic stability of MB₂ monolayers, the phonon dispersion curve was calculated using a finite displacement method approach as implemented in the PHONOPY code with convergence accuracy consistent with the electronic calculation.³⁶ With the help of complete Linear Synchronous Transit (LST)/Quadratic Synchronous Transit (QST) method and nudged elastic band (NEB) tools in the Dmol³ code,³⁷ the transition state and diffusion pathways were predicted.³⁸ The double numerical atomic orbital augmented by a polarization function (DNP) is chosen as the basis set.³⁹ A smearing of 0.005 Ha (1 Ha = 27.21 eV) to the orbital occupation is applied to achieve accurate electronic convergence. In transition state search calculations, the convergence tolerances of energy, maximum force and displacement are 1.0 × 10⁻⁵ Ha, 0.002 Ha Å⁻¹ and 0.005 Å, respectively. Incorporating spin polarization was necessary to achieve precise determination of the ground state energy. The Bader charge analysis serves to measure and evaluate the transfer of charges.

The adsorption energy equation of a single Li atom on the surface of MB₂ can be given as follows:

E_ad = E_MB2+Li − E_MB2 − E_Li (1)

where E_MB2+Li and E_MB2 denote the total energy of MB₂ with and without adsorbed Li atom, respectively, E_Li refers to the energy per Li in the bulk lattice.

The charge density difference can be given as follows:

Δρ = ρ_LiMB2 − ρ_MB2 − ρ_Li (2)

where ρ_LiMB2 and ρ_MB2 represent the charge density of MB₂ monolayers after and before Li atom adsorption, respectively, and ρ_Li refers to the charge density of an individual Li atom.

The charge/discharge process follows the usual half-cell reaction in an aqueous solution, as follows:⁴⁰

MB₂ + xLi⁺ + xe⁻ ↔ Li_xMB₂ (M = Sc, Ti, V, Nb, Mo, W)

The average adsorption energy, which is defined as the difference between the total energy of the system before and after the Li atom adsorption process, can be expressed as:

where E_LinMB2 and E_MB2 are the total energies of MB₂ with n adsorbed Li atoms and the pristine MB₂ monolayer, respectively. E_Li denotes the energy per Li atom derived from the bcc bulk metal, and n stands for the number of Li atoms on both sides of the MB₂ monolayer.

On the premise of ignoring the influence of volume, pressure and entropy, the open circuit voltage (OCV) can be given as follows:⁴¹

where , and E_Li represent the energy of Li_n1MB₂, Li_n2MB₂ and per atom for bulk Li metal, respectively, and n₁ and n₂ denote the number of adsorbed Li atoms.

The estimate of theoretical specific capacity (TSC) is obtained by the following equation:

where x_max refers to the maximum adsorption concentration of Li atoms on MB₂ monolayers, F represents the Faraday constant with a value of 26 801 mA h mol⁻¹, and m_MB2 stands for the molecular mass of MB₂.

3. Results and discussion

3.1 Adsorption of Li atom on MB₂ monolayer

To explore the potential of MB₂ (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn) as an anode material, we have investigated the adsorption of Li atom on the surface of MB₂. According to the symmetry of MB₂ structure, there are six adsorption sites for Li atom (sites 1 to 6 in Fig. 1), which can be divided into two parts based on the location of Li atom, whether it is closer to the B atomic side or the metal side. The adsorption energy of a single Li atom on MB₂ was calculated using eqn (1). The calculation results are shown in Table S2.† For MB₂ (M = Sc, Ti, V, Cr, Cu, Zn, Y, Nb, Mo, Hf, Ta and W), Li tends to be close to B atom of MB₂, especially the site 1 in the center of the B-layer six-membered ring is the most favorable adsorption position for Li atom. However, in the case of FeB₂ and MnB₂, Li atoms tend to be close to metal atoms of MB₂, and the best adsorption site is site 5. In addition, it is found that CoB₂, NiB₂ and ZrB₂ systems adsorb a single Li atom and exhibit obvious structural distortion, indicating that these systems are not suitable anode candidates.

Based on the preferential adsorption site, the adsorption energy (E_ad) of Li atom the B atomic side in fourteen MB₂ structures was computed (Fig. 2a). Our computed values are in the range of −0.11 to −2.31 eV, which are close to the reported boride results (−0.36 to −2.70 eV).^42–46 Furthermore, the results reveal that the strongest adsorption of Li atom occurs on MoB₂ surface (−2.31 eV), while MnB₂ exhibits relatively weak binding with Li atom (−0.11 eV). At the same time, the Li atom is difficult to attach to the B atom side of FeB₂. As shown in Fig. S1,† the adsorption energy of Li atom on the metal side of TiB₂ and NbB₂ becomes positive, indicating that Li also is difficult to adsorb on the metal side. For FeB₂ and MnB₂, the adsorption strength of Li atom on the metal side is stronger than the B atomic side. However, the adsorption strength on the metal side for other structures is weaker than the adsorption strength on the B atomic side.

Fig. 2 (a) The adsorption energy of Li atom on the B atomic side of MB₂ monolayers; (b) top and side views of differential charge density diagram of Li atom adsorbed on TiB₂ monolayer, where the green region indicates charge depletion and yellow region indicates charge accumulation and blue, orange, and green colors denote the Ti, Li and B atoms, respectively.

To gain a deeper understanding of the adsorption mechanism between Li atom and MB₂ surface, we have calculated the charge density difference (Δρ) between Li and MB₂ using eqn (2), where the result of TiB₂ is shown in Fig. 2b. It is worth noting that the charge transfers from Li to B. Subsequently, the charges deplete around the Li atom and accumulate near the B atomic layer, respectively. This can be attributed to the electronegativity of B atoms. In addition, Bader charge analysis was performed to quantify the charge transfer effect.⁴⁷ About 0.85 charges were observed to transfer from the Li atom to the B atomic layer. Similarly, the charge density difference and Bader charge of the most stable adsorption sites on the surface of other structures are shown in Fig. S6, Tables S3 and S4.† The amount of charge transfer from the Li atom to the nearest B atom of MB₂ varies slightly within the range of 0.837 to 0.874, indicating that the charge transfer of the Li atom is almost complete, which is consistent with other borophenes⁴⁸ and borides.²⁵ Moreover, the strong attractive electrostatic interactions between Li atom and MB₂ monolayer can avoid the formation of Li clusters, thereby ensuring better electronic conductivity throughout the charge/discharge process and improving the safety of LIBs.

According to the previous reports,^49–52 there is a certain correlation between the p-band center of non-metal atoms and the adsorption energy (E_ad) of the investigated system. The p-band center is of particular interest for application in energy storage and conversion. As an illustration, they can be used as electrode materials in LIBs,⁵¹ offering the potential for a dependable and high-energy-density cathode or anode. Consequently, we have explored the correlation between the p-band center of B atoms and the adsorption energy of Li atom on the surface of MB₂ (Fig. 3). Fig. 3 illustrates an approximate trend where, as the p-band center shifts toward the Fermi level, the values of E_ad gradually increase. Moreover, the location change of p-band center is the result of atomic orbital hybridization. Herein, it is worth noting that the p-band center is affected by the hybridization of the B atom p-orbitals in the MB₂. When B atom p-orbitals are hybridized for the MB₂ with different M elements, they become more diffused and less strongly bound to the atomic nucleus. We have compared the correlation between the p-band center and the diffusion energy barrier of the B-side Li atom and the adsorption energy and diffusion energy barrier of the metal-side Li atom, respectively, and found that the relationship between them is not obvious (Fig. S2†). According to the report of Zheng et al.,⁵³ there is a correlation between the d-band center of transition metal atoms and the adsorption energy, and we have also analyzed this, and the results are shown in Fig. S3,† which shows that the correlation is not strong enough.

Fig. 3 The relationship between the p-band center and the adsorption energy of Li atom on the B atomic side of MB₂ monolayers and p-orbital projected density of states (PDOS). The curve in the inset diagram is the PDOS curve of the B atom in TiB₂.

3.2 Diffusion of Li atom on MB₂ monolayers

Besides the adsorption, the diffusion of Li atoms needs to be considered for the charge and discharge efficiency.⁵⁴ There are four possible diffusion paths for the Li atom on the B atomic and metal surfaces of the MB₂ monolayer (Fig. S4a and b†). On the B atomic side, a direct path through two neighboring sites 1 (path I) and an indirect path through two neighboring sites 1 (through site 2, path II) (Fig. S4a†). On the metal side, a direct path through two adjacent sites 5 (path III) and a indirect path through two adjacent sites 5 (through site 4, path IV) (Fig. S4b†). The diffusion barrier of Li atom along paths I–IV on the MB₂ monolayer is calculated (Tables S5 and S6†). We compare the diffusion energy barriers of the four diffusion paths of Li atom on the B atomic side and metal side. It can be seen that the Li atom is more inclined to diffuse through paths II and IV on the B atomic side and the metal side. This is consistent with the previously reported diffusion path of Li atom on the surface of BeB₂ and MgB₂.²⁴

Fig. 4a and S5† present the diffusion barrier profiles for the Li atom and the diffusion path of Li atom on the B atomic side and metal side of MB₂ monolayer, respectively. According to the previous reports, the diffusion energy barrier of most 2D materials is in the range of 0.03 to 0.78 eV.^55–60 Herein, the computed values are comparable to those of previously studied electrode materials. The computed diffusion energy barrier for Li atom on the B atomic side of MoB₂ is 0.63 eV, which is comparable to the previously reported value of 0.52 eV,²⁵ showing the reliability of current results. Furthermore, to explore the law of diffusion energy barrier of different metal elements on the surface of MB₂, the relevant discussions were carried out. For Group 1 in Fig. 4b, it can be clearly observed that the smaller adsorption energy leads to a higher diffusion energy barrier. However, for Group 2, the relationship between adsorption energy and diffusion energy barrier is not obvious.

Fig. 4 (a) Diffusion energy barrier and diffusion path of Li atom on the B atomic side; (b) adsorption energy and diffusion energy barrier of Li atom on the B atomic side; the diffusion energy barrier of Li atom along path II with the amount of charge transfer of (c) Li atom and (d) metal atom at the most stable position, respectively.

The charge transfer of Li atom and metal atom in the most stable position is strongly related to the corresponding value of diffusion energy barrier (Fig. 4c and d). With the increase of diffusion energy barrier, the charge transfer of Li atom decreases gradually, while the charge transfer of metal atom increases. There is no obvious relationship between the number of transferred charges and adsorption energy, which may be ascribed to the presence of the metal atoms in the structure of MB₂.

3.3 Open circuit voltage and theoretical charge storage capacity

From the viewpoint of practical applications and theoretical research, the OCV serves as a crucial parameter for evaluating the performance of LIBs. The E_ave and OCV of different MB₂ structures for Li atoms can be calculated by eqn (3) and (4), respectively. As a negative electrode material for LIBs, MB₂ structure should exhibit a certain stability. Notably, the phonon spectra of ScB₂, MoB₂, and WB₂ monolayers exhibits tiny imaginary frequencies only at the gamma point, which should be consistent with the previous findings of Zhang et al.²⁸ In addition, according to previous reports, the tiny virtual frequency of the gamma point does not affect the stability of the material.^20,23,61,62 Based on the above, we think these three materials are also dynamically stable. Herein, MB₂ (M = Sc, Ti, V, Nb, Mo, W and Ta) structures are selected via the relevant band structure and phonon spectra owing to their stability and meta metallicity (Fig. S6 and S7†). Referring to Fig. 4b, the diffusion energy barrier and adsorption energy of these six systems are located in the middle region. Hence, appropriate diffusion energy barrier and adsorption strength are also conducive to the insertion and extraction of Li-ions in the anode material.

Before calculating OCV, we need to first consider the average adsorption energy of MB₂. We strive to systematically build a stable configuration by progressively adsorbing multiple Li atoms on both sides of the MB₂ monolayer (Fig. 5a). On the 2 × 2 × 1 supercell of MB₂ monolayers, four Li atoms are adsorbed in each layer, and the Li atoms from the first layer to the fourth layer are adsorbed at site 1, site 5, site 2 and site 5, respectively, in which the Li atomic layers of the second and fourth layers are staggered. As the adsorption concentration of each structure increases gradually, the adsorption strength decreases and the rate of adsorption strength reduction becomes smaller (Fig. 5b). Among them, the adsorption strength of ScB₂ changes the most when adsorbing the first layer to the second layer.

Fig. 5 (a) The structural configuration of four layers of Li atoms adsorbed on the surface of MB₂; (b) the change in average adsorption energy and (c) the open circuit voltage of Li_xMB₂ (M = Sc, Ti, V, Nb, Mo, W and Ta) as a function of Li concentration.

Based on the average adsorption energy, we can further calculate the OCV of MB₂ monolayer (Fig. 5c). With the increase of adsorption concentration, the value of OCV decreases gradually and the decrease rate becomes smaller and smaller. Specifically, ScB₂ exhibits the most significant changes and its OCV decreases by 43.0% from the first to the second layer, 21.7% from the second to the third layer, and 8.5% from the third to the fourth layer. With the increase of adsorbed Li atom content, the average adsorption energy of each structure is positive and the change of OCV is further calculated. It can be found that the OCV of MB₂ (M = Sc, Ti, V, Nb, Mo, W and Ta) ranges from 0.48–1.87, 0.42–1.28, 0.39–1.12, 0.44–1.39, 0.51–2.31, 0.48–1.96 and 0.58–2.25 V, respectively. For LIBs systems, the average OCV calculations are 0.87, 0.81, 0.73, 0.77, 0.89, 0.80 and 1.07 V, respectively. Among them, the operating voltage ranges of ScB₂, TiB₂, VB₂, NbB₂, MoB₂ and WB₂ materials are the most suitable for anode materials (0.10–1.00 V). Previous research studies have shown that the excessively low average OCV leads to metallic electroplating, whereas an exorbitantly high average OCV impedes to achieve a high energy density.^22,63

Theoretical specific capacity (TSC) is a frequently used index to evaluate the performance of anode materials in LIBs. The TSC of different MB₂ structures for Li atoms can be calculated by eqn (5). The correlation between the diffusion energy barrier of the Li atom on B atomic side of MB₂ monolayers and theoretical specific capacity is not strong (Fig. 6a). Herein, we divide MB₂ structures into three groups. Group 1 exhibits high TSC values with a wide range of diffusion energy barrier for Li atom on MB₂ monolayers. In the case of Group 2, the diffusion energy barrier for Li atom on MB₂ monolayers is higher than Group 1, but the TSC values are lower. Group 3 exhibits minimum TSC values and a moderate diffusion energy barrier.

Fig. 6 The theoretical specific capacity of different MB₂ monolayers with respect to (a) diffusion energy barrier and (b) adsorption energy of Li atoms on B atomic side.

Fig. 6b presents the correlation between the adsorption energy of the Li atom on B atomic side of MB₂ monolayers and theoretical specific capacity of the Li–MB₂ systems. Overall, the correlation is relatively complicated. The adsorption energy of most MB₂ structures is relatively high (Group 1). When the adsorption strength is relatively weak, the theoretical specific capacity is in the upper middle position (Group 2).

According to the maximum adsorption concentration, we find that MB₂ (M = Sc, Ti, V, Nb, Mo, and W) adsorbing Li atoms has an ultra-high theoretical specific capacity (521.77–1610.20 mA h g⁻¹). The specific capacity is higher than that of several reported anode materials, such as graphite (372 mA h g⁻¹), pillared MnO₂ (588 mA h g⁻¹),⁶⁴ Ti₃C₂ (488 mA h g⁻¹),⁶⁰ and Nb₂CS₂ (194.36 mA h g⁻¹).⁵ What's more, the specific theoretical capacity is comparable to that of graphene monolayer (1116 mA h g⁻¹),⁶⁵ Hf₂S (1378 mA h g⁻¹)⁶⁶ and BN-doped reduced graphene oxide (BN-rGO) (1583 mA h g⁻¹).⁶⁷ They are expected to become negative electrode materials for LIBs in the future.

Considering that the change of MB₂ geometry after the adsorption of Li atom will lead to the change of electronic structure, the band structure and PDOS of Li₄MB₂ are further analyzed (Fig. 7a–f). It can be seen that the contribution of the states around the Fermi level is mainly from the atoms in the MB₂ monolayer, while the contribution of the Li atoms is very small. The above results show that the metal properties of MB₂ monolayer can be well maintained before and after the insertion of Li atom into MB₂, so that the electronic conductivity of MB₂ monolayer can meet the requirements of LIBs during the whole charge and discharge process. Therefore, we believe that MB₂ monolayer has very excellent storage capacity and electronic properties, and is a very ideal anode material. In addition, we evaluated the structural thermal stability of the MB₂ monolayer at 300 K before and after adsorption of Li atoms using AIMD simulation method (Fig. S8 and 9†). The mean values of potential energy and temperature remained almost constant at room temperature of 300 K throughout the AIMD simulation, and MB₂ held the initial structural framework well, indicating that they have good thermal stability. These results indicate that MB₂ monolayers have high synthetic potential under certain experimental conditions. In particular, maintaining stability at this temperature is sufficient to ensure the safe operation of LIBs.

Fig. 7 (a–f) Energy band structures and PDOS of the Li₄MB₂ (M = Sc, Ti, V, Nb, Mo, and W) monolayers. The horizontal black dashed lines represent Fermi levels.

4. Conclusions

Based on the first-principle calculations, we have investigated the potential of MB₂ (M = Sc, Ti, V, Nb, Mo and W) as anode materials for LIBs. The surface of MB₂ exhibited medium adsorption strength and diffusion energy barrier for Li atom, which is conducive to the insertion and extraction of Li-ions during the charge/discharge process. For different adsorption structures, the mean adsorption energy of MB₂ is negative. The correlation between the p-band center of the B atomic atom and the adsorption energy of Li atom on the surface of MB₂ is quite obvious. As the p-band center undergoes an upward shift towards the Fermi level, a diminution in the adsorption energy is observed. Finally, MB₂ can spontaneously adsorb Li atoms, forming Li₄MB₂. The theoretical specific capacity range of Li atoms adsorbed on MB₂ monolayers (M = Ti, V, Nb, Mo and W) ranges from 521.77 to 1610.20 mA h g⁻¹. These values surpass those of Ti₃C₂ (488 mA h g⁻¹), V₂CS₂ (301.12 mA h g⁻¹) and other conventional anode materials. The average voltage range of MB₂ falls within the ideal energy range for anode materials (0.10–1.00 V). The results confirm that MB₂ monolayer is a promising LIBs electrode material. Additionally, these studies can provide some clues for the design of other high-capacity metal diboride anodes, and inject new impetus for further experimental and theoretical research.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51701152, 22002116), the Natural Science Foundation of Shaanxi Province (2018JQ5181).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra00287c

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