Research on metallic chalcogen-functionalized monolayer-puckered V2CX2 (X = S, Se, and Te) as promising Li-ion battery anode materials

Chunmei Tang *a, Xiaoxu Wang a and Shengli Zhang *b
aCollege of Science, Hohai University, Nanjing, Jiangsu 210098, P. R. China
bKey Laboratory of Advanced Display Materials and Devices, and Ministry of Industry and Information Technology, College of Material Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China

Received 17th March 2021 , Accepted 16th April 2021

First published on 30th April 2021


Abstract

Two-dimensional MXene nanomaterials are promising anode materials for Li-ion batteries (LIBs) due to their excellent conductivity, large surface area, and high Li capability. The chalcogen-terminated monolayer-puckered V2CX2 (X = S, Se, and Te) structures are expected to embody remarkable properties for Li storage and diffusion in this paper. The metallic property enables the V2CX2 anodes to have a fast electron transport rate in the charge/discharge process. The phonon spectra prove their dynamical stabilities. The adsorption energy and energy diffusion barrier of the Li atom on the V2CX2 (X = S, Se, and Te) surface both decrease with increasing atomic number of the terminated element. The monolayer V2CSe2 shows higher Li capacity (394.41 mA h g−1), relatively low Ebarrier (0.21 eV) and a small volume expansion ratio (6.1%) when compared with those of V2CX2 (X = O, S, and Te) monolayers, indicating that they should be the most promising LIB anodes. These excellent properties indicate that monolayer chalcogen-terminated V2CX2 (X = S, Se, and Te) structures have promising applications as LIB anodes.


1. Introduction

Clean energy is a perpetual topic for environment protection with increasing air pollution brought by the burning of fossil fuels for decades. The high energy density and excellent cycling performance of Li-ion batteries (LIBs) enable them to become the major power supplier for electric vehicles and portable electronics, which extensively depend on clean energy storage devices.1 The small volume expansion ratio (VER) of the anode and low diffusion energy barrier (Ebarrier) of Li+ in the anode will promote the excellent cycling performance of LIBs, which can retain the remarkable electrode capacity in the charge and discharge cycle.2,3

Numerous two-dimensional (2D) materials, such as graphene, graphdiyne, and transition metal chalcogenides, show promising prospects as LIB anodes.4–6 Recently, MXenes, as a new family of 2D materials,7 have shown promising performance as LIB anodes,8,9 attributed to their high conductivities and large surfaces. The formula for MXene is Mn+1AXn (n = 1, 2 or 3), where M stands for the transition metal (Ti, V, Cr, etc.), A comes from group IIIA or IVA (Al, Si, Sn, etc.) and X is C or N.10 There are more than 20 types of MXenes that have been synthesized experimentally and more than 50 types of MXenes that have been predicted by computer simulations.7,11 The drive for exploring MXenes as LIB anodes is mainly attributed to the high Li capacity and low Ebarrier of the Li atom, which result in high cycling rate.12 Importantly, most MXene structures can provide high electrical conductivity.13 For example, the 2D MXene material Ti3C4 monolayer shows a metallic band characteristic and has high capacity as the anode of Na- ion batteries.14 Such as in the monolayer V4C3,15 the Fermi energy (Ef) of the MXene materials is mainly contributed by the d orbitals of transition metals. The monolayer Mn2C shows low Ebarrier (0.05 eV) and high theoretical Li capacity (879 mA h g−1).16 After 100 charging/discharging cycles, Nb4C3Tx has increased specific Li capacity to 380 mA h g−1 and exhibited excellent rate capability.17 The Li capacity of monolayer V2NS2 and monolayer Ti2NS2 can reach 299.5 mA h g−1 and 308.3 mA h g−1, respectively.18

Experimentally, MXene structures are always terminated with functional groups –F, –OH or –O on the surface in their synthesis process.19–22 Bare MXenes are unstable in aqueous solution or open air.23 Recently, the –OH group can be replaced by a –S (or S2−) group during the heat treatment as known from the XPS analysis experiment.24 Moreover, it has been theoretically demonstrated that the substitution of –O(–F and –OH) groups on the MXene by the –S group is indeed possible12 and the S-terminated MXene structures show excellent performance as LIB anodes in previous research.25–27

Recent experimental and theoretical investigations have suggested that the energy capacities of MXene structures are strongly dependent on the functional groups.26,28 The monolayer V2C, as a new member of the MXenes, has been experimentally synthesized,29 which is one promising anode for LIBs and has a high theoretical Li capacity of 940 mA h g−1 and low Ebarrier for Li+.30 The Li capacity of the V2CO2 monolayer was theoretically 376 mA h g−1,31,32 while its experimental Li capacity was 260 mA h g−1 under the high charge/discharge rate.9 Therefore, more research is needed to explore the monolayer V2C with better electrochemical performance.

The S-terminated V2C monolayer has shown excellent performance as the LIB anode or when applied in Li–S batteries in previous research.33–35 In the periodic table of elements, since S, Se, Te and O are in the same column, the chalcogen-terminated puckered V2CX2 (X = S, Se, and Te) monolayers are expected to embody remarkable performance when used as the LIB anodes. Tin+1Cn (n = 1, and 2) MXenes terminated with selenium and tellurium have been synthetized experimentally in 2020.36 Sulfidation can replace oxygen with sulfur element.37 Until now, the Se- and Te-terminated V2C monolayers have not been reported. We will systematically investigate the possibilities of chalcogen-terminated monolayer-puckered V2CX2 (X = S, Se, and Te) and explore their application as LIB anodes in this paper. Our calculated results reveal that the monolayer V2CSe2 shows a higher Li capacity than V2CX2 (X = O, S, and Te) monolayer and the reduced Ebarrier for the Li+ ion compared with that of the monolayer V2CO2.

2. Computational methods

The Vienna ab initio simulation package (VASP) software, in which the electron-ion interaction is described by the projector augmented wave (PAW) method,38,39 is used to perform all the first-principles calculations in this paper.40 Based on the generalized gradient approximation (GGA), the Perdew–Burke–Ernzerhof (PBE) functional is used to treat the electron correlation interaction.41,42 The cutoff energy of the plane wave is set to be 600 eV. In order to avoid the interlayer interaction caused by the period boundary condition, in the direction perpendicular to the surface, a vacuum layer of 30 Å is applied. In the optimization process, the Brillouin zone is sampled by the 4 × 4 k-point for the 3 × 3 super cell with the convergence criteria of 0.01 eV Å−1 for the force and 10−6 eV for the energy. The van der Waals (vdW) interaction is taken into consideration by using the Grimme DFT-D2 dispersion correction method.43 The phonon spectra are calculated by using the Phonopy code44 interfaced with the VASP software. By using both the climbing image nudged elastic band (CI-NEB) and nudged elastic band (NEB) method,45,46 the diffusion of the Li+ along many pathways on monolayer V2CX2 (X = S, Se, and Te) are considered. Additionally, the ab initio molecular dynamics (AIMD) simulations are carried out to examine the thermal stability of the 3 × 3 super cell for Li atom embedded monolayer V2CX2 (X = S, Se, and Te) at 300 K. The AIMD method is based on the finite temperature of the local density approximation (with free energy as a variable) and diagonal to each MD step with effective matrix solution and effective Pulay mixed solution of instantaneous electronic ground state. The NVT ensemble is carried out to balance the system at 300 K for 5 ps.47–49

The monolayer V2C is firstly calculated to examine the computational method used here. The calculated lattice constant and the V–C bond length for the unit cell are 2.90 Å and 1.99 Å, respectively, which are consistent with the 2.91 Å and 2.00 Å in the previous research.50 Thus, the computation method used in this paper is reliable.

3. Results and discussion

3.1 The structures and stabilities of the V2CX2 monolayer

The structure of the monolayer V2CX2 (X = Se and Te) is the same as that of monolayer V2CS2, in which the S atom is replaced by Se and Te atoms. The optimized lattice constants of the unit cell for three monolayer structures (V2CS2, V2CSe2 and V2CTe2) are 3.05 Å, 3.12 Å and 3.24 Å, respectively, which are larger than those of 2.91 Å50 and 2.90 Å51 for V2C and V2CO2 monolayers. The geometric structures of monolayer V2CX2 (X = S, Se, and Te) have up-to-down five X, V, C, V, and X layers, and the two layers of atoms above the C layer are symmetric to those below the C layer, as shown in Fig. 1(a). The band structures of monolayer V2CX2 (X = S, Se, and Te) plotted in Fig. 1(b–d) reveal their metallic characteristics and the ultrafast electrical conductivity, which are critical to the LIB anode materials and similar to the case of monolayer Ti3C2S2.26 In order to judge whether monolayer V2C can form stable new structure of V2CX2, the formation energy (Ef) is calculated as follows:34
 
Ef(X) = 2E(X) + E(V2C) − E(V2CX2)(1)
where E(X) refers to the total energy per atom of the S8 molecule (when X = S), Se8 molecule (when X = Se) and metal Te (when X = Te), respectively. The E(V2C) and E(V2CX2) are the total energies of V2C and V2CX2, respectively. The calculated Ef of monolayer V2CS2 is 5.09 eV, which is in agreement with the previous research.34 The Ef are 4.80 eV and 3.25 eV for X = Se and X = Te, respectively. The positive values mean that the formation process of the V2CX2 monolayer is exothermic and the phase of the V2CX2 monolayer is stable against the separation for all X.

image file: d1qm00422k-f1.tif
Fig. 1 (a) Top view and side view of chalcogen-functionalized monolayer V2CX2 (X = S, Se, and Te). Electronic band structures of (b) monolayer V2CS2, (c) monolayer V2CSe2 and (d) monolayer V2CTe2. The yellow, blue and gray balls represent chalcogen, V, and C atoms.

In order to further check the stabilities of monolayer V2CX2 (X = S, Se, and Te), the phonon dispersion calculation is performed. The dynamical stability of monolayer V2CX2 (X = S, Se, and Te) is obviously verified by the none imaginary frequency in their phonon spectra as shown in Fig. 2(a–c). It can be noticed that, in the phonon spectra of monolayer V2CS2, three acoustic branches and twelve optical branches are observed, which are the same as those in the previous research.35 In the vicinity of the Γ point, there is linear dispersion of two in-plane acoustic branches and a quadratic out-of-plane acoustic branch, which is consistent with that of multilayer materials.26,52 Thus, these structures should be all dynamically stable. In addition, the thermal stabilities of them are checked by the AIMD simulation. The free energy of V2CX2 fluctuates around a constant value during 5 ps of total time at a temperature of 300 K. The time step is 1 fs, as presented in Fig. 2(d–f). The intact structure without any apparent distortion indicates that monolayer V2CX2 is thermally stable at room temperature. It should be noticed that the free energy curve of monolayer V2CS2 is consistent with that in the previous research.35


image file: d1qm00422k-f2.tif
Fig. 2 The calculated phonon band structures of the unit cells of the monolayers (a) V2CS2, (b) V2CSe2 and (c) V2CTe2. The free energy variations of (d) monolayer V2CS2, (e) monolayer V2CSe2 and (f) monolayer V2CTe2 within 5 ps during the AIMD simulation at 300 K.

The mechanical properties are further calculated to check the mechanical stability of the monolayer-puckered V2CX2 (X = S, Se, and Te). The four independent elastic constants, C11, C12, C22, and C44, are calculated and presented in Table 1. Clearly, they comply within the Born criteria53,54 that C11, C22, C44, and C11C22C122 should be all positive, implying that V2CX2 (X = S, Se, and Te) monolayer has feasible strength to store numerous Li atoms. According to the calculated elastic constant,55,56 the Young's modulus Y and Poisson’ ratio v along the x and y directions are calculated as follows:57

 
Yx = (C11C22C122)/C22(2)
 
Yy = (C11C22C122)/C11(3)
 
vx = C12/C22(4)
 
vy = C12/C11(5)

Table 1 The C11, C12, C22 and C44, Yx,Yy, vx, and vy of the monolayer V2CX2 (X = S, Se, and Te)
C 11 (N m−1) C 12 (N m−1) C 22 (N m−1) C 44 (N m−1) Y x (N m−1) Y y (N m−1) v x v y
V2CS2 243.67 38.94 243.67 102.36 237.45 237.45 0.16 0.16
V2CSe2 220.84 49.78 220.84 85.53 209.62 209.62 0.23 0.23
V2CTe2 166.70 75.47 166.70 45.61 132.53 132.53 0.45 0.45


The calculated Yx and Yy are equal for these 2D monolayer materials, which are 237.45 N m−1 for V2CS2, 209.62 N m−1 for V2CSe2 and 132.53 N m−1 for V2CTe2, respectively. Obviously, the Young's modulus Y of the V2CX2 (X = S, Se, and Te) monolayer is greater than those of some other 2D materials, such as monolayer WS2 (106.4 N m−1), indicating that monolayer-puckered V2CX2 (X = S, Se, and Te) has better mechanical strength. The decreased Y of monolayer V2CX2 means that the flexibility will increase with the larger mass of X. The calculated vx and vy for monolayer V2CX2 (X = S, Se, and Te) are 0.16, 0.23, and 0.45 respectively.

3.2 The adsorption and diffusion of the Li atom on monolayer V2CX2

Based on the time-cost consideration, the 3 × 3 super cell of monolayer V2CX2 (X = S, Se, and Te) is used to study the Li adsorption behavior. As shown in Fig. 5(a), there are three different sites, that is, upon the V atom of the lower layer (site A), upon the C atom (site B), and upon the X atom (site C). The average adsorption energy (Ead) of the Li atom at the above three different sites on monolayer V2CX2 is defined in the following:30
 
Ead = ELi + EV2Cx2EV2CX2+Li(6)

We take ELi as the energy of bcc-Li. If the Ead is larger than 0 eV, the adsorption strength should be ideal. The strong binding strength between the Li atom and monolayer V2CX2 can be reflected by the large positive Ead. Table 2 shows the Ead of one Li atom on the chalcogen-terminated monolayer V2C. It is found from the Ead that site A should be the most stable. Moreover, the calculated Eads at site A, B and C decrease with the increased atomic number of chalcogen elements. The Bader charges58 are calculated to explore the adsorption properties and are shown in Table 3. Obviously, the electron transfer between Li and monolayer V2CX2 decreases with the increased electronic layers of chalcogen elements. The three functional groups have little effect on the electron transfer between V and C. It can be known from the table that the heavier chalcogen element has less charge transfer. The difference charge densities are defined in the following:59

 
Δρ = ρ(V2CX2 + Li) − ρ(V2CX2) − ρ(Li)(7)
where ρ(V2CX2 + Li), ρ(V2CX2), and ρ(Li) represent the charge densities of one Li-adsorbed V2CX2, pristine V2CX2 and one Li atom, respectively.

Table 2 The Ead of single Li adsorbed on three different sites on the monolayer V2CX2 (X = S, Se, and Te)
E ad (eV) V2CS2 V2CSe2 V2CTe2
A 1.48 0.94 0.27
B 1.35 0.82 0.19
C 0.66 0.16 −0.45


Table 3 The Bader charges of Li(QLi), C(QC), and V(QV) and X (QX) in the monolayer V2CX2 (X = S, Se, and Te)
Li site Q Li Q C Q V Q X
V2CS2Li A 0.892 −1.461 1.285 −0.604
B 0.901 −1.461 1.285 −0.605
C 0.927 −1.460 1.282 −0.604
V2CSe2Li A 0.888 −1.504 1.209 −0.506
B 0.898 −1.506 1.207 −0.504
C 0.921 −1.505 1.207 −0.506
V2CTe2Li A 0.882 −1.549 1.110 −0.385
B 0.889 −1.549 1.110 −0.395
C 0.911 −1.550 1.108 −0.383


As shown in Fig. 4, the charge transfer mainly happens between Li and chalcogen atoms. Moreover, Li on the monolayer V2CX2 transfers more charge than that from the Li atom to monolayer V2C (0.80 e),30 which explores the stronger adsorption strength for the Li on the monolayer V2CX2 than that on the monolayer V2C. This can be in good agreement with the larger Ead on monolayer V2CX2 than that on monolayer V2C (0.16 eV).30 Thus, the heavier chalcogen element can obviously increase the binding strength between Li and the V2C monolayer. Obviously, it is known from Table 3 that the Li atom on site C loses most electrons. We further investigate the PDOS of one Li atom-adsorbed monolayer V2CS2. As shown in Fig. 3. The Ef is at 0 eV. The apparent overlapping of the PDOS of V(d), C(p), and X(p) indicates the strong hybridization among them. The overlapping between the PDOS of the Li atom and monolayer V2CX2 indicates the hybridization interaction between them. The PDOS around Ef mainly consists of V(d) orbitals, which is the same as that of monolayer V2CO2.31 The area between the PDOS and the energy axis is increasing with the increased electronic layers of chalcogen elements, which is consistent with the Bader charge analysis.58


image file: d1qm00422k-f3.tif
Fig. 3 (a) The structure of one Li-adsorbed V2CX2 at site A. The calculated partial density of states of one Li-adsorbed (b) monolayer V2CS2, (c) monolayer V2CSe2 and (d) monolayer V2CTe2, where the Ef is set to 0 eV.

image file: d1qm00422k-f4.tif
Fig. 4 The calculated difference charge densities of one Li-adsorbed (a) monolayer V2CS2, (b) monolayer V2CSe2 and (c) monolayer V2CTe2, with isosurface 0.008 e/borh3; the blue region represents electron depletion, while the yellow region represents electron accumulation.

image file: d1qm00422k-f5.tif
Fig. 5 (a)Top view of three adsorption sites and the most favorable diffusion path upon the monolayer V2CS2. (b) Energy profile of the A → B → A path upon monolayer V2CS2. (c) Column plots of Ebarrier from A → B for V2CX2 (X = S, Se, and Te).

To explore the promising properties of monolayer V2CX2 as high-rate LIB anode materials, the Ebarrier along diffusion paths has been investigated for one Li+ on the monolayer V2CX2 by both the CI-NEB and NEB methods.45 The Ebarrier is calculated in the following:60

 
Ebarrier = ETSESS(8)
Here, ETS and ESS are the total energies of the transitional state (TS) and the most stable state (SS) of the Li adsorbed membrane, respectively. We only consider the diffusion path between two neighboring most stable A sites (A → A). The Li atom will go past site B when travelling from site A to another nearest site A (A → B → A) according to the CI-NEB method. The Li atom will shift through site B when travelling from site A to another nearest site A, which is similar to the previous research of monolayer Ti3C2S226 and monolayer V2CS2.33 As shown in Fig. 5(c), the calculated Ebarrier values along the A → B diffusion path for V2CS2, V2CSe2 and V2CTe2 monolayers are 0.23 eV, 0.21 eV and 0.19 eV via the NEB method, respectively. We can note that the Ebarrier for the Li atom along the A → A diffusion path is equal to that along the A → B → A path, which are both 0.23 eV based on the CI-NEB method and can indicate the freedom of Li atoms diffusing on the V2CS2 substrate. This result is consistent with the previous research of monolayer V2CS2 (0.22 eV).33 Clearly, with the increased electronic layers of chalcogen elements, the Ebarrier is decreasing continuously. In addition, the Ebarrier of the chalcogen element-terminated V2C monolayer is lower than that of monolayer V2CO2 (0.30 eV). The higher Ebarrier of the Li atom on the monolayer V2CO2 compared with that on the V2CS2 monolayer is consistent with the previous research,33 which means the faster migration of Li on the monolayer V2CX2 (X = S, Se, and Te) than that in the corresponding path on the monolayer V2CO2.

3.3 The Li storage capability of chalcogen terminated V2CX2 (X = S, Se, and Te)

In order to compare the Li capability of monolayer V2CX2, the Ead values of the Li atoms on each layer are calculated using the following formula30 to analyze the effect of chalcogen element on the Li capability:
 
Ead(m) = (EV2CX2Li(m−1) + 18ELiEV2CX2Lim)/18(9)
where Ead(m) is the Ead of Li in the mth layer, and EV2CX2Li18m is the total energy of monolayer V2CX2 with m Li layers on both sides. The energy per bcc-Li is calculated as 2.03 eV in this paper. Therefore, the Ead can check the possibility of phase separation of the composite V2CX2Lim.61,62 The positive Ead larger than the cohesive energy of bcc-Li indicates that the Li atoms can avoid the formation of dendrites and collapse of anode materials can be avoided. All Li atoms in the first layer are located on the most stable site A on the monolayer V2CX2. When putting a second layer of Li atoms in site C (i.e. beyond the chalcogen atoms) of the substrate, the total energy of the formed composite V2CX2Li4 is the lowest for all X values. It can be known from Table 4 that monolayer V2CS2 can only adsorb one layer of Li atoms with the Ead of 0.77 eV, which is consistent with previous research.33 The Ead (−0.04 eV) of the second layer Li atoms in the V2CS2 substrate becomes negative. The V2CSe2 and V2CTe2 monolayers can both adsorb two layers of Li atoms with the adoptable Ead. The Ead of the first Li layer on site A on both sides of monolayer V2CSe2 is 0.39 eV, and the Ead of second Li layer on site C is 0.02 eV. The Ead of first Li layer on site A in each side of monolayer V2CTe2 is 0.07 eV, and the Ead of second Li layer on site C is 0.04 eV. When monolayers V2CSe2 and V2CTe2 adsorb more Li layers, the Ead becomes negative, indicating that the clustering of Li atoms will happen.61 The Ead of the second Li layer on the monolayers V2CSe2 and V2CTe2 is comparable with that on monolayer Mo2C(0.01 eV),63 monolayer Nb2C(0.02 eV)64 and monolayer MoC2(0.04 eV).65 As shown in Table 5, according to Bader charge58 analysis, when adsorbing second Li layer atoms, the charge transfer from one Li atom in the second layer to monolayer V2CS2, monolayer V2CSe2 and monolayer V2CTe2 is 0.29 e, 0.78 e and 0.79 e, respectively, which is consistent with their Eads (−0.04 eV, 0.02 eV and 0.04 eV) of second layer Li atoms and the same as that in the previous research about zirconium carbide MXene.66 When comparing the data shown in Tables 5 and 3, it can be found that the charge transfer mainly happens between Li and chalcogen atoms, while the charges of other elements in the substrate rarely change during the lithiation process. Meanwhile, the maximum theoretical Li capacities (mA h g−1) are calculated using the following formula:67
 
C = nνF103/M(10)
where, M is the atomic mass of monolayer V2CX2, n is the number of adsorbed Li atoms, ν is the valency of the Li atom, and F is the Faraday constant (26.801 A h mol−1).67 Obviously, the calculated Li capacity of monolayer V2CS2 (301.08 mA h g−1) is the same as that in previous research.33 The Li capacities of monolayers V2CSe2 and V2CTe2 are 394.41 mA h g−1 and 290.45 mA h g−1, respectively. Importantly, monolayer V2CSe2 can adsorb two Li layer atoms, thus resulting in the highest capacity, which is larger than that of graphite (372 mA h g−1) (Table 6).68
Table 4 The Ead, the area A, and the VER for the 3 × 3 super cell of monolayer V2CX2(X = S, Se, and Te)
E ad (eV) A (Å) VER (%)
V2CS2Li2 0.77 9.68 11.6
V2CSe2Li2 0.39 9.83 10.5
V2CSe2Li4 0.02 9.63 6.1
V2CTe2Li2 0.07 10.01 6.3
V2CTe2Li4 0.04 9.92 4.4


Table 5 The Bader charges of Li(QLi), C(QC), and V(QV) and X(QX) for the 3 × 3 super cell of the monolayer V2CX2(X = S, Se, and Te) with 36 Li atoms (two layers)
V2CS2 V2CSe2 V2CTe2
V 1.269 1.191 1.087
C −1.490 −1.152 −1.548
X −1.152 −0.973 −0.802
Li (1st layer) 0.337 −0.240 −0.300
Li (2nd layer) 0.291 0.780 0.789


Table 6 The lattice constants, area and VER of the 3 × 3 super cell of the monolayer V2CX2(X = S, Se, and Te)
3 × 3 supercell a (Å) b (Å) Area (Å2) VER (%)
V2CS2 9.16 9.16 72.59
V2CS2Li2 9.68 9.68 81.15 12
V2CS2Li4 9.55 9.55 78.98 8.8


Large VER will damage the cycle life of LIBs in practical applications. The lattice constants of the unit cell of three structures are 3.05 Å, 3.12 Å and 3.24 Å after the full lattice optimization, respectively. The VERs of the fully lithiated monolayer V2CX2(X = S, Se, and Te) shown in Table 4 are much lower than that of graphite (12%)69 and far smaller than those of other ultrahigh capacity anodes such as germanium(370%),70 Si(323%)71 and Tin(259%).72 They are also much smaller than those of the experimental critical values for the standards (<25%) for most LIB anodes.73,74 Therefore, the VER of monolayer V2CX2 is rather ideal and it should be adoptable for the LIB anodes. In addition, the VERs of three structures are decreasing with the increased mass of chalcogen elements.

Finally, the AIMD simulation of monolayer V2CSe2Li4 in the 3 × 3 super cell is performed, as shown in Fig. 6. The free energy fluctuates around a constant during the 5 ps total time at 300 K with the 1 fs time step. The structure remains intact without any apparent distortion, indicating its excellent thermodynamic stabilities. The Ebarrier, Li capacity and VER show that monolayer V2CSe2 should be more feasible for LIB anodes than monolayer V2CS2.


image file: d1qm00422k-f6.tif
Fig. 6 (a) The OCVs of monolayer V2CX2 (X = S, Se, and Te) during lithiation when all composites are fully optimized. (b) The curve of free energy within 5 ps at 300 K with time step of 1 fs of lithiated V2CSe2Li4 (left) and the snapshot after 5 ps (right).

The open circuit voltage (OCV) is an important quantity for the LIB anode materials. Commonly, the OCV for anodes should be positive and low to get a maximum capacity for the battery cell.75 The OCV of monolayer V2CX2Lim can be estimated by the following formula:76

 
OCV = (EV2CX2Li(m−1) + 18ELiEV2CX2Lim)/18e(11)

The positive OCV ensures that the adsorption of Li atoms on the anode material is feasible, which can prevent the formation of metallic states.77 The OCVs of monolayer V2CX2 plotted in Fig. 6(a) are positive. Obviously, the OCV of V2CSe2Lix ranges from 0.38 V to 0.02 V. Therefore, monolayer V2CSe2 has relatively low average OCV (0.2 V vs. Li/Li+), indicating that it is suitable for serving as the anode materials.14 Obviously, the average OCV of monolayer V2CSe2 of 0.2 V is comparable with that of monolayer W2C (0.27 V).78 Thus, monolayer V2CSe2 has high Li capacity, low Ebarrier for the Li+ ion, and small OCV when compared with those of graphite and other anode materials and should be the most ideal LIB anode material.

3.4 Chalcogen-terminated monolayer Ti2CX2 (X = S, Se, and Te)

In order to check whether the above results apply to other MXenes, we investigated the case of monolayer Ti2CX2 (X = S, Se, and Te). The previous research79 has shown that the phonon spectrum of monolayer Ti2CS2 has no imaginary frequency and the AIMD demonstrates the thermodynamic stability of monolayer Ti2CS2. As shown in Fig. 7(a and b), the phonon spectra of Ti2CSe2 and Ti2CTe2 monolayers also have no imaginary frequency, indicating the dynamical stability of the Ti2CSe2 and Ti2CTe2 substrates. In addition, the AIMD is calculated to check the thermodynamic stabilities of monolayer Ti2CSe2 and monolayer Ti2CTe2. After 5 ps, the structure remains intact without apparent distortion when the temperature is set as 300 K and the time step is set as 1 fs. The free energy fluctuates around a constant value, as shown in Fig. 7(c and d). Thus, monolayer Ti2CSe2 and monolayer Ti2CTe2 show high thermodynamic stabilities.
image file: d1qm00422k-f7.tif
Fig. 7 The phonon spectra of the (a) monolayer Ti2CSe2 and (b) monolayer Ti2CTe2. The free energy variations of (c) monolayer Ti2CSe2 and (d) monolayer Ti2CTe2 within 5 ps during the AIMD simulation at 300 K.

We next calculate the Li storage capacity of monolayer Ti2CX2 (X = S, Se, and Te). However, monolayer Ti2CX2 (X = S, Se, and Te) can only store one Li layer on each side of the substrate, which is different from the case of monolayer V2CX2 (X = S, Se, and Te), with the Eads of 1.5 eV, 0.78 eV and 0.26 eV, respectively. The corresponding Li storage capacities are 311.84 mA h g−1, 208.83 mA h g−1 and 147.68 mA h g−1 for monolayer Ti2CX2 (X = S, Se, and Te). Amongst all, monolayer Ti2CSe2 adsorbs only one Li layer on each side and has low Li storage capacity when compared with that of monolayer V2CSe2.

4. Conclusion

The metallic properties of monolayer V2CX2 (X = S, Se, and Te) can ensure fast electron transport in the charge/discharge process when used as the LIB anode. The dynamical stabilities are verified by the phonon spectra. The thermodynamic stabilities are verified by AIMD simulation. The Ead and Ebarrier of V2CX2 (X = S, Se, and Te) decrease with the increasing atomic number of the terminated element. Monolayer V2CSe2 shows higher Li capacity (394.41 mA h g−1), relatively low Ebarrier (0.21 eV) and small VER(6.1%) when compared with that of monolayer V2CX2 (X = O, S, and Te). Thus, monolayer V2CSe2 has high Li capacity, low Ebarrier for the Li+ ion, and small OCV when compared with graphite and other anode materials and should be the most ideal LIB anode material. Monolayer V2CS2 can adsorb one-layer Li atoms and then achieve the Li capacity of 301.08 mA h g−1, while monolayer V2CTe2 can adsorb multi-layer Li atoms and then achieve the Li capacity of 290.45 mA h g−1. The composite V2CX2Lin shows high thermodynamic stability during the AIMD at 300 K after 5 ps, which is consistent with its small VER. These excellent properties indicate that monolayer chalcogen-terminated V2C has promising applications as a LIB anode.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially sponsored by the Fundamental Research Funds for the Central Universities (Grant No. B200202001), the Natural Science Foundation of Jiangsu Province (Grant No. BK20161501), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX18_0521), Six talent peaks project in Jiangsu Province (Grant No. 2015-XCL-010), and the Open Subject of National Laboratory of Solid State Microstructures (Grant No. M32055).

References

  1. A. Manthiram, X. Yu and S. Wang, Lithium battery chemistries enabled by solid-state electrolytes, Nat. Rev. Mater., 2017, 2(4), 16103 CrossRef CAS.
  2. W. J. Zhang, A review of the electrochemical performance of alloy anodes for lithium-ion batteries, J. Power Sources, 2011, 196(1), 13–24 CrossRef CAS.
  3. H. Wu and Y. Cui, Designing nanostructured Si anodes for high energy lithium ion batteries, Nano Today, 2012, 7(5), 414–429 CrossRef CAS.
  4. A. L. M. Reddy, A. Srivastava, S. R. Gowda, H. Gullapalli, M. Dubey and P. M. Ajayan, Synthesis of nitrogen-doped graphene films for lithium battery application, ACS Nano, 2010, 4(11), 6337–6342 CrossRef CAS PubMed.
  5. C. Huang, S. Zhang, H. Liu, Y. Li, G. Cui and Y. Li, Graphdiyne for high capacity and long-life lithium storage, Nano Energy, 2015, 11, 481–489 CrossRef CAS.
  6. L. Zhou, S. Yan, L. Pan, X. Wang, Y. Wang and Y. Shi, A scalable sulfuration of WS 2 to improve cyclability and capability of lithium-ion batteries, Nano Res., 2016, 9(3), 857–865 CrossRef CAS.
  7. K. R. G. Lim, A. D. Handoko, S. K. Nemani, B. Wyatt, H. Y. Jiang, J. Tang, B. Anasori and Z. W. Seh, Rational design of two-dimensional transition metal carbide/nitride (MXene) hybrids and nanocomposites for catalytic energy storage and conversion, ACS Nano, 2020, 14(9), 10834–10864 CrossRef CAS PubMed.
  8. M. Naguib, J. Halim, J. Lu, K. M. Cook, L. Hultman, Y. Gogotsi and M. W. Barsoum, New two-dimensional niobium and vanadium carbides as promising materials for Li-ion batteries, J. Am. Chem. Soc., 2013, 135(43), 15966–15969 CrossRef CAS PubMed.
  9. F. Liu, J. Zhou, S. Wang, B. Wang, C. Shen, L. Wang, Q. Hu, Q. Huang and A. Zhou, Preparation of high-purity V2C MXene and electrochemical properties as Li-ion batteries, J. Electrochem. Soc., 2017, 164(4), A709–A713 CrossRef CAS.
  10. J. Halim, S. Kota, M. R. Lukatskaya, M. Naguib, M. Q. Zhao, E. J. Moon, J. Pitock, J. Nanda, S. J. May, Y. Gogotsi and M. W. Barsoum, Synthesis and characterization of 2D molybdenum carbide (MXene), Adv. Funct. Mater., 2016, 26(18), 3118–3127 CrossRef CAS.
  11. A. D. Handoko, S. N. Steinmann and Z. W. Seh, Theory-guided materials design: two-dimensional MXenes in electro-and photocatalysis, Nanoscale Horiz., 2019, 4(4), 809–827 RSC.
  12. J. Zhu, A. Chroneos, J. Eppinger and U. Schwingenschlögl, S-functionalized MXene as electrode materials for Li-ion batteries, Appl. Mater. Today, 2016, 5, 19–24 CrossRef.
  13. Y. Lee, S. B. Cho and Y. C. Chung, Tunable indirect to direct band gap transition of monolayer Sc2CO2 by the strain effect, ACS Appl. Mater. Interfaces, 2014, 6(16), 14724–14728 CrossRef CAS PubMed.
  14. Q. Meng, A. Hu, C. Zhi and J. Fan, Theoretical prediction of MXene-like structured Ti3C4 as a high capacity electrode material for Na ion batteries, Phys. Chem. Chem. Phys., 2017, 19(43), 29106–29113 RSC.
  15. L. Bai, H. Yin and X. Zhang, Energy storage performance of Vn+1Cn monolayer as electrode material studied by first-principles calculations, RSC Adv., 2016, 6(60), 54999–55006 RSC.
  16. Y. Zhou and X. Zu, Mn2C sheet as an electrode material for lithium-ion battery: A first-principles prediction, Electrochim. Acta, 2017, 235, 167–174 CrossRef CAS.
  17. S. Zhao, X. Meng, K. Zhu, F. Du, G. Chen, Y. Wei, Y. Gogotsi and Y. Gao, Li-ion uptake and increase in interlayer spacing of Nb4C3 MXene, Energy Storage Mater., 2017, 8, 42–48 CrossRef.
  18. V. Shukla, N. K. Jena, S. R. Naqvi, W. Luo and R. Ahuja, Modelling high-performing batteries with MXenes: The case of S-functionalized two-dimensional nitride MXene electrode, Nano Energy, 2019, 58, 877–885 CrossRef CAS.
  19. F. Liu, A. Zhou, J. Chen, H. Zhang, J. Cao, L. Wang and Q. Hu, Preparation and methane adsorption of two-dimensional carbide Ti2C, Adsorption, 2016, 22(7), 915–922 CrossRef CAS.
  20. M. H. Tran, T. Schäfer, A. Shahraei, M. Dürrschnabel, L. Molina-Luna, U. I. Kramm and C. S. Birkel, Adding a new member to the MXene family: synthesis, structure, and electrocatalytic activity for the hydrogen evolution reaction of V4C3Tx, ACS Appl. Energy Mater., 2018, 1(8), 3908–3914 CrossRef CAS.
  21. M. Ghidiu, J. Halim, S. Kota, D. Bish, Y. Gogotsi and M. W. Barsoum, Ion-exchange and cation solvation reactions in Ti3C2 MXene, Chem. Mater., 2016, 28(10), 3507–3514 CrossRef CAS.
  22. O. Mashtalir, M. R. Lukatskaya, M. Q. Zhao, M. W. Barsoum and Y. Gogotsi, Amine-assisted delamination of Nb2C MXene for Li-ion energy storage devices, Adv. Mater., 2015, 27(23), 3501–3506 CrossRef CAS PubMed.
  23. K. D. Fredrickson, B. Anasori, Z. W. Seh, Y. Gogotsi and A. Vojvodic, Effects of applied potential and water intercalation on the surface chemistry of Ti2C and Mo2C MXenes, J. Phys. Chem. C, 2016, 120(50), 28432–28440 CrossRef CAS.
  24. X. Liang, A. Garsuch and L. F. Nazar, Sulfur cathodes based on conductive MXene nanosheets for high-performance lithium–sulfur batteries, Angew. Chem., 2015, 127(13), 3979–3983 CrossRef.
  25. J. Zhu, A. Chroneos, J. Eppinger and U. Schwingenschlögl, S-functionalized MXene as electrode materials for Li-ion batteries, Appl. Mater. Today, 2016, 5, 19–24 CrossRef.
  26. Q. Meng, J. Ma, Y. Zhang, Z. Li, C. Zhi, A. Hu and J. Fan, The S-functionalized Ti3C2 MXene as a high capacity electrode material for Na-ion batteries: a DFT study, Nanoscale, 2018, 10(7), 3385–3392 RSC.
  27. V. Shukla, N. K. Jena, S. R. Naqvi, W. Luo and R. Ahuja, Modelling high-performing batteries with MXene: The case of S-functionalized two-dimensional nitride MXene electrode, Nano Energy, 2019, 58, 877–885 CrossRef CAS.
  28. M. Lu, H. Li, W. Han, J. Chen, W. Shi, J. Wang, X. M. Meng, J. Qi, H. Li, B. Zhang, W. Zhang and W. Zheng, 2D titanium carbide (MXene) electrodes with lower-F surface for high performance lithium-ion batteries, J. Energy Chem., 2019, 31, 148–153 CrossRef.
  29. M. Wu, B. Wang, Q. Hu, L. Wang and A. Zhou, The Synthesis Process and Thermal Stability of V2C MXene, Materials, 2018, 11(11), 2112 CrossRef PubMed.
  30. J. Hu, B. Xu, C. Ouyang, S. A. Yang and Y. Yao, Investigations on V2C and V2CX2(X = F, OH) monolayer as a promising anode material for Li ion batteries from first-principles calculations, J. Phys. Chem. C, 2014, 118(42), 24274–24281 CrossRef CAS.
  31. D. Sun, Q. Hu, J. Chen, X. Zhang, L. Wang, Q. Wu and A. Zhou, Structural transformation of MXene (V2C, Cr2C, and Ta2C) with O groups during lithiation: a first-principles investigation, ACS Appl. Mater. Interfaces, 2015, 8(1), 74–81 CrossRef PubMed.
  32. Y. Xie, M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, X. Yu, K. W. Nam, X. Q. Yang, A. I. Kolesnikov and P. R. C. Kent, Role of surface structure on Li-ion energy storage capacity of two-dimensional transition-metal carbides, J. Am. Chem. Soc., 2014, 136(17), 6385–6394 CrossRef CAS PubMed.
  33. Y. M. Li, Y. L. Guo and Z. Y. Jiao, The effect of S-functionalized and vacancies on V2C MXenes as anode materials for Na-ion and Li-ion batteries, Curr. Appl. Phys., 2020, 20(2), 310–319 CrossRef.
  34. B. Yan, C. Lu, P. Zhang, J. Chen, W. He, W. Tian, W. Zhang and Z. Sun, Oxygen/sulfur decorated 2D MXene V2C for promising lithium ion battery anodes, Mater. Today Commun., 2020, 22, 100713 CrossRef CAS.
  35. Y. Wang, J. Shen, L. C. Xu, Z. Yang, R. Li, R. Liu and X. Li, Sulfur-functionalized vanadium carbide MXene (V2CS2) as a promising anchoring material for lithium–sulfur batteries, Phys. Chem. Chem. Phys., 2019, 21(34), 18559–18568 RSC.
  36. V. Kamysbayev, A. S. Filatov, H. Hu, X. Rui, F. Lagunas, D. Wang, R. F. Klie and D. V. Talapin, Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes, Science, 2020, 369(6506), 979–983 CrossRef CAS PubMed.
  37. K. R. G. Lim, A. D. Handoko, L. R. Johnson, X. Meng, M. Lin, G. S. Subramanian, B. Anasori, Y. Gogotsi, A. Vojvodic and Z. W. Seh, 2H-MoS2 on Mo2CTx MXene Nanohybrid for Efficient and Durable Electrocatalytic Hydrogen Evolution, ACS Nano, 2020, 14(11), 16140–16155 CrossRef PubMed.
  38. G. Kresse and D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59(3), 1758 CrossRef CAS.
  39. P. E. Blöchl, Projector augmented-wave method, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50(24), 17953 CrossRef PubMed.
  40. G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54(16), 11169 CrossRef CAS PubMed.
  41. J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson, D. J. Singh and C. Fiolhais, Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation, Phys. Rev. B: Condens. Matter Mater. Phys., 1992, 46(11), 6671 CrossRef CAS PubMed.
  42. J. P. Perdew, K. Burke and M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett., 1996, 77(18), 3865 CrossRef CAS PubMed.
  43. S. Grimme, Semiempirical GGA-type density functional constructed with a long-range dispersion correction, J. Comput. Chem., 2006, 27(15), 1787–1799 CrossRef CAS PubMed.
  44. K. Parlinski, Z. Q. Li and Y. Kawazoe, First-principles determination of the soft mode in cubic ZrO2, Phys. Rev. Lett., 1997, 78(21), 4063 CrossRef CAS.
  45. G. Henkelman, B. P. Uberuaga and H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys., 2000, 113(22), 9901–9904 CrossRef CAS.
  46. G. Henkelman and H. Jónsson, Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points, J. Chem. Phys., 2000, 113(22), 9978–9985 CrossRef CAS.
  47. S. Nosé, A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys., 1984, 81(1), 511–519 CrossRef.
  48. S. Nose, Molecular dynamics simulations, Prog. Theor. Phys., 1991, 103, 1–117 CrossRef CAS.
  49. D. M. Bylander and L. Kleinman, Energy fluctuations induced by the Nosé thermostat, Phys. Rev. B: Condens. Matter Mater. Phys., 1992, 46(21), 13756 CrossRef PubMed.
  50. X. Ji, K. Xu, C. Chen, B. Zhang, H. Wan, Y. Ruan, L. Miao and J. Jiang, Different charge-storage mechanisms in disulfide vanadium and vanadium carbide monolayer, J. Mater. Chem. A, 2015, 3(18), 9909–9914 RSC.
  51. L. Feng, X. H. Zha, K. Luo, Q. Huang, J. He, Y. Liu, W. Deng and S. Du, Structures and mechanical and electronic properties of the Ti2CO2 MXene incorporated with neighboring elements (Sc, V, B and N), J. Electron. Mater., 2017, 46(4), 2460–2466 CrossRef CAS.
  52. H. Zabel, Phonons in layered compounds, J. Phys.: Condens. Matter, 2001, 13(34), 7679 CrossRef CAS.
  53. M. Born and K. Huang, Dynamical theory of crystal lattices, Clarendon press, 1954 Search PubMed.
  54. J. Wang, S. Yip, S. R. Phillpot and D. Wolf, Crystal instabilities at finite strain, Phys. Rev. Lett., 1993, 71(25), 4182 CrossRef CAS PubMed.
  55. K. H. Michel and B. Verberck, Theory of elastic and piezoelectric effects in two-dimensional hexagonal boron nitride, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80(22), 224301 CrossRef.
  56. T. Zhao, S. Zhang, Y. Guo and Q. Wang, TiC2:a new two-dimensional sheet beyond MXene, Nanoscale, 2016, 8(1), 233–242 RSC.
  57. R. C. Andrew, R. E. Mapasha, A. M. Ukpong and N. Chetty, Mechanical properties of graphene and boronitrene, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 85(12), 125428 CrossRef.
  58. G. Henkelman, A. Arnaldsson and H. Jónsson, A fast and robust algorithm for Bader decomposition of charge density, Comput. Mater. Sci., 2006, 36(3), 354–360 CrossRef.
  59. B. Zhang, L. Fan, J. Hu, J. Gu, B. Wang and Q. Zhang, MnB2 nanosheet and nanotube: stability, electronic structures, novel functionalization and application for Li-ion batteries, Nanoscale, 2019, 11(16), 7857–7865 RSC.
  60. S. T. Oyama, D. Lee, P. Hacarlioglu and R. F. Saraf, Theory of hydrogen permeability in nonporous silica membranes, J. Membr. Sci., 2004, 244(1-2), 45–53 CrossRef CAS.
  61. E. Lee and K. A. Persson, Li absorption and intercalation in single layer graphene and few layer graphene by first principles, Nano Lett., 2012, 12(9), 4624–4628 CrossRef CAS PubMed.
  62. X. Fan, W. T. Zheng, J. L. Kuo and D. J. Singh, Adsorption of single Li and the formation of small Li clusters on graphene for the anode of lithium-ion batteries, ACS Appl. Mater. Interfaces, 2013, 5(16), 7793–7797 CrossRef CAS PubMed.
  63. Q. Sun, Y. Dai, Y. Ma, T. Jing, W. Wei and B. Huang, Ab initio prediction and characterization of Mo2C monolayer as anodes for lithium-ion and sodium-ion batteries, J. Phys. Chem. Lett., 2016, 7(6), 937–943 CrossRef CAS PubMed.
  64. J. Hu, B. Xu, C. Ouyang, Y. Zhang and S. A. Yang, Investigations on Nb 2 C monolayer as promising anode material for Li or non-Li ion batteries from first-principles calculations, RSC Adv., 2016, 6(33), 27467–27474 RSC.
  65. Y. Yu, Z. Guo, Q. Peng, J. Zhou and Z. Sun, Novel two-dimensional molybdenum carbides as high capacity anodes for lithium/sodium-ion batteries, J. Mater. Chem. A, 2019, 7(19), 12145–12153 RSC.
  66. Q. Meng, J. Ma, Y. Zhang, Z. Li, A. Hu, J. J. Kai and J. Fan, Theoretical investigation of zirconium carbide MXene as prospective high capacity anode materials for Na-ion batteries, J. Mater. Chem. A, 2018, 6(28), 13652–13660 RSC.
  67. D. Datta, J. Li and V. B. Shenoy, Defective graphene as a high-capacity anode material for Na-and Ca-ion batteries, ACS Appl. Mater. Interfaces, 2014, 6(3), 1788–1795 CrossRef CAS PubMed.
  68. M. Broussely and G. Archdale, Li-ion batteries and portable power source prospects for the next 5-10 years, J. Power Sources, 2004, 136(2), 386–394 CrossRef CAS.
  69. K. Persson, Y. Hinuma, Y. S. Meng, A. Van der Ven and G. Ceder, Thermodynamic and kinetic properties of the Li-graphite system from first-principles calculations, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82(12), 125416 CrossRef.
  70. C. K. Chan, X. F. Zhang and Y. Cui, High capacity Li ion battery anodes using Ge nanowires, Nano Lett., 2008, 8(1), 307–309 CrossRef CAS PubMed.
  71. J. C. Arrebola, A. Caballero, J. L. Gómez-Cámer, L. Hernán, J. Morales and L. Sánchez, Combining 5 V LiNi0.5Mn1.5O4 spinel and Si nanoparticles for advanced Li-ion batteries, Electrochem. Commun., 2009, 11(5), 1061–1064 CrossRef CAS.
  72. P. Meduri, C. Pendyala, V. Kumar, G. U. Sumanasekera and M. K. Sunkara, Hybrid tin oxide nanowires as stable and high capacity anodes for Li-ion batteries, Nano Lett., 2009, 9(2), 612–616 CrossRef CAS PubMed.
  73. S. Zhao, W. Kang and J. Xue, The potential application of phosphorene as an anode material in Li-ion batteries, J. Mater. Chem. A, 2014, 2(44), 19046–19052 RSC.
  74. V. V. Kulish, O. I. Malyi, C. Persson and P. Wu, Phosphorene as an anode material for Na-ion batteries: a first-principles study, Phys. Chem. Chem. Phys., 2015, 17(21), 13921–13928 RSC.
  75. D. Das, R. P. Hardikar, S. S. Han, K. R. Lee and A. K. Singh, Monolayer BC2: an ultrahigh capacity anode material for Li ion batteries, Phys. Chem. Chem. Phys., 2017, 19(35), 24230–24239 RSC.
  76. K. T. Chan, J. B. Neaton and M. L. Cohen, First-principles study of metal adatom adsorption on graphene, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77(23), 235430 CrossRef.
  77. Q. Zhang, J. Ma, M. Lei and R. Quhe, Metallic MoN layer and its application as anode for lithium-ion batteries, Nanotechnology, 2018, 29(16), 165402 CrossRef PubMed.
  78. V. Shukla, N. K. Jena, S. R. Naqvi, W. Luo and R. Ahuja, Modelling high-performing batteries with Mxenes: The case of S-functionalized two-dimensional nitride Mxene electrode, Nano Energy, 2019, 58, 877–885 CrossRef CAS.
  79. X. Liu, X. Shao, F. Li and M. Zhao, Anchoring effects of S-terminated Ti2C MXene for lithium-sulfur batteries: A first-principles study, Appl. Surf. Sci., 2018, 455, 522–526 CrossRef CAS.

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