GeP3/NbX2 (X=S, Se) Nano-Heterostructures: Promising Isotropic Flexible Anodes for Lithium-Ion Batteries with High Lithium Storage Capacity.

A new trend is emerging that flexible batteries will play an indispensable role in the progress of social science and technology. However, flexibility exists only in a single direction for the existing electrode material. Searching for flexible battery materials has attracted more and more attention from researchers. In this article, the lattice structural stability, electronic structure modulation, and the Li adsorption properties of the heterostructures designed by assembling GeP3 and NbX2 (X = S, Se) together were methodically explored based on van der Waals. We found that diffusion barrier of the GeP3/NbS2 heterostructure with metallic properties is 0.21 eV for Li. It greatly improves the charge and discharge performance of the battery. The predicted heterostructure shows quite high theoretical specific capacity with 540.24 mA h/g, which is higher than the traditional graphite anode (372 mA h/g). It demonstrates superior isotropic flexibility with a considerable small Young's modulus (151.98-159.02 N/m), which has promising application as flexible electrodes for rechargeable battery equipment.

Introduction

The burgeoning demand for flexible devices having high softness, for example, the folding screen, implantable medical devices, wearable sensors, and computers that require bending, folding, twisting or even stretching, has attracted tremendous attention recently.[1−4] Rechargeable Li-ion batteries (LIBs) are dominating components in the portable device market because of their high energy density, relatively long lifespan, environmentally friendly operation, and design flexibility currently outperforming other systems.[5−7] The current choice of anode materials for LIBs is graphite.[8] However, the graphite anode still encounters some unsolved problems, such as considerably low storage capacity (372 mA h/g), safety issues related to lithium deposition, and the insufficient mechanical strength fall off the flexible substrate during the mechanical deformation process. The challenges of the graphite anode mentioned above impede its extensive application.[9−11] To meet the increasing global pliable demand of electronic devices, designation of flexible LIBs electrode is one of the essential strategies.[12−14] Selecting electrode materials with both excellent electrochemical and mechanical properties is the key to realize flexible energy storage.[15,16] There is a series of exploration on the advanced flexible electrode materials during last decades. The other members of the family of carbon attracted extensive investigations accordingly, for example, carbon nanotubes, graphene, and carbon cloth. Stimulated by the graphene with diverse functional properties, the family of many other two dimensionals (2D) nanomaterials and their composites or hybrids have been studied both extensively and intensively, thus plenty of potential candidates for exploration and assessment, which is monolayer GeP3,[17,18] phosphorus allotropes, and transition-metal dichalcogenides (TMDCs).[19−21] The single-layer 2H–NbS2 (hexagonal) with stability is chosen to form heterojunction electrode materials for LIB. It can be prepared by alkali metal naphthalene solution.[21−23] More importantly, NbS2 has a suitable open-circuit voltage range and phase-maintaining as an anode material during alkali-ion adsorption.[23] In addition, the NbSe2 monolayer exhibits metallic properties before and after Li atom adsorption, which is a necessary electrical conductivity requirement for anode materials. It has comparatively low diffusion barrier of approximately 0.21 eV for Li atoms, which guarantees outstanding cycling performance of the NbSe2 monolayer as a battery electrode.[24] Recently, Zhang et al. predicted that monolayer GeP3 with a puckered honeycomb structure, can be used as a promising anode material for rechargeable LIBs with ultrahigh capacity and superior ionic conductivity.[18,25] GeP3 monolayer has an exfoliation energy of 1.14 J/m2, which suggests mechanical cleavage and liquid exfoliation approaches can be used to fabricate the GeP3 monolayer from the bulk material.[26,27]

Various ways have been explored to improve graphene properties of 2D nanomaterials, including doping[28,29] defect[30,31] strain[32,33] and building heterojunctions with other 2D materials.[34,35] Heterojunctions have been deeply studied as potential anode materials, such as MXene/Blue Phosphorene, Blue Phosphorene/MS2, Black Phosphorene/TiC2, and so forth.[9,36−39] Heterojunctions are constructed by combining more than one artificially layered materials through interlayer weak van der Waals (vdW) forces. The heterojunctions are potentially used in electronics and optoelectronics, particularly 2D semiconductor heterostructures, forming p–n junctions with atomic thickness, may play important roles in flexible integrated circuits field in the near future.[40−44] Inspired by these, in this article, we systematically studied the atomic structure, electronic structure, mechanical stability, adsorption, and diffusion properties of Li atoms in GeP3/NbX2 (X = S, Se) vdW heterostructures using first-principles calculations based on vdW-amended density functional theory.[43] The article is mainly composed of four parts, namely, research background, simulation method, analysis and discussion, and conclusion.

Results and Discussion

Geometry and Stability

There are many different combination arrays of the two monolayers NbX2 and GeP3. According to the atomic and hole locations on the monolayer surface, six typical combinations are selected. The method is to fix one layer first and then rotate the other layer, rotating 60° at a time, which is 1/6 of the circle as illustrated. The most stable configuration is presented in Figure . The views of the structures of GeP3 and NbX2 (X = S, Se) monolayers are shown in Figure S1. Optimized lattice constants and the M–X (M = Nb, Ge; X = S, Se, P) and P–P bond length are listed in Table S1. Considering the lattice matching, a 2 × 2 × 1 supercell of NbS2 was applied to construct two monolayers heterojunction with GeP3, where the lattice mismatching of the two monolayers is achieved 0.15%, so it is compatible for constructing GeP3/NbX2 nano-heterostructures. All the possible configurations are shown in Figure S2. Relative energy based on the most stable structure, the space distance between the layers and bond lengths are shown in Table S2. The Nb–X and P–P bond lengths barely change except the Ge–P bond. Additionally, the energy released by the combination of the two layers for GeP3/NbS2 and GeP3/NbSe2 heterostructures are −1807.80 and −1926.30 meV. Therefore, good lattice matching and negative formation can be conducive to the formation of GeP3/NbX2 heterostructures. It was computed that the calculated binding energies between GeP3 and NbX2 monolayers are 28.30 meV/Å2 for GeP3/NbS2 and 26.80 meV/Å2 for GeP3/NbS2 heterostructures, not far from the known vdW heterojunction binding energy (20 meV/Å2).[45]

Figure 1

Most stable configuration of GeP3/NbX2 (X = S, Se). (a) Top view, (b) side view.

Electronic Properties

The electronic band structures of hybrid systems are plotted in detail in Figure a,b, and it is found that the electronic structures of GeP3/NbX2 vdW heterostructures retain the metallic characteristics of NbX2. Another notable finding is that the electron belts of GeP3/NbX2 vdW heterostructures around the Fermi Energy are denser than their monolayers. The GeP3/NbX2 vdW heterostructures have the advantages of electrical conductivity over TMDCs. The brillouin zone with high-symmetry points labeled and band structures of the single-layer involved in this study are shown in Figure S3, from which we can see that the GeP3 monolayer is an obvious indirect gap band semiconductor and NbX2 has metallic character.[21,46]

Figure 2

Band structures of GeP3/NbS2(a) and GeP3/NbSe2 (b) heterostructures, where the energy is scaled with respect to the Fermi Energy EF, and the size of the pink, blue, red, and green circles illustrates the projected weight of Ge-p, P-p, Nb-d, and X-p (X = S, Se) electrons, respectively.

In order to analyze the electronic energy band structure of the NbX2 single-layer and GeP3/NbX2 vdW heterostructures further, the total density of states (DOS) and orbital-resolved projected DOS are depicted in Figure . The valence bands of both single layer NbS2 and NbSe2 are from −4 to −0.70 eV (S-3p) and from −4 to −0.60 eV (Se-4p), respectively. Above the Fermi level, the conduction bands of NbS2 and NbSe2 are composed of Nb-4d orbitals. However, there are no electronic state displays in the range from −0.70 to −0.30 (−0.60 to −0.30) eV and from 0.90 to 2.10 (0.60 to 1.90) eV for NbS2 (NbSe2). For the GeP3/NbS2 and GeP3/NbSe2 vdW heterostructures, the valence bands S-3p orbitals and Se-4p orbitals also play an important role, while the Nb-4d orbitals are located at the conduction bands. It is worth noting that the heterostructures increased an additional DOS level at the edge of the valence band compared with the corresponding single-layer NbS2 and NbSe2. More significantly, bands of different locations die out and are reborn, specifically for the vdW heterostructures of GeP3/NbS2 from 0.70 to 1.90 eV and GeP3/NbSe2 from 0.40 to 1.70 eV. The strengthened DOS and modulation of bands in GeP3/NbS2 and GeP3/NbSe2 heterostructures could be beneficial to improve the charge transfer, the rate which suggests the potential application of GeP3/NbX2 vdW heterostructures as LIBs electrodes.

Figure 3

Total DOS and orbital-resolved partial DOS: (a) NbS2, (b) NbSe2 monolayers, (c) GeP3/NbS2, and (d) GeP3/NbSe2 heterostructures. Inset red arrows denote the increased DOS in heterostructures.

The change of electron density is shown in Figure . Apparently, the primary reason for the differential charge distribution is the interlayer coupling effect. Electrons cross the heterojunction interface from the GeP3 side to the NbX2 side. Correspondingly, a hole is created in the place of departure. In order to estimate the charge distribution and transfer quantification, the Bader charge analysis was used.[47] The results show that the GeP3 side lost 0.51 and 0.28 electrons for NbS2 and NbSe2 side, respectively. Meanwhile, the electron localization functions (ELF) was used to identify chemical bond changes of the single layer and double layer. The ELF contour plots projected in the (110) plane are exhibited in Figure .[48] The values of ELF at the S, Se, and P sites are localized. On the contrary, the interlayer is negligible (they are 0.15 and 0.16 for GeP3/NbS2 and GeP3/NbSe2 heterostructures in the area). This aspect explains the weak vdW force between two layers.

Figure 4

Plane-averaged electron density difference along the direction perpendicular to the interface: (a) GeP3/NbS2 and (b) GeP3/NbSe2 heterostructures. The positions of the Ge, P, Nb, and X (X = S, Se) atoms are indicated by blue, pink, gray, and green solid circles, respectively. The magenta and cyan regions indicate electron accumulation and depletion, respectively.

Figure 5

ELF contour plots projected on the (110) plane: (a) NbS2, (b) NbSe2, (c) GeP3/NbS2, and (d) GeP3/NbSe2.

Mechanical Properties

The mechanical stability and omnidirectional tensile properties of the GeP3/NbX2 heterostructures were indirectly obtained by calculating the elastic constants.[49,50] For the 2D rectangular crystal structures, the predicted four independent elastic constants, C11, C22, C12, and C44, were summarized in Table . All the calculated elastic constants fulfil Born’s mechanical stability criteria (see eq for details), signifying the mechanical stability of GeP3, NbS2, and NbSe2 monolayers and the GeP3/NbX2 heterostructures.[9] The C11 and C22 of the five materials are similar, designating the compressive and tensile properties that are extraordinarily similar with respect to the x and y directions. The results indicated that the in-plane elastic constants of GeP3/NbX2 heterostructures are greater than those of the corresponding monolayer systems. Under the action of a certain force, the strain of the GeP3, NbS2, and NbSe2 monolayer is greater than that of the GeP3/NbX2 heterostructure, resulting in the relative slip between materials in the vdW heterostructure is more likely to occur in the deformation process than the corresponding monolayer.

Table 1

Predicted Elastic Constants C11, C22, C12, and C44 (N/m) for the GeP3, NbS2, and NbSe2 Monolayers and GeP3/NbX2 Heterostructures

system	C11	C12	C22	C44	source
GeP3	48.76	6.9	45.37	19.76	this work
NbS2	112.79	38.58	109.20	33.91	this work
GeP3/NbS2	167.03	35.76	159.64	63.26	this work
NbSe2	83.27	30.64	87.96	32.42	this work
	78.92	19.83	79.63		(55)
GeP3/NbSe2	144.88	31.92	141.82	56.96	this work

Comparing with the monolayer counterpart, the Young’s modulus and the stiffness are increased by constructing the GeP3/NbX2 heterostructure.[9,37,42] The Young’s modulus was further calculated by using two independent methods to verify again our predicted isotropy in GeP3/NbX2 heterostructures. In the first approach, the Young’s modulus in each direction was calculated for all the 2D systems according to eq (labeled as the C method). The polar diagrams are shown in Figure . The Young’s modulus of GeP3, NbS2, and NbSe2 monolayers, as well as GeP3/NbS2 and GeP3/NbSe2 heterostructures has a value between 44.39–60.96, 93.19–99.16, 72.60–83.39, 151.98–159.02, and 134.79–138.78 N/m, respectively (see Figure S4). The average values of the Young’s modulus of GeP3, NbS2, and NbSe2 monolayers, as well as GeP3/NbS2 and GeP3/NbSe2 heterostructures among all directions are 52.94, 95.42, 78.81, 155.09, and 137.16 N/m, respectively.However, in the second calculating approach, the stress–strain curve was mainly used.[51,52] The Young’s moduli are shown in Table . But as a rule of thumb, the results are almost always the same.[37] The data of X-axis and Y-axis are very close, confirming that they are isotropic, which have supported our previous analysis of the elastic constants. The full range isotropy and omnidirectional stretch ability of the GeP3, NbS2, and NbSe2 monolayers and GeP3/NbX2 heterostructures are illustrated in Figure . In fact, the GeP3/NbX2 heterostructures demonstrate superior flexibility with a much smaller Young’s modulus. The reference material is graphene (342.20 N/m)[53,54] and BN (275.80 N/m).[55] Notably, the GeP3/NbX2 heterostructures with omnidirectional flexibility have great potential for development in flexible electronic devices and efficient electrodes.

Figure 6

Polar diagrams of the Young’s modulus E(θ) (N/m): (a) GeP3/NbS2 and (b) GeP3/NbSe2. The angle θ identifies the extension direction with respect to the armchair direction. Isotropic (anisotropic) behavior is associated with the circular (noncircular) shapes of the E(θ).

Table 2

Predicted Young’s Moduli E (N/m) along Armchair (x), Zigzag (y) Directions for the GeP3, NbS2, and NbSe2 Monolayers, as well as GeP3/NbS2 and GeP3/NbSe2 Heterostructures

system	Ex	Ey	method or source
GeP3	47.71	44.39	tension
	38.11	40.96	Cij
NbS2	99.15	96.00	tension
	83.24	86.80	(36)
	81.70	86.70	Cij
NbSe2	72.61	76.68	tension
	71.35	71.01	(36)
	71.70	82.60	Cij
GeP3/NbS2	159.02	151.98	tension
	132.97	140.52	Cij
GeP3/NbSe2	137.70	134.79	tension
	122.91	132.50	Cij

Li-Adsorption Properties

For the GeP3/NbX2 heterostructure composed by 1 × 1 × 1 cell of GeP3 and 2 × 2 × 1 cell of NbX2, expand it into a supercell (Nb4Se8/Li0.25), and adsorb li atoms on the NbX2 side. For the adsorption of one Li atom on the NbSe2 sheet, the Li atom preferentially occupies on top of a metal atom (TNb).[56] For the adsorption of one Li atom on a sole NbS2 monolayer (Nb4S8/Li0.25), four typical adsorption sites were considered, as shown in Figure , including the top point above the metal atom (TNb), the hollow point above the hexagonal center, the top point above the sulfur compound atom (TS), and the bridge point above the mid-point bond (TB). By optimizing the structure, the bridge point was unstable and Li would automatically move to the top of the metal atoms. The adsorption energy sequence is Ead(TNb) < Ead(HNbS) < Ead(TS). Therefore, the priority of TNb site is higher for the adsorbed atoms. For the supercell of GeP3/Li0.25, four typical adsorption sites were also considered, as shown in Figure , including the top site directly on top of a Ge or P atom (TGe, TP1, and TP2) and the top site directly on the center of a hexagon (HP). By optimizing the structure, the TP1 adsorption structure will disappear and turn into the TGe adsorption structure. The adsorption energy sequence is Ead(TGe) < Ead(TP2) < Ead(HP). Therefore, the priority of TGe site is higher, agreeing well with other available theoretical data.[18] Nine adsorption sites of monatomic Li fromGeP3/NbS2 heterostructure were considered, which were the outside surface of NbS2 (TNb, TS, and HNbS), the outside surface of GeP3(TGe, TP2, and HP), and the position embedded into the vdW gap with a GeP3/Nb4S8/Li0.25 stoichiometry (TS/HP, TNb/TGe, and TNb/TP2). After structural optimization, the top site directly above a chalcogenide atom did not exist. Because the Li atom moved to the hollow site above the center of a hexagon. To sum up, after removing the unstable adsorption sites, we determined a total of 16 adsorption sites and made the heterojunction into a 2 × 2 × 1 supercell for adsorption and obtained the final model Figure . In the rest eight models, the lithium atoms did not deviate significantly, and the diverse adsorption data are shown in Table . The negative adsorption energy indicates the stability of Li adsorption structure and thus as a potential candidate serving as a flexible anode. The adsorption energy sequence is Ead(TNb) < Ead(HNbS) < Ead(TNb/TGe) < Ead(TNb/TP2) < Ead (TGe) < Ead(TP2) < Ead(HP) < Ead(TS/HP). Thus, the top of the metal atom (TNb) has the highest priority for adsorbed atoms. The interlayer distances of the GeP3/NbS2 heterostructure are listed in Table S3. The interlayer distance of the GeP3/NbS2 heterostructure increases after inserting the Li atom in the inner part and when inserting the Li atom in the TNb or TGe, the interlayer distance became even bigger. It may be that the existence of lithium atoms breaks the stable state of vdW force. According to Pauli’s incompatibility principle, the electrons are fermions, and two fermions can never occupy the same quantum state in the same system. When a lithium atom is inserted the site near the Nb or Ge atom, a partial of electron cloud will overlap, resulting in greater interlayer distance. The adsorption energy of atom (Li) can be described as eq ,[48] including atomization energy (1.60 eV/Li, which is consistent with the previous calculated value with 1.63 eV), heterotopic lattice micro deformation energy (shown in Table ), and osmotic adsorption energy. It is obvious that the chemical bond energy between lithium atoms is reduced because the value of lithiated heterostructures tends to be smaller.

Figure 7

GeP3/NbS2 supercell (2 × 2 × 1) lattice structure of Li-adsorption sites in GeP3/NbS2 heterostructure: (a–c) Top and side views for the occupied site, the red circles are theoretical vacancies; (d) side view accommodating up to 64 Li atoms; the red atoms are the adsorbed lithium atoms; (e) ELF contour plot projected on the (110) plane of 16 Li atom adsorption on the GeP3/NbS2 heterostructure.

Table 3

Diverse Adsorption Data of Li Atom Adsorption on 4 × 4 × 1 Supercell of NbS2 and 2 × 2 × 1 Supercell of GeP3 Monolayers and the 2 × 2 × 1 Supercell of the GeP3/NbS2 Heterostructurea

system	site	Ead	OCV	Ef	Es	E–b	capacity	ΔQLi	ΔQT	ΔQP
Nb4S8/Li0.25	TNb	–3.652	2.048	–2.048	0.417	–4.069	10.637	+0.989	–0.989
Nb4S8/Li0.25	HNbS2	–3.587	1.983	–1.983	0.392	–3.979	10.637	+0.992	–0.992
Nb4S8/Li0.25	TS	–3.042	1.438	–1.438	0.400	–3.442	10.637	+0.986	–0.986
GeP3/Li0.25	TGe	–2.914	1.310	–1.310	0.464	–3.378	20.130	+0.989	–0.989
GeP3/Li0.25	TP2	–2.819	1.215	–1.215	0.557	–3.376	20.130	+0.991	–0.991
GeP3/Li0.25	HP	–2.658	1.054	–1.054	0.509	–3.167	20.130	+0.993	–0.993
GeP3/Nb4S8/Li0.25	TNb	–3.465	1.860	–1.860	0.421	–3.885	6.972	+0.988	–0.920	–0.068
GeP3/Nb4S8/Li0.25	HNbS2	–3.386	1.781	–1.781	0.426	–3.811	6.972	+0.992	–0.925	–0.067
Li0.25/GeP3/Nb4S8	TGe	–2.790	1.186	–1.186	0.431	–3.221	6.972	+0.990	–0.033	–0.957
Li0.25/GeP3/Nb4S8	TP2	–2.657	1.052	–1.052	0.436	–3.092	6.972	+0.989	–0.036	–0.953
Li0.25/GeP3/Nb4S8	HP	–2.521	0.917	–0.917	0.442	–2.963	6.972	+0.989	–0.024	–0.965
GeP3/Li0·25/Nb4S8	TNb/TGe	–3.190	1.586	–1.586	0.628	–3.818	6.972	+0.999	–0.493	–0.506
GeP3/Li0·25/Nb4S8	TNb/TP2	–2.850	1.246	–1.246	0.75	–3.600	6.972	+0.999	–0.494	–0.505
GeP3/Li0·25/Nb4S8	TS/HP	–2.198	0.594	–0.594	1.01	–3.208	6.972	+0.998	–0.299	–0.699

The adsorption energy, Ead (eV/Li); open circuit voltage, OCV (V); formation energy, Ef (eV/Li); strained energy cost, Es (eV/Li); E– (eV/Li), in Li-intercalated systems; theoretical gravimetric capacity (mA h/g); and the charge the reverse binding energy transfer of Li atoms (ΔQLi, |e|), NbS2 (ΔQT, |e|), and GeP3 (ΔQP, |e|) layers.

In the two-layer heterostructure, according to the calculation of total energy, Li atoms preferentially occupy the TNb position on the surface of NbS2 and the TGe position on the surface of GeP3. Then, the diffusion behavior of Li atom from TNb1 to TNb2 sites or TGe1 to TGe2 was calculated by using the nudged elastic band (NEB) method[57] to analyze the diffusion energy barrier on the surface of pure monolayer NbS2 surface and GeP3/NbS2 heterostructure, and the diffusion paths and diffusion barriers are shown in Figures and S5, respectively. According to Figure a, it is seen that for the monolayer NbS2 surface as the diffusion matrix, the optimal path diffusion barrier of the Li atom is 0.48 eV. For the monolayer GeP3 surface as diffusion matrix, the optimal path diffusion barrier of the Li atom is 0.50 eV, which is shown in Figure S5a. The calculated results are very close to the general theoretical results, which are 0.47 and 0.50 eV, respectively. The calculated results are shown in Figures b and S5(b) that the diffusion behavior of Li atom from TNb1 to TNb2 sites or TGe1 to TGe2 sites in the heterogeneous junction by the NEB method. The diffusion barriers are 0.49 and 0.56 eV for NbS2 and GeP3 layers in the heterogeneous junction, respectively, which are larger than the two layers involved. It is worth mentioning that the optB86b vdW functional used in this paper has a lower energy barrier than Perdew–Burke–Ernzerhof (PBE) functional. Consequently, the formation of GeP3/NbS2 heterostructure increases the diffusion barrier from 0.01 to 0.09 eV. It is better than traditional materials such as TiO2-based polymorphs (0.30–0.65 eV)[53,54] and silicon (0.57 eV).[58] Therefore, it has great application prospects in anode materials.

Figure 8

Migration paths from TNb1 to TNb2 sites and the corresponding diffusion energy barrier profiles for a Li atom adsorbed on the (a) NbS2 monolayer and (b) GeP3/NbS2 heterostructure. Inset red arrows denote the Li atom diffusion path.

The calculated diverse adsorption data of Li atom adsorption involved in this study are summarized in Table . It is 1.86 V for the heterojunction OCV between its two monolayer components (NbS2 2.05 V and GeP3 1.31 V). The article also found that for lithiated heterojunctions, the best adsorption site is the top of TNb. The theoretical speculation is that the Es of the Li atom on the TNb site is less than those at the other sites. In addition, the adsorption of Li atoms at the TNb position will make the Li bond energy larger and E– more negative. If adsorption occurs, this will be the first place where the Li atom resides. The relationship between the number of Li atoms adsorbed and OCV and adsorption energy is shown in Table S4. The calculated results show that one GeP3/NbS2 cell can stably adsorb 16 Li atoms. The model is Li4/GeP3/Li4/Nb4S8/Li8. In this case, the heterojunction cell is still stable, Li-adsorption energy is −2.10 eV/Li, and Li storage capacity is as high as 540.24 mA h/g, higher than the previously reported graphene value of 372 mA h/g.[59,60] It is more important to note that in addition to these excellent performances, the GeP3/NbS2 heterostructure also has omnidirectional flexibility. Furthermore, Figure e shows the ELF of the (110) section of the GeP3/NbS2 heterostructure with 16 Li atoms, which indicates that the absorbed Li atoms are stable.

Next, charge transfer, used model of GeP3/Nb4S8/Li0.25 with TNb site, was applied to characterize the chemical bond strength between lithium atoms and heterojunction atoms,[47] and the data are shown in Table . The analysis results found that the charge of the Li atom was transferred to its corresponding adsorption layer (NbS2), which is same for Li0.25/GeP3/Nb4S8 with the TGe site. When the Li atom is embedded between two monolayers (GeP3/Li0·25/Nb4S8, TNb/TGe site), the charge transferred from Li to NbS2 and GeP3 are −0.49|e| and −0.50|e| which reveals the strong ionic interaction between the Li atom and the heterojunction.[40] Furthermore, various energy changes for the NbSe2 monolayer and the GeP3/NbSe2 heterostructure are summarized in Table S5. Compared with the GeP3/NbS2 heterostructure, the OCV and capacity of GeP3/NbSe2 heterostructure is lower.

The above charge transfer analysis can also be well explained with images, as illustrated Figure . It is the charge density difference of lithiated GeP3/Nb4S8/Li0.25 (TNb site), Li0.25/GeP3/Nb4S8 (TGe site), and GeP3/Li0·25/Nb4S8 (TNb/TGe site) systems. Obviously, the charge loss of lithium atoms corresponds to their strong ionic bonds. The charge of lithium atoms on one side of the monolayer flow to the monolayer (Li0.25/GeP3/Nb4S8, see Figure b). The charge of lithium atoms between the two monolayers flow to the two monolayers (GeP3/Li0·25/Nb4S8, see Figure c) which is a strong mutual confirmation for the result and the above analysis.

Figure 9

Top and side views of the charge density difference of a Li atom (a) adsorption on the out-surface of NbS2; (b) adsorption on the out-surface of GeP3; (c) inset into the interlayer of GeP3/NbS2, where the loss of electrons is indicated with cyan and gain of electrons is indicated with magenta.

Conclusions

We have constructed a stable GeP3/NbX2 heterojunction model and found that the metal properties are mainly derived from the d orbital contribution of Nb atoms, and the interlayer coupling effect makes the charge transfer from GeP3 to NbX2. The calculation results show that the heterostructure is isotropic flexibility. Its diffusion barrier (0.21 eV) is lower than well-known anode materials, for example, silicon (0.57 eV). The lithium atom storage capacity of GeP3/NbS2 up to 540.24 mA h/g is considerably higher than graphite (372 mA h/g). Based on the assessment of structural stability, omnidirectional flexibility and lithium storage energy, we recommend GeP3/NbS2 for a potential lithium-ion flexible battery anode nanomaterial applied in flexible electronic device in the future, while GeP3/NbSe2 is not the case because of its rather lower theoretical gravimetric capacity although both NbS2 and NbSe2 belong to TMDCs.

Computational Methods

Calculations were carried out using the VASP package with the projector augmented wave method.[61−63] The exchange–correlation function was described in the PBE scheme with generalized gradient approximation and modified by vdW density functional optB86b.[47,57,64] Cut-off energy of 500 eV was set for the plane-wave expansion. All structures were fully relaxed until the forces acting on all atoms are minor than 0.01 eV Å–1, and the energy is less than 10–5 eV per atom.[40,65] Besides, we used Bader charge analysis to quantitatively study the charge distribution and transfer and used the “nudge elastic band” method to calculate the diffusion barrier value.[51,66,67] Same characteristics have emerged in the GeP3 and NbX2 monolayers and the GeP3/NbX2 heterostructures, which is their high symmetry in the zigzag and armchair directions. It is necessary to calculate the stress–strain curves in these two directions and derive the Young’s modulus.[51,52] The tensile strain is defined as eq .

The Young’s modulus (E(θ)) was deduced by formula eq . (C method),[68] where Δ = C11C22 – C122, c = cos θ, and s = sin θ.

After optimizing crystal geometry, we calculated the formation energy of six models as mentioned above by eq .[69]

Meanwhile, the vdW forces between the two monolayers were evaluated by eq ,[37] where is the sum of the total energy of mutually independent single-layered GeP3 and NbX2 (S,Se) fixed in the corresponding heterostructure lattice.

The theoretical capacity for adsorbed lithium atoms was calculated by eq ,[70] where n is the number of Li atom, F is Faraday constant, and M is total mass of the adsorption panel.

The formula of open-circuit voltage is eq , where μ is chemical potential of the Li atom.

The adsorption energy of Li intercalation was calculated based on eq , where ELi is energy of the isolated Li atom.

The adsorption formation energy was defined as eq , and the parameters involved have been mentioned above.

The rate of charge transfer was calculated by following eq .[43]