Due to the growing need for high-capacity ion storage batteries, researchers are exploring new categories of electrode materials with superior electrochemical properties. In this investigation, a tetragonal symmetry of carbon named T-graphene has been taken into consideration as an anode material for Mg and Ca ion batteries for its superior electrical characteristics. To comprehend the adsorption behavior, charge transfer, and anodic properties of T-graphene, density functional theory has been employed. Initially, the most favorable adsorption sites for Mg/Ca insertion on the T-graphene have been identified and it found that bridge and octagonal hollow sites show high adsorption behavior with energy of about −1.99 and −4.86 eV, respectively. A significant amount of charge (Hirshfeld) about 0.24e and 0.79e transfer from Mg and Ca to T-graphene, respectively. When Mg/Ca is inserted, the electronic structure calculations demonstrate that T-graphene acts metallically. A very high specific capacity is found at about 1737.13 and 1985.30 mAh/g for Mg and Ca ion batteries, respectively. Moreover, the average open-circuit voltages for Mg are 0.65 V and 1.51 V for Ca. Therefore, it is assumed that T-graphene may be used as high-capacity anode material for Mg and Ca ion battery.

In the most recent study, rechargeable ion batteries have drawn much interest. Because of the over-reliance on fossil fuels energy, which is mostly responsible for climate change, renewable energy conversion and storage devices are highly required. For that, the world has started large-scale renewable energy initiatives, such as wind and solar energy systems, as they could meet global energy demands in the future. Although the energy produced by these sources is inconsistent and dependent upon the weather, the energy generated from these sources must also be stored to ensure continuous supply. As a result, electrical storage technologies have become critical for storing additional energy in the context of rising global energy demand. On the other hand, with the quick development of electric vehicles and mobile communication devices, there is a strong need for rechargeable batteries with high energy and power densities. The ongoing instability of fossil fuel costs, resources, and the potential threat of global warming caused by CO2 emission are the motivation to develop cutting-edge efficient electrical energy storage systems, particularly rechargeable ion storage batteries.

Over the past few years, lithium-ion batteries (LIBs) have dominated the market in small- and medium-sized portable gadgets because of having important advantages, including strong reversible capacity, long cycle life, and high energy storage efficiency.1 The use of LIBs is expanding, and the prices are rising due to a scarcity of lithium mineral supplies. Its 35 ppm estimated concentration is in the upper continental crust.2 Therefore, it has been worrying in recent years that the amount of Li resources would not be enough to supply the constantly growing demand for LIBs. These worries have prompted an ongoing search for acceptable alternatives. In particular, the multivalent ions of alkali earth metals, such as calcium-ion (Ca2+) batteries (CIBs)3 and magnesium-ion (Mg2+) batteries (MIBs), have received much interest because of their availability, affordability, and non-toxicity.4,5 Mg and its compounds are often less hazardous and safer than Li-based chemicals since they are stable when exposed to air.5 It could be possible candidates for future electrode materials due to a number of benefits over Li- and Na-ion batteries, notably (a) their inherent sufficiency, (b) their fast diffusion rate of Mg and Ca ion than Na and Li for its smaller radius, and (c) their divalent nature, which results in tighter bonding allowing a larger theoretical–volumetric capacity, such as two electrons per Mg/Ca atom vs only one for Li and Na6 and specific-capacity as comparable to Li.7 

Graphene is a divine intervention material of the twenty-first century. After the discovery by Andre Geim and Konstantin Novoselov in 2004, there have been numerous developments in 2D anode materials, including graphene-based networks, the well-known graphite, graphene, C-60 fullerene,8 and diamond,9 as well as the more recently theoretically predicted T-graphene (T-G) by Liu et al.10 T-G is a carbon allotrope, a single sheet of sp2 hybridized carbon atoms densely compacted into a two-dimensional honeycomb-like lattice structure11 with some outstanding properties, such as zero bandgap,12 high electron transport,13 and high mechanical/tensile strength.14 Its vast range of properties has enabled T-G to be implemented in various applications, including rechargeable batteries,15 hydrogen storage by Saedi et al.,16–18 gas sensors by Liu et al.,19,20 optoelectronic devices by Yoo et al.,21 solar cells, current rectifiers by Bandyopadhyay et al.,22 etc. According to Wang et al., it has been discovered that the T-G has a greater density of states (DOS) at the Fermi level than the graphyne.15 The theoretical capacity for NIBs in previous research for T-G by Junping et al. is as high as 2232 mAh/g (Na6C6), which is six times that of graphite. The theoretical capacities for LIBs and KIBs anodes are 744 and 1116 mAh/g, respectively, and coincide with the chemical formulas Li2C6 and K3C6, respectively.23 

Encouraged by the preceding research on T-G due to its superior electrochemical performance, high surface-to-volume ratio, and rapid ion migration properties,24 we have been motivated to conduct the first-principle simulations to investigate the characteristics of monolayer T-G and proposed as our efficient/potential candidate to become an anode material for rechargeable Ca and Mg ion storage batteries. In this study, density functional theory (DFT) calculations have been done using the GGA-PBE exchange–correlation functional to investigate the geometric, electrical, adsorption, and electrochemical (specific charge capacity and open-circuit voltage) characteristics of T-G for MIB and CIB.

All calculations were carried out using the Biovia Material Studio Simulation Package inside DFT via the Dmol3 technique. For characterizing the interactions between electron exchange and correlation, the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional was taken into consideration. With periodic boundary conditions, the cut-off energy for the plane wave was fixed at 4.6 eV. The vacuum separation in the model structure was adjusted to 20 Å in order to prevent the improper coupling effect between periodic graphene layers along the normal direction.

The cohesive energy of the T-G was determined from the following equation, which may be considered as the necessary energy to divide a crystal into neutral atoms, in order to measure the structural stability:25,
ECoh=ETGnECn,
(1)
where ETG is the total energy of the T-G nanosheet, EC is the energy of the corresponding isolated carbon atom, and n be the total number of atoms in the nanosheet. The open-circuit voltage (OCV) is an important component in characterizing the performance of ion storage batteries. By using the following equation in the concentration range of x1 < x < x2, we have calculated the OCV of Mg/CaxT-G:26 
VEMg/Cax1T  GEMg/Cax2T  G+x2x1EMg/Ca2x2x1e,
(2)
where EMg/Cax1T-G and EMg/Cax2T-G are the total energies of the T-G with x1 and x2 Mg/Ca atoms.

The tetragonal symmetry of carbon is such a kind of congenital graphene named T-graphene. A pristine T-G nanosheet has been examined for generating anode material.

A figurative explanation of T-G is provided in Fig. 1. All the carbon atoms are furnished by two geometrical configurations. It is composed of 36 carbon atoms distributed among four octagonal and nine tetragonal rings. There is an average C–C bond length of 1.32 Å and bond angle of 90° in the octagonal ring and a comparatively high bond length of 1.50 Å and bond angle of 135° due to its large strain in the tetragon site, which is consistent with the previous study.25 

FIG. 1.

(a) Optimized T-G nanosheet and (b) its band graph.

FIG. 1.

(a) Optimized T-G nanosheet and (b) its band graph.

Close modal

The cohesive energy of T-G was found −7.18 eV per atom, which is in good agreement with the previous study.24 A metallic band structure is visible in T-graphene. It is evident from Fig. 1(b) that the valance band and conduction overlap at the Fermi level, which is proved by previous explanation.12 Because of its clear metallic conductivity, T-graphene has the potential to be used as an electrode material in ion storage batteries.

1. Investigating preferable adsorption sites

Finding the most consistent adsorption location is crucially important for introducing Mg/Ca on a T-G nanosheet to examine adsorption behavior. We have considered four adsorption locations:

  • Tetragon sites (T): midpoint of carbon tetragon;

  • Bridge sites (B): above the midpoint of C–C bonds;

  • Octagon sites (O): midpoint of the carbon octagon;

  • Top sites (T1): above the top of carbon atoms.

To identify the appropriate adsorption site, at first, the adsorption energy has been calculated by placing a single Mg/Ca atom on the T-G at each of the four locations to investigate the most ideal adsorption site.

According to the findings, all adsorption sites are predicted to have negative adsorption energy, i.e., an exothermic reaction and attractive contact ensue between the Mg/Ca atom and T-G. We have calculated adsorption energy per unit atom using the following equation:25 
Eads=EMg/canTGETGnEMg/can,
where EMg/CanTG, ETG and nEMg/Ca represent the total energy of T-G with n being the number of Mg/Ca atoms, the energy of T-G nanosheet, and a single Mg/Ca atom. The list of adsorption sites and associated adsorption energies and charge transfer from Mg/Ca to T-G are mentioned in Table I and in Fig. 2, and the top and side views of the typical Mg/Ca adsorption sites are displayed.
TABLE I.

Calculated values of adsorption energies Eads. in (eV), distance from Mg and Ca to T-G, d(Mg/Ca-TG) in (Å), and charge (Hirshfeld and Mulliken) of Mg/Ca atom Q in (e).

Eads.d(Mg/Ca-TG)QHQM
Adsorption siteMgCaMgCaMgCaMgCa
−1.95 −4.45 2.97 2.17 0.27 0.83 0.34 1.29 
−1.99 −4.45 3.08 2.16 0.24 0.82 0.55 1.28 
−1.79 −4.86 2.77 1.74 0.27 0.79 0.63 1.44 
T1 −1.78 −4.45 2.75 2.19 0.28 0.82 0.34 1.29 
Eads.d(Mg/Ca-TG)QHQM
Adsorption siteMgCaMgCaMgCaMgCa
−1.95 −4.45 2.97 2.17 0.27 0.83 0.34 1.29 
−1.99 −4.45 3.08 2.16 0.24 0.82 0.55 1.28 
−1.79 −4.86 2.77 1.74 0.27 0.79 0.63 1.44 
T1 −1.78 −4.45 2.75 2.19 0.28 0.82 0.34 1.29 
FIG. 2.

Front and side views of Mg/Ca adsorbed T-G (a) T, (b) B, (c), and (d) T1 sites, respectively. The green color represents Mg atom, and red color represents Ca atom.

FIG. 2.

Front and side views of Mg/Ca adsorbed T-G (a) T, (b) B, (c), and (d) T1 sites, respectively. The green color represents Mg atom, and red color represents Ca atom.

Close modal

The adsorption energies were computed and showed T-G nanosheet resulted in negative values for the adsorption energies, which proves the attractive contact between the Mg/Ca atom and the T-G nanosheet. Higher negative energy values suggest an inherently more favorable reaction of T-G with Mg/Ca atom. For a single Mg adsorption on a T-G, the adsorption energies are found −1.95, −1.99, −1.79, and −1.78 eV, respectively, at the T, B, O, and T1 sites, respectively. It was observed that in the B site, the adsorption energy is relatively high with respect to other sites, which indicates that higher interaction took place toward with Mg atom and T-G. In a previous study, Yang claimed that the adsorption energy for Mg to VS2 nanosheet was found −0.58, −0.61, and −2.69 eV, respectively.27 The minimum adsorption distance for single Mg atom from the nanosheet was 2.97, 3.08, 2.77, and 2.75 Å at the T, B, O, and T1 sites. Furthermore, the Hirshfeld and Mulliken charge transfer analyses have been performed in which it is found that electrons are transferred from Mg to T-G about 0.27e, 0.24e, 0.27e, 0.28e and 0.34e, 0.55e, 0.63e, 0.34e, respectively. Therefore, Mg atom acts as electron donor and T-G acts as electron acceptor.

Again, the adsorption energy for Ca is found −4.45, −4.45, −4.86, and −4.45 eV, respectively, at the T, B, O, and T1 sites. In the case of Ca adsorption on the T-G, adsorption energies are almost similar at T, B, and T1 sites except O site. The calculated adsorption energies are very consistent with a previous study by Zeng et al. for Li adsorption on T-G, which is −3.14 eV.20 The adsorption distance for single Ca atom from the nanosheet are 2.17, 2.16, 1.74, and 2.19 Å at the T, B, O, and T1 sites. The Hirshfeld and Mulliken charge transfer analyses suggested that, during adsorption of Ca on the T-G, electrons are transferred from Ca to T-G about 0.83e, 1.29e, 0.82e, 1.28e and 0.79e, 1.44e, 0.82e, 1.29e, respectively.

Electron density structures show the distribution of electrons in the overall structures. We analyze the Electron Density Difference (EDD) of the T-G by inserting a single Mg/Ca atom by using the following equation:28 
Δρ=ρMg/CaTG(ρTG+ρMg/Ca),
where ρMg/CaTG is the electron density of total systems with Mg and Ca atoms, and ρTG and ρMg/Ca are the bare electron densities of the T-G nanosheets and adsorbing Mg/Ca atom. The 3D EDD maps also demonstrated to identify the charge transferred from Mg/Ca to graphene substrates. The graph indicates that the charge drained from Mg/Ca to C–C bonds primarily falls at the system’s electronic cloud, which is shown in Fig. 3. The blue color (Δρ > 0) denotes electron accumulation, and the green color (Δρ < 0) describes electron depletion. The iso-surface was set to 0.015 Å−3. Thus, charge analysis supported these findings, demonstrating that the charge was transferred from the Mg/Ca to the T-G after adsorption at all four sites.
FIG. 3.

Top and side views of EDD maps of T-G with adsorbed single Mg and Ca atom. The blue color denotes electron accumulation, and the green color describes electron depletion.

FIG. 3.

Top and side views of EDD maps of T-G with adsorbed single Mg and Ca atom. The blue color denotes electron accumulation, and the green color describes electron depletion.

Close modal

2. Diverse number of Mg and Ca adsorption on T-G

Following the investigation of several adsorption sites, a diverse quantity of Mg/Ca atoms was adsorbed on the T-G according to their favorable adsorption sites. More Mg/Ca atoms are added to the T-G to evaluate the maximum adsorption capacity. As Mg/Ca adsorption is correlated with anode material’s efficiency, which is crucial to measure the performance of the anode material of Mg/Ca-ion batteries, we adsorbed several numbers of atoms. Due to the most favorable adsorption site, the Mg atoms were initially absorbed in the B site and Ca on the O site. But as the concentration increased, the Mg/Ca atoms were distributed across two or more distinct layers, i.e., they took place at the second most favorable adsorption sites. To prevent the repulsion caused by Mg/Ca absorption, we adsorbed on the other hand of the nanosheet after latching the Mg/Ca atom to every T site. In this case, we gradually increased the number of Mg/Ca atoms on the T-G. When the number of Mg atoms were increased, i.e., 6, 8, 10, 14, and 16 Mg inserted, the adsorption energy was found to be −1.40, −1.34, −1.31, −1.30, and −1.29 eV, and the Hirshfield charge transfer occurred about 0.17e, 0.15e, 0.14e, 0.15e, and 0.14e from Mg to T-G, respectively (Table II). We noticed that after adsorbing 16 and more Mg on T-G, the nanosheet is noticeably distorted and we stopped absorbing Mg. Figure 4 shows top and side views of T-G with diverse amounts of Mg and Ca atoms.

TABLE II.

Obtained values of adsorption energy (Eads.) in eV, Hirshfeld (QH) and Mulliken (QM) charge transfer in (e).

Number of atomsEads. (eV)QH (e)QM (e)
MgCaMgCaMgCa
−1.70 −3.85 0.37 0.77 0.60 1.44 
−1.45 −3.08 0.22 0.36 0.31 0.80 
−1.40 −3.02 0.17 0.35 0.22 0.80 
−1.34 −2.96 0.15 0.29 0.19 0.66 
10 −1.31 −2.92 0.14 0.25 0.17 0.61 
12 −1.30 −2.90 0.15 0.21 0.24 0.52 
14 −1.30 −2.98 0.15 0.19 0.15 0.49 
16 −1.29 −3.01 0.14 0.17 0.13 0.45 
18 −1.35 −3.05 0.13 0.15 0.166 0.44 
20 −1.40 −2.99 0.12 0.14 0.165 0.41 
Number of atomsEads. (eV)QH (e)QM (e)
MgCaMgCaMgCa
−1.70 −3.85 0.37 0.77 0.60 1.44 
−1.45 −3.08 0.22 0.36 0.31 0.80 
−1.40 −3.02 0.17 0.35 0.22 0.80 
−1.34 −2.96 0.15 0.29 0.19 0.66 
10 −1.31 −2.92 0.14 0.25 0.17 0.61 
12 −1.30 −2.90 0.15 0.21 0.24 0.52 
14 −1.30 −2.98 0.15 0.19 0.15 0.49 
16 −1.29 −3.01 0.14 0.17 0.13 0.45 
18 −1.35 −3.05 0.13 0.15 0.166 0.44 
20 −1.40 −2.99 0.12 0.14 0.165 0.41 
FIG. 4.

Front and side views of optimized T-G structures adsorbed with diverse numbers (a) 2, (b) 4, (c) 6, (d) 8, (e) 12, and (f) 14 of Mg and Ca atoms. The green sphere represents Mg, and the red represents the Ca atom.

FIG. 4.

Front and side views of optimized T-G structures adsorbed with diverse numbers (a) 2, (b) 4, (c) 6, (d) 8, (e) 12, and (f) 14 of Mg and Ca atoms. The green sphere represents Mg, and the red represents the Ca atom.

Close modal

Similarly, we start adsorbing the Ca atom gradually. After adsorbing 6, 8, 12, 14, and 16 Ca on T-G, the adsorption energy was found to be −3.02, −2.96, −2.90, −2.98, and −3.01 eV, and charge transfer occurred at about 0.35e, 0.29e, 0.21e, 0.19e, and 0.17e, respectively, from Ca to T-G. It was found that the T-G structure has undergone a significant amount of distortion after the adsorption of 16 Ca atoms,29 which suggests that a high Ca loading can impair the stability of the carbon network. Figure 5 illustrates the change in adsorption energy per atom and the charge transfer from Mg/Ca to T-G with respect to increasing number of Mg/Ca atoms on the T-G. We found the sharp decreasing of adsorption energy per atom except some point and the charge transfer from Mg/Ca atoms to T-G. In the adsorption process, we examined the volume expansion of the nanosheet with Mg/Ca atoms by calculating the lattice parameters a , b of the nanosheet. It was found that during adsorption of Mg/Ca atoms on the T-G, the lattice parameters remain almost constant. In the case of Mg adsorption on the T-G, the lattice parameter a varies from 3.44 to 3.45 Å and b varies from 3.44 to 3.46 Å, and in the case of Ca adsorption, a and b varies from 3.45 to 3.51 Å and 3.45 to 3.52 Å, respectively. During the adsorption process, we did not find any clustering effect of the Mg/Ca after adsorbing maximum number of atoms. It was confirmed by calculating the distances between Mg–Mg and Ca–Ca adsorbing atoms where we found that the average distances are 3.31 and 3.52 Å, respectively. The obtained distances are higher than the minimum bond lengths of the Mg–Mg (2.85 Å) and Ca–Ca (3.50 Å) atoms in the cluster.30,31 Furthermore, to investigate the buckling effect with the increasing coverage of Mg/Ca atoms, the buckling height has been determined and it is observed that the buckling height gradually increases after increasing the concentration of the Mg/Ca. The calculated values are 0.046, 0.053, and 0.058 Å for 4, 8, and 12 Mg atoms adsorbed T-G and 0.028, 0.031, and 0.038 Å for 4, 8, and 12 Ca atoms adsorbed T-G, respectively. Our calculated values are so much smaller compared to the other 2D anode materials in the case of ion storage batteries.32 Therefore, we can predict that the studied T-G may not degrade mechanically during intercalation.

FIG. 5.

Illustrating (a) the change in adsorption energy per atom and (b) the charge transfer from Mg/Ca to T-G with respect to increasing number of Mg/Ca atoms on the T-G.

FIG. 5.

Illustrating (a) the change in adsorption energy per atom and (b) the charge transfer from Mg/Ca to T-G with respect to increasing number of Mg/Ca atoms on the T-G.

Close modal

In order to get more understanding of the interactions between the Mg/Ca cation and the T-G nanosheet, the electronic characteristics of the Mg/Ca-adsorbed T-G were examined. The electronic properties have been studied by investigating the band structure as well as the bandgap with DOS spectra analysis. The band structure provides the band shape, valence, and conduction band separation distance, and the DOS plots show the number of different states at a particular energy level.

Graphene is a zero-band gap semiconductor, and its conduction and valence bands are in contact with the hexagonal Brillouin zone’s two asymmetrical vertices.33 When Mg/Ca atoms are adsorbed on T-G, the bandgap drops significantly, overlapping the conduction and valence bands and acting as a conductor, as shown in Fig. 6. So, it shows metallic properties due to the adsorption of Mg/Ca atoms on the T-G nanosheet. The total density of states (TDOS) is a useful mathematical concept that can be calculated by adding all bands together, and the integral of TDOS from minus infinity to the Fermi level gives the total amount of electrons in the unit cell. A high DOS at a certain energy level indicates that there are numerous states accessible for occupancy. On the other hand, a DOS of zero indicates that no states can be possessed at that energy level. To analyze the electronic structure, the TDOS and partial DOS of these optimized structures are plotted and compared in Fig. 7.

FIG. 6.

Band structures of T-G nanosheet with (a) Mg atoms and (b) Ca atoms having 8 and 14 atoms.

FIG. 6.

Band structures of T-G nanosheet with (a) Mg atoms and (b) Ca atoms having 8 and 14 atoms.

Close modal
FIG. 7.

DOS spectra of (a) 8, (b) 14 Mg and Ca adsorbed T-G. Left column for Mg and right column for Ca atom.

FIG. 7.

DOS spectra of (a) 8, (b) 14 Mg and Ca adsorbed T-G. Left column for Mg and right column for Ca atom.

Close modal

It is visible in the DOS spectra that the Fermi levels for every DOS have been set to zero. The DOS of the Mg/Ca-adsorbed graphene, however, shifted down, and their intensity toward the Fermi level enhanced after Mg/Ca adsorption, suggesting that more electronic states can lie at the Fermi level, which could be explained by the fact that electrons from Mg/Ca are transferred to T-G. Additionally, after Mg/Ca adsorption, the DOS intensity near the Fermi level significantly increased and the fermi level gets steeper as the number of Mg/Ca atoms on the nanosheet increases, which is also proved by a previous study.26 It is evident from the DOS spectra that once Mg/Ca atoms are adsorbed on the nanosheets, dominating peaks are created in the Fermi level, and the number of states increases as the quantity of Mg/Ca atoms increases.

For an electrode material, the specific capacity and open circuit voltage are the crucial factors for an ion storage battery. Specific capacities indicate the number of atoms that can be stored per unit mass34 and are computed by the following equation:35,
Specificcapacity=zeFM,
where F is the Faraday constant, 96 485 C/mol, and z is the number of atoms participating in the electrochemical process, e is the elementary charge of atom, and M is the mass of the T-G. The specific capacity is based on how many Mg/Ca it can hold. The specific capacity and open circuit voltage are roughly related to adsorption energy, that is, the greater the negative adsorption energy toward Mg/Ca, the greater the storage capacity. Hence, the value of the specific capacity of an ion storage battery increases with the amount of Mg/Ca stored on the anode material during the charging process and it can provide additional electricity throughout the discharge process. For that, we gradually increase the concentration of Mg/Ca atoms on the T-G by adding numbers of Mg/Ca until a noticeable amount of structural deformation is seen in the geometric configuration. We discovered that T-G can hold 14 Mg and 16 Ca atoms without considerable distortion. Therefore, the specific capacity of T-G is 1737.13 mAh/g for MIB and 1985.30 mAh/g for CIB, which are consistent with the previous studies, i.e., Xiaoming et al., 2233.2 mAh/g (for Li storage battery) and 2357.2 mAh/g (for Na storage battery),35 twice for N-doped graphene 1043 mAh/g from Shamim et al.25 and almost three times from 751 mAh/g (B doped graphene).36 Our investigated results are higher than many other 2D anode materials reported five times compared to a commercial graphite anode. Those findings strongly imply that the development of T-G as a super anode material for MIB/CIB with an ultrahigh ion storage capacity is very promising.

The open-circuit voltage (OCV) is a key measure for defining the performance of an electrode. It makes predictions about the Mg/Ca implantation rates from the cathode to anode during the charging process. Figure 8 shows the voltage profile with respect to Mg and Ca content on T-G. Up to 12 adsorbed Mg and Ca atoms on the T-G, the voltage profile gradually decreases, but after that, it has increased due to the increase in adsorption energies. In the T-G, when the adsorbed atoms are more than 12, then the structure is slightly deformed, which is responsible for enhancing the adsorption energy. Therefore, the calculated average OCV for the Mg ion battery is 0.65 V, which is aligned with the previous studies wherein it is 0.54 V for N doped graphene oxide,37 0.93 V for VS2 monolayer,38 and 0.86 V for B2C monolayer.39 Again, the OCV calculated for the Ca ion battery is 1.51 V, which is very near to 1.56 V for the BC2NNT nanosheet investigated by Jaber et al.40 and 1.8 V for Ca–Sn alloy.41 Our investigated OCV values are substantially below 1.6 V and are regarded as a modest value for anode material applications.10 

FIG. 8.

Voltage profile with respect to (a) Mg content and (b) Ca content.

FIG. 8.

Voltage profile with respect to (a) Mg content and (b) Ca content.

Close modal

By employing the DFT with GGA-PBE exchange–correlation functional, theoretical studies have been employed for investigating the electronic properties, adsorption properties, and electrochemical properties of MIB/CIB, where T-G is used as the anode material. Investigations have been done on the geometric, electrical, adsorption, and electrochemical (specific charge capacity and open-circuit voltage) characteristics of T-G nanosheets. Initially, the most favorable adsorption site for Mg/Ca insertion on the T-G has been identified and the bridge site and octagonal hollow site for Mg and Ca atoms show high interaction behavior with the adsorption energy about −1.99 and −4.86 eV, respectively. A significant amount of charge (Hirshfeld) of about 0.24e and 0.79e transfers from Mg and Ca to T-G, respectively. Therefore, Ca atom is highly ionized than Mg ion. When Mg/Ca is inserted, the electronic structure calculations demonstrate that T-G acts metallically. A very high specific capacity is found to be about 1737.13 and 1985.30 mAh/g for MIB and CIB, respectively, which are four and five times higher than our conventional Li ion battery. Moreover, the average open-circuit voltages for Mg are 0.65 V and 1.51 V for Ca. Therefore, we made a conclusion that the high specific capacity with low OCV, metallic character during the Mg/Ca insertion process, and good structural integrity of our suggested nanosheet point to the possibility of the nanosheet serving as potential anode candidates for MIB/CIB.

We thankfully acknowledge the Research Cell of Mawlana Bhashani Science and Technology University funded by UGC of Bangladesh (Grant No. 3631108) under the Ministry of Science and Technology (MOST).

The authors have no conflicts to disclose.

Obaidullah: Data curation (equal); Formal analysis (equal); Writing – original draft (equal). Umme Habiba: Data curation (equal); Investigation (equal); Writing – review & editing (equal). Afiya Akter Piya: Data curation (equal); Investigation (equal); Writing – review & editing (equal). Siraj Ud Daula Shamim: Conceptualization (equal); Investigation (equal); Methodology (equal); Software (equal); Supervision (equal); Validation (equal).

The data that support the findings of this study are available within the article.

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