Two-dimensional Janus BeSeCl as a potential anode material for sodium- and potassium-ion batteries

Nidhi Verma and Ashok Kumar*
Department of Physics, Central University of Punjab, Bathinda, India-151401. E-mail: ashokphy@cup.edu.in

First published on 5th August 2025

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1. Introduction

The world is currently confronting two primary energy challenges, namely, switching from fossil fuels to sustainable energy sources for electricity production and switching from internal combustion engines (ICEs) to electrical propulsion for road transport.¹ In order to address the significant concerns of resource wastage, rising urbanization, and climate change in the years to come, collaborative efforts are needed to establish low-emission and sustainable energy storage systems. The development of energy storage technologies, including batteries, is essential for enhancing the security of the energy supply and to guarantee the access to continuous and low-cost electricity.

As the most developed and widely used energy preservation technology, lithium-ion batteries (LIBs) are being widely used in a variety of industries such as large-scale power plants, electric cars, and portable electronic devices.² However, the scarcity and uneven distribution of lithium resources have sparked the invention and industrialization of innovative batteries like magnesium-ion batteries (MIBs), sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) based on Mg, Na and K ions, respectively, because of their plentiful supply, apparent efficiency, and affordability.³ In light of their chemical resemblance with lithium, sodium and potassium have started to emerge as potential alternatives to lithium.^4,5 Although Na and K have the same operating mechanism as Li, still conventional anode of LIBs cannot be employed for SIBs and PIBs because of the larger ionic radii of Na (1.66 Å) and K (2.03 Å), leading to the exploration of anode materials. Eventually, the emergence of novel electrode materials with desired electrochemical efficiency is crucial.

Nowadays, two-dimensional (2D) materials have garnered considerable attention as anode electrodes for secondary batteries because of their large surface area, high electrical conductivity, more active sites and fast ion-transportation, which enhance interactions between the electrolyte and electrode at the interface.⁶ Several 2D materials, including graphene, MXenes, TMDs, and Xenes, have been evaluated experimentally and computationally in order to develop anode electrodes.^7,8 In this category, a rising class of 2D materials, namely, Janus materials, has been explored widely for various applications like optoelectronics,⁹ spintronics,¹⁰ thermoelectrics,¹¹ batteries,¹² solar cells¹³ and water-splitting.^14,15 These Janus structures can be used in robust energy storage devices because of their dual functionality and ability to develop various surface attributes.¹⁶ Implementing diverse features into the Janus configuration improves performance and stability, resulting in more efficient and stable metal-ion batteries.¹⁷

Following the experimental fabrication of Janus graphene¹⁸ and Janus TMDs,¹⁹ these materials have attracted particular scrutiny from researchers both experimentally and computationally owing to the remarkable physical features that are brought about by breaching the mirror symmetry of conventional 2D materials.²⁰ For instance, 2D MoS₂ (MoSe₂) exhibits a specific capacity of 669 mAh g⁻¹ (ref. 21) (422 mAh g⁻¹),²² while Janus MoSSe exhibits a higher specific capacity of 776.5 mAh g⁻¹ (ref. 23) for LIBs. The Janus TiSSe monolayer of experimentally synthesized TiS₂ (ref. 24) exhibits a better specific capacity in the range of 337–665 mAh g⁻¹ for LIBs, SIBs and MIBs, with a low diffusion barrier (0.10–0.94 eV).²⁵ Theoretically explored Janus TiSSe (VSSe) monolayers were reported to have a low OCV of 0.15 V (0.72 V) and a specific capacity of 337 mAh g⁻¹ (331 mAh g⁻¹).²⁶ Additionally, the H-phase VSeTe monolayer is also reported to exhibit fast ion transport, with a diffusion barrier of 0.15 eV and 0.18 eV at Se and Te layers, respectively, and a specific capacity of 416 mAh g⁻¹.²⁷ Recently, Janus V₂COS has been theoretically described with a moderate OCV of 0.76 V and a specific capacity of 663 mAh g⁻¹ for SIBs.²⁸ Thus, the elimination of symmetry in Janus structures can establish an intrinsic dipole moment and electric field, causing different adsorption aspects in both upper and lower layers.

Next, to alter the various characteristics of 2D materials, substitutional doping can be a crucial approach.²⁹ Merging a second type of atom into the basic structure can alter the inherent crystal structure, stability, electronic–ionic states, band gap, etc. Modifying these characteristics can also change the fundamental reaction kinetics, specific capacity, charging rate and energy density. More significantly, controlled doping is crucial for efficiently improving the performance of anode materials.³⁰ Earlier studies on Me-graphene (C₅₆₈) showed a specific capacity of 344 mAh g⁻¹ while Ge, P, Si and Al-doped Me-graphene exhibited specific capacities of 379 mAh g⁻¹, 613 mAh g⁻¹, 779 mAh g⁻¹ and 1097 mAh g⁻¹, respectively.³¹ Transition metal (Cr, Mn, Fe, Co and Ni) doped phosphorene also confirmed that Fe-doped phosphorene owns a high adsorption energy of −2.95 eV, specific capacity of 828 mAh g⁻¹ and fast transportation with diffusion barrier of 0.09 eV.³² Also, O-doped VS₂ monolayer exhibits higher specific capacity of 1419 mAh g⁻¹ and diffusion barrier of 0.02 eV for LIBs.³³ These results confirm that doping may strengthen the electrochemical performance of anode materials.

Following the successful fabrication of BeN₄,³⁴ various beryllium-based 2D materials have been examined. Similar to BeN₄,³⁵ transition-metal atoms like Ir, Cu, Ni, Rh, Pd, Pt and Au have also been reported to theoretically favour geometrical structure co-ordination with Be. Consequently, following the experimental fabrication of a 2D RhSeCl monolayer,³⁶ in this work, we have theoretically explored the novel 2D Janus BeSeCl monolayer, since beryllium-based Janus structures have remained theoretically and experimentally unexplored. The desirable characteristics of improved electrode materials with a high surface area, low ion diffusion barriers, high conductivity, and a long cycling life can be fulfilled by Be-based materials, which feature light atomic weight, mechanical robustness and higher conductivity. We have also incorporated the anodic performance of various Be-based materials that exhibit exceptional storage capacity, fast ion-transportation and low voltage. Considering these desirable characteristics of beryllium, we selected the Be-based novel Janus structure BeSeCl. After confirming its structural, dynamic, mechanical and thermodynamic stability, we studied the electronic characteristics of the BeSeCl monolayer. To examine the electrochemical properties of the BeSeCl anode material for Na- and K-ion batteries, we determined their adsorption characteristics, specific capacity and migration barriers. To enhance the anodic performance of 2D BeSeCl, we introduced substitutional doping of Al and Si atoms. The results demonstrate that 2D BeSeCl has the potential to operate as an anode material for SIBs and PIBs.

2. Computational details

The Vienna Ab Initio Simulations Package (VASP)³⁷ software, based on density functional theory (DFT), was used to evaluate the properties. The exchange–correlation interactions were analysed via generalized gradient approximation (GGA). Implementing the Perdew–Burke–Ernzerhof (PBE) functional,³⁸ valence electrons for Be, Se and Cl are considered as 2s², 4s²4p⁴ and 3s²3p⁵ electrons, respectively. The periodic cell replicated the monolayers having a substantial vacuum of 20 Å along the out-of-plane direction to avoid interactions between layers. Following the structural optimization, the Hellmann–Feynman convergence limits for force and energy of 10⁻⁴ eV Å⁻¹ and 10⁻⁶ eV were obtained, respectively. The thermal viability of the monolayer structure was verified using ab initio molecular dynamics simulations (AIMD)³⁹ with a total computation time of 5 ps with a timestep of 1 fs. The Nosé–Hoover thermostat⁴⁰ was employed to regulate temperature at 300 K and 500 K during AIMD simulations. To analyse the electrochemical performance of the anode, a 3 × 3 × 1 supercell was adopted. The DFT-D3 approach⁴¹ was applied to explain the weak vdW interactions between metal atoms and the BeSeCl surface. The Bader charge approach⁴² was incorporated to analyse the charge distribution between atoms. Using the initial and final state of Na-/K-atom migration, the nudged elastic band method with climbing-image (CI-NEB) was adopted to evaluate the minimum energy profile, which was incorporated using the VASP transition state tool (VTST) code.^43,44 All the convergence parameters are consistent with previous studies.^45,46

3. Results and discussion

3.1 Structure and stability of 2D BeSeCl

The intrinsic structural characteristics of the 2D Janus BeSeCl monolayer comprised three subatomic layers, as pictured in Fig. 1(a). The hexagonal unit cell of the monolayer features three atoms arranged in a tetragonal crystal framework with space group P3m1. The relaxed lattice parameters of the BeSeCl monolayer are a = b = 3.17 Å, with a thickness of 2.88 Å. Notably, the Be–Se bond length of BeSeCl is 2.46 Å, which is somewhat longer than the Be–Cl bond length of 2.21 Å. The presence of different atoms (Se and Cl) over the BeSeCl surface disrupts the out-of-plane symmetry, manifesting an intrinsic dipole moment and electric field. This electric field can be obtained by a slope between the minima of the electrostatic potential of both surface atoms, as displayed in Fig. 1(b). The electric field of 1.03 eV Å⁻¹ is calculated with a direction from the Se to Cl side, which is primarily caused by the electronegativity difference of the Se (2.55) and Cl (3.16) atoms. The difference in potential between both sides is calculated to be 0.36 eV.

Experiments suggest that mechanical exfoliation may be used to synthesize 2D materials with modest layer-to-layer interactions from their bulk equivalents.^47,48 To fabricate the 2D Janus BeSeCl monolayer, the cleavage energy (E_cl) was simulated using a slab model containing four layers with AB stacking as portrayed in Fig. S1, SI. To provide a defined separation interval, the cleavage process was executed using constrained atomic positions. Fig. 1(c) shows that as the separation interval increases, the energy variation, (E_d − E_d0), correspondingly increases and converges at a value of 0.28 J m⁻². The obtained E_cl is smaller than for the previously studied graphene (0.36 J m⁻²)⁴⁹ and MoS₂ (0.29).⁵⁰ The low E_cl was confirmed using the cleavage strength (first order gradient of E_cl) in Fig. 1(c), which tends to zero value at the convergence.

Next, to ensure the energetic stability of 2D BeSeCl, we obtained the cohesive energy using the expression:

	(1)

Here, E_Be, E_Se and E_Cl symbolise the total energy of isolated Be, Se and Cl systems, respectively, and E_BeSeCl denotes the total energy of the BeSeCl monolayer. The cohesive energy is calculated to be 3.50 eV per atom, which is comparable to or higher than previously reported Janus BiXY (X = S, Se and Te and Y = F, Cl, Br and I), having a range of 2.46–3.58 eV per atom,⁵¹ revealing the energetic stability of the BeSeCl monolayer.

We then examined the dynamic stability of the BeSeCl monolayer using a phonon dispersion plot. In Fig. 1(d), a total of nine modes of phonon dispersion are included, including three acoustic modes (ZA, TA and LA) and six optical modes. The lack of any sort of imaginary frequency with respect to the Brillouin zone confirms the credible dynamic stability of the monolayer. To better understand the thermodynamic stability of the BeSeCl monolayer, we performed AIMD simulations at 300 K and 500 K using the Nose–Hoover thermostat for a total timestep of 5 ps. The plots for temperature and energy (with ±0.02 eV per atom) fluctuations versus timesteps at 300 K (Fig. S2, SI) and 500 K, shown in Fig. 1(e), clearly illustrate that following the thermal relaxation, no structural deformation and discernible bond breaking in the final BeSeCl monolayer configuration were observed, which attests to its stability over varying temperatures.

We then evaluated the mechanical characteristics to establish the mechanical stability of the BeSeCl monolayer. The non-zero elastic constants, Poisson's ratio, and Young's modulus were simulated, which also satisfy the Born–Huang criteria⁵² of C₁₁ > 0, C₁₁ > |C₁₂| and , having values of C₁₁ = 90.22 N m⁻¹, C₁₂ = 28.29 N m⁻¹ and C₆₆ = 30.99 N m⁻¹. By using these constants, we calculated the mechanical strength in terms of the as 81.39 N m⁻¹ (Fig. 1(f)), which confirms the elastic flexibility of the BeSeCl monolayer. Likewise, the fraction of transverse to axial strain, , with a value of 0.31 (Fig. S3, SI), suggests the brittle nature of the monolayer. Additionally, the polar plots of Y and ν show a perfect circle, which confirms the isotropic nature of the BeSeCl monolayer.

3.2 Electronic properties

Following the stability validation, we studied the electronic band structure of the BeSeCl monolayer (Fig. S5(a), SI), which manifests a metallic nature due to the crossing of the valence band above the Fermi level at the Γ point. To gather more comprehensive information, partial density of states (PDOS) and total density of states (TDOS) were examined. The PDOS clearly emphasizes that whereas Cl atoms dominate in the valence states, the conduction states are influenced by Be atoms.

To obtain in-depth details about electronic phenomena, we also calculated an electron localization function (ELF) plot, which indicates the likelihood of discovering an electron with the same spin close to another electron at an appropriate location. The probability of localization of a reference electron is quantified using the ELF range, where ELF = 0 indicates delocalized electrons, ELF = 0.5 represents an ionic bond, and ELF = 1 shows completely localized electrons. As depicted in Fig. S5(b), SI, the ELF plot shows that covalent bonding exists between Be, Se and Cl atoms due to the presence of strong localization characteristics of electrons between the Be–Se and Be–Cl bonds.

3.3 BeSeCl monolayer as an anode for SIBs and PIBs

Following the study of fundamental features, we investigated the adsorption mechanism of Na- and K-atoms over the BeSeCl surface. The electrochemical effectiveness of SIBs and PIBs is determined by the robust and stable adsorption of metal atoms. For this, we designed a 3 × 3 × 1 supercell of the monolayer and assumed six distinct sites for adsorption; namely, A₁, A₂, A₃, B₁, B₂ and B₃, as displayed in Fig. 2(a). Here, A₁, A₂ and A₃ are the sites above Be, Se, and Cl atoms, respectively. In the same way, B₁, B₂ and B₃ are the bridge sites above Be–Se, Se–Cl, and Be–Cl bonds, respectively. Due to the presence of inherent asymmetry in the Janus structure, the metal atoms were adsorbed on each side (above and below) of the BeSeCl surface. Correspondingly, we scrutinized all six sites by adsorbing Na- and K-atoms below the BeSeCl surface. To evaluate an appropriate adsorption site of Na and K atoms, the adsorption energy (E_ad) of all potential configurations of the Na-/K-inserted site was calculated using the formula:

Ead = EBeSeCl(Na/K) − (EBeSeCl + ENa/K)	(10)

where E_BeSeCl(Na/K) is energy of Na and K adsorbed BeSeCl surface, E_BeSeCl depicts the energy of the BeSeCl surface and E_Na/K refers to the energy of isolated Na and K systems. By using this formula, all the calculated adsorption energies of various sites are listed in Table S1, SI. This Table clearly illustrates that of all the sites, the A₁ site is highly viable, with energies of −2.91 eV and −3.12 eV for Na and K atoms, respectively. Fig. 2(b) portrays both Na and K atom adsorption above and below sites, with the above sites exhibiting higher adsorption energies. Analogous to above adsorption sites, the below adsorption sites also have A₁ sites as the most favourable, with energies of −2.40 eV and −2.59 eV for Na and K atoms, respectively.

For an extensive study of the interaction involving the BeSeCl surface and metal atoms during adsorption, we also calculated the charge density difference (CDD) plot. The CDD plot was obtained using the formula:

Δρ = ρBeSeCl(Na/K)(r) − ρBeSeCl(r) − ρNa/K(r)	(11)

where ρ_BeSeCl(Na/K)(r) depicts the charge density of the Na-/K-adsorbed BeSeCl surface, ρ_BeSeCl(r) shows the charge density of the pristine BeSeCl surface, and ρ_Na/K(r) defines the charge density of Na and K atoms. In the CDD plot as portrayed in Fig. 2(c) and (d), yellow and cyan colors indicate accumulation and dissipation of charges, respectively. The yellow color around the BeSeCl surface shows the charges are moving from the metal atom to the surface and accumulating around it. The interaction between metal atoms (Na and K) and the BeSeCl surface is reinforced due to the greater electrostatic attraction of constituent elements of BeSeCl than the metal atoms. We examined this data in more detail by simulating the Bader charge to observe how much of the charges are transferred from the metal atom to the surface. On adsorbing Na and K atoms over the BeSeCl surface, 0.87e and 0.83e charges are transferred from Na and K atoms, respectively, to the BeSeCl surface. Furthermore, we also computed the band structure of Na(K)-atom adsorbed BeSeCl surfaces at the favourable site, as displayed in Fig. 2(e) and (f). The PDOS clearly illustrates that both Na(K) atoms are dominating in states above the Fermi level, resulting in enhanced metallicity of the BeSeCl surface.

3.4 Aluminium (Al) and silicon (Si)-doped BeSeCl monolayer

After evaluating the electrochemical performance parameters for SIBs and PIBs, we studied these characteristics by incorporating the surface functionalization technique. To integrate this technique, we studied p-block doping for the BeSeCl surface. These p-block dopants could generate stronger structures due to their valence electron configuration, enabling improved results.⁵⁹ Introducing p-block doping enhances mechanical strength, charge and ionic conductivity, and can also alter ion diffusivity, which are the prominent factors for enhancing the electrochemical performance of an anode.⁶⁰ Among the p-block elements, we selected aluminium (Al) and silicon (Si).^61–63 Note that the atomic radii of Al (143 pm) and Si (111 pm) are comparable to those of the respective BeSeCl monolayer constituent elements Be (112 pm), Se (120 pm) and Cl (99 pm). Also, aluminum maintains structural integrity, exhibits higher ionic conductivity, and minimizes lattice strain, which provides more space for Na and K intercalation.⁶⁴ Si-based anode materials are well known for their higher theoretical specific capacity of the order of 4200 mAh g⁻¹ for LIBs.⁶⁵ We used a 3 × 3 × 1 supercell and adopted substitutional doping of Al and Si atoms by creating vacancies for the individual atoms. More details regarding vacancy creation are given in the SI (Section 6 and Fig. S11).

We next studied the substitutional doping of Al and Si atoms in place of Se. In general, doping can change a number of additional aspects like covalent radii, valence configuration of dopants and structural changes, which can have a significant impact on the chemical reactivity of the BeSeCl surface. To gain comprehensive information on how doping contributes to the chemical composition of BeSeCl, we first relaxed all the atomic coordinates and analysed its bonding aspects. During optimization, Al and Si atoms get slightly shifted out of the plane of Se atoms by 1.16 Å and 0.49 Å, respectively, as represented in Fig. 5(a) and (c). The calculated bond lengths of Al–Se and Al–Be are 3.24 Å and 3.41 Å, respectively, while the bond lengths of Si–Se and Si–Be are 3.0 Å and 2.81 Å, respectively.

Additionally, we examined the mechanical stability of Al- and Si-doped BeSeCl systems by calculating the Poisson's ratio (ν) and Young's modulus (Y). The calculated values of the elastic constants are C₁₁ = 119.46 N m⁻¹, C₁₂ = −100.92 N m⁻¹ and C₆₆ = 110.19 N m⁻¹, which results in Y = 34.213 N m⁻¹ and ν = −0.85, confirming the auxetic nature and mechanical stability of the Al-doped BeSeCl system (Fig. S12, SI). Likewise, the Si-doped BeSeCl system has elastic constants C₁₁ = 165.36 N m⁻¹, C₁₂ = 6.54 N m⁻¹ and C₆₆ = 79.41 N m⁻¹ following the higher values of Y = 165.10 N m⁻¹ and ν = 0.04 (Fig. S13, SI). These results also confirm that doping with Si enhances the mechanical strength and stability of the 2D BeSeCl system. The exact circle in the polar graph of Y and ν indicates that the mechanically isotropic nature of the BeSeCl monolayer is maintained after doping of Al and Si atoms, contributing to the enhanced mechanical strength and stability.

We then investigated the band structure and associated DOS of the doped systems. The computed band structure, as featured in Fig. 5(b) and (d), reveals that doping with Al and Si atoms does not alter the metallic nature of BeSeCl. The PDOS also indicates that Al incorporation contributes to states below and above the Fermi level equally, whereas Si contributes more to the states above the Fermi level.

To explore the adsorption phenomena of Na and K, we adsorbed Na and K atoms over the doped surface at different sites. The calculated adsorption energies, adsorption height and transferred charge of all sites are shown in Fig. S14, SI. Among all the sites, the A₁ site was preferable for both cases, with adsorption energies of −2.84 eV (−3.05 eV) for SIBs (PIBs) and −2.82 eV (−3.02 eV) for SIBs (PIBs) with Al- and Si-doped BeSeCl surfaces, respectively. The calculated adsorption height and charge transfer of adsorbed Na (K) atoms are listed in Table S4, SI.

3.5 Diffusivity analysis of pristine and doped BeSeCl anode surfaces

For metal-ion batteries, the diffusion coefficient is a significant parameter because it directly affects how quickly ions travel across the electrode and electrolytes, ultimately influencing the power efficiency, voltage window, charging speed and cyclic life.⁷⁹ By plotting diffusion coefficients versus temperature, activation energies and an estimation of the efficiency of a battery at operational temperatures can be extracted. A higher diffusion coefficient allows structural stability during ion insertion, faster ion insertion and extraction, and capacity utilization, resulting in increased power densities and shorter charging times.⁸⁰ To gain in-depth insight into the diffusion of metal atoms (Na and K) over the BeSeCl surface, the diffusion coefficients (D) of Na and K were obtained using the Arrhenius equation as:⁸¹

D = l02C	(15)

where l₀ is the migration distance between two neighbouring stable sites, C represents the kinetic rate constant for diffusion, which can be calculated as follows:

	(16)

where k_B denotes Boltzmann constant, T is temperature, and h is the Planck constant. ΔG_f is the Gibbs free energy for activation, which can be written as:⁸²

ΔGf = Eb + ΔEZPE − TΔS	(17)

E_b is the diffusion barrier of the respective metal atom on the BeSeCl surface, ΔE_ZPE is the variation in zero-point energy, which can be obtained by the total of all vibrational frequencies of metal atoms in its adsorbed state, and TΔS is the change in entropy between the initial and transition state. Usually, entropy includes configurational and vibrational components; here, only the vibration component is used for the Gibb's free energy calculation as the configurational component is negligible.⁸³ Hence, the following equation provides an approximation of the Gibbs free energy:

ΔGf ≈ Eb + Hvib	(18)

where H_vib is the vibrational Helmholtz free energy, which can be evaluated using the thermodynamic equation as:⁸⁴

	(19)

In the above expression, ℏ is reduced Planck's constant, ω_k is the kth vibrational frequency of Na and K atoms. The obtained plot of diffusion coefficient as a function of temperature for pristine and doped (Al and Si-atom) BeSeCl monolayer is shown in Fig. 9. The diffusion coefficients for Na and K over the BeSeCl surface at room temperature are 1.03 × 10⁻⁷ cm² s⁻¹ and 4.79 × 10⁻⁷ cm² s⁻¹, respectively. The calculated diffusion coefficient offers substantially better outcomes in comparison with other theoretically studied 2D anode materials, such as Si₃N (2.26 × 10⁻⁷ cm² s⁻¹ for Na),⁸⁵ Y₂C(OH)₂ (1.52 × 10⁻¹² cm² s⁻¹ for Na);⁸⁶ however, they are lower than for Nb₂Sc₂C (9.5 × 10⁻⁵ and 2.89 × 10⁻⁴ for Na and K, respectively).⁸⁷ The corresponding findings are also comparable to traditional graphite, for which both simulation and experimental results reveal that K has a higher diffusivity than Na.⁸⁸ Experimentally, the diffusion coefficient for Na in hard carbon was reported to be 10⁻⁹ cm² s⁻¹ during charging and discharging processes.⁸⁹ Examples of experimentally determined diffusion coefficients for various anode materials are: MoSe₂ (1.31 × 10⁻¹³ cm² s⁻¹ for Li),⁹⁰ Ti₂C_0.5N_0.5T_x (2.3 × 10⁻¹³ cm² s⁻¹ for Na);⁹¹ commercially used hard carbon (1.73 × 10⁻⁹ cm² s⁻¹ for K)⁹² and graphite (10⁻⁸ cm² s⁻¹ for K).⁹³

These results confirm that Na and K atoms disperse quickly, facilitating charge/discharge rates on the BeSeCl monolayer. We also obtained the diffusion coefficient on Al- and Si-doped BeSeCl surfaces. The calculated diffusion coefficient values are 7.03 × 10⁻⁷ cm² s⁻¹ and 3.26 × 10⁻⁷ cm² s⁻¹ for Na atom diffusion, and 1.51 × 10⁻⁶ cm² s⁻¹ and 1.03 × 10⁻⁶ cm² s⁻¹ for K atom diffusion, on Al- and Si-doped BeSeCl surfaces, respectively. These results indicate that K atoms diffuse faster than Na atoms for both Al- and Si-doped BeSeCl surfaces. We have also analysed the diffusion coefficient for diffusion of Na and K atoms below the pristine and doped-BeSeCl surface (Fig. S18, SI) and found values of 1.50 × 10⁻⁹ cm² s⁻¹ (pristine), 6.98 × 10⁻⁹ cm² s⁻¹ (Al-doped) and 3.23 × 10⁻⁹ cm² s⁻¹ (Si-doped) at 300 K for Na atoms. For K atom diffusion, the diffusion coefficients are 4.75 × 10⁻⁹ cm² s⁻¹ (pristine), 3.25 × 10⁻⁸ cm² s⁻¹ (Al-doped) and 6.98 × 10⁻⁹ cm² s⁻¹ (Si-doped) at 300 K. These outcomes confirm that the diffusion of Na and K atoms is faster above the surface than below the surface. The diffusion coefficient values of pristine and doped BeSeCl systems at different temperatures are given in Table S7, SI. The fast ion-transportation of Na and K over the BeSeCl surface indicates that it can be a potential candidate as an anode material.

3.6 Adaptability between BeSeCl anode material and various electrolytes

To evaluate the overall anode performance in the electrochemical environment of a battery, the susceptibility of the BeSeCl anode material and the electrolyte plays an essential role. The anode–electrolyte interface stimulates the fundamental electrochemical processes of metal-ion batteries, ensuring stable interfaces that also permit fast ion transport while retarding electrolyte breakdown and reducing battery efficiency. To gain deeper insights into the anode–electrolyte interaction, we utilize typical solvent molecules and metal salts for SIBs and PIBs as electrolytes with the BeSeCl anode surface. The electrolytes used include metal salts such as NaClO₄, KClO₄, NaFSI, KFSI, NaPF₆, and KPF₆, and solvent molecules like ethylene carbonate (C₃H₄O₃), dimethyl carbonate (C₃H₆O₃), diethyl carbonate (C₅H₁₀O₃) and propylene carbonate (C₄H₆O₃).

To determine the compatibility of these electrolytes with the BeSeCl surface, we adsorbed the various electrolytes on the surface (Fig. S19, SI), and the adsorption energy (E_ad) was obtained as:

Ead = EBeSeCl+electrolyte − (EBeSeCl + Eelectrolyte)	(21)

where, E_{BeSeCl+electrolyte} is the energy of the electrolyte adsorbed on the BeSeCl surface and E_electrolyte is the energy of the isolated electrolyte systems. The obtained adsorption energies, having negative values, are listed in Table 1, which show that the electrolytes interact strongly with the BeSeCl surface (Se side). The adsorption energies of the metal salts and solvent molecules range from −0.34 eV to −1.01 eV and −0.72 to −2.90 eV, respectively. Among the metal salts, NaPF₆ does not show stability with the BeSeCl surface, with an E_ad value of +0.47 eV; in contrast, NaFSI interacts more strongly than other metal salts. Likewise, ethylene carbonate (C₃H₄O₃) exhibits higher E_ad, with a value of −2.90 eV, than other carbonate molecules. The obtained results are comparable to those of previous studies on graphene (−0.54 eV to −0.87 eV), B₂S (−0.44 eV to −0.69 eV)⁹⁴ and BeN (−0.46 eV to −0.66 eV)⁹⁵ monolayer. Overall, most of the electrolytes considered are compatible with the BeSeCl surface, indicating that the monolayer anode is suitable for use in an electrochemical environment.

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We have also examined the stability of Al- and Si-doped systems with solvent electrolytes. Both Al-doped (Fig. S20, SI) and Si-doped (Fig. S21, SI) BeSeCl surfaces exhibit higher interactions with DMC and DEC as compared with metal-salt electrolytes; however, the Si-doped system suggests higher stability than the Al-doped BeSeCl system when employed in an electrochemical environment.

To analyse the effect of electrolyte adsorption on the Janus BeSeCl, we also studied the adsorption behaviour below the BeSeCl surface (Cl side), as the environments on the upper and lower atomic layers of the Janus structure differ. The relaxed structures of all the solvent molecules and metal salts adsorbed on pristine and doped-BeSeCl surfaces (Cl side) are shown in Fig. S22–S24, SI. For the pristine BeSeCl surface, most of the adsorption energies show negative values ranging from −0.74 eV to −2.84 eV for solvent molecules and −0.05 eV to −0.67 eV for metal salts (Table S8, SI). The Al- and Si-doped BeSeCl are also stable, with favourable adsorption energies as listed in Table S8, SI, for electrolytes when used in an electrochemical environment.

4. Conclusions

We have studied the electronic, mechanical and electrochemical characteristics of a novel 2D Janus BeSeCl monolayer using first-principles calculations. This metallic monolayer shows high adsorption energies of −2.91/−3.12 eV, making it suitable as an anode electrode material in Na/K-ion batteries. The BeSeCl monolayer exhibits specific capacity values of 699.65 mAh g⁻¹ and 434.26 mAh g⁻¹ and OCVs of 0.91 V and 0.52 V for SIBs and PIBs, respectively. The material promotes fast ion transport, having energy barriers of 0.08 eV and 0.04 eV and higher diffusion coefficients of the order of 10⁻⁷ cm² s⁻¹ for Na and K atoms, respectively, which makes it suitable as an anode electrode material. Furthermore, to enhance the electrochemical storage phenomenon of the BeSeCl monolayer, a substitutional doping strategy with Al and Si atoms in place of Se was used. After introducing doping, the specific capacity increases to 916.87 mAh g⁻¹ (687.65 mAh g⁻¹) and 839.59 mAh g⁻¹ (534.28 mAh g⁻¹) for Al- and Si-doped BeSeCl, respectively, for SIBs (PIBs). Conversely, OCVs for BeSe_0.89Al_0.11Cl and BeSe_0.89Si_0.11Cl tend to decrease compared to the pristine BeSeCl monolayer. The energy barrier for Na and K atom migration over the surface reduces to 0.03 eV (0.01 eV) and 0.05 eV (0.02 eV) for the Al- and Si-doped BeSeCl systems, respectively. Both Al- and Si-doped systems also promote fast ion-transportation, with higher diffusion coefficients of the order 10⁻⁷ cm² s⁻¹ and 10⁻⁶ cm² s⁻¹ for Na and K-atoms, respectively, over the BeSeCl surface. The obtained results suggest that Al- and Si-doping of BeSeCl enhances its anodic performance, enabling the BeSeCl monolayer to serve as an anode electrode for SIBs and PIBs.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included in this article and in the SI.

Different stacking pattern of bulk BeSeCl, AIMD simulation of BeSeCl at room temperature, polar diagram of Poisson's ration, phase diagram concerning chemical potential, electronic properties, adsorption energy table for Na and K-atom, multilayer adsorption and distortion structures of Na and K-atoms at different sites of BeSeCl surface, Diffusion barrier and energy profile below BeSeCl surface, electrochemical performance table of BeSeCl with other 2D materials, vacancy and doping structure and mechanical stability in BeSeCl surface, adsorption energy and respective plots for doped BeSeCl, diffusion energy and plots of metal atoms below doped structures, diffusion coefficient and plots for pristine and doped systems, adsorption energy table and respective structures of various solvent molecules and metal-salts on pristine and doped BeSeCl system. See DOI: https://doi.org/10.1039/d5ta04648c.

Acknowledgements

NV is thankful to the Council of Scientific and Industrial Research (CSIR) for providing the financial aid in the form of a Senior Research Fellowship. PARAM Rudra, a national supercomputing facility, at the Inter-University Accelerator Centre (IUAC), New Delhi, was used to obtain the results presented in this manuscript.

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