Thi Nhan Tran*a,
Nguyen Hoang Sonbc,
Nguyen Minh Hieuc,
Thuy Trang Nguyen
bd,
Yoshiyuki Kawazoe
efg,
Luong Huu Duch,
Minh Triet Dang
i,
Phi Long Nguyen
a and
Viet Bac Phung Thi
*c
aHanoi University of Industry, 298 Cau Dien Street, Bac Tu Liem, Hanoi 100000, Vietnam. E-mail: tran.nhan@haui.edu.vn
bFaculty of Physics, University of Science, Vietnam National University – Hanoi, Hanoi, Vietnam
cCenter for Environmental Intelligence and College of Engineering & Computer Science, VinUniversity, Hanoi 100000, Vietnam
dKey Laboratory for Multiscale Simulation of Complex Systems, University of Science, Vietnam National University – Hanoi, Hanoi, Vietnam
eNew Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8579, Japan
fDepartment of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur 603203, Tamil Nadu, India
gSchool of Physics, Institute of Science, Suranaree University of Technology, 111 University Avenue, Nakhon Ratchasima 30000, Thailand
hLaboratory for Chemistry and Life Science (CLS), Institute of Integrated Research (IIR), Institute of Science Tokyo (Science Tokyo), R1-25, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8501, Japan
iSchool of Education, Can Tho University, Can Tho City 900000, Vietnam
First published on 14th July 2025
Heterostructures composed of graphene (G) and WS2 have recently been proposed as a promising new two-dimensional carbon allotrope for an anode material in sodium-ion batteries. Actively controlling material defects by substituting sulfur (S) atoms on the surface of WS2 with alternative dopants is anticipated to be a potential strategy for enhancing the electrochemical performance of WS2/G heterostructures. Here, we employ first-principles density functional theory (DFT) calculations to systematically investigate the impact of boron (B) and carbon (C) doping on the sodium intercalation and diffusion mechanisms within the heterostructures. The results reveal that doped WS2/G heterostructures show electronic characteristics of metallic materials, which are beneficial for their application as high-performance anode materials. The introduction of B/C dopants significantly enhance the binding affinity for sodium intercalation at active sites, both on the surface and at interfacial region, with binding energies reaching up to −1.702 eV, which can mitigate sodium dendrite formation during electrochemical cycling. Notably, the presence of B/C dopants can create energetically favorable diffusion pathway both on the surface and in the interfacial region of the WS2/G bilayers for sodium ions with energy barriers ranging from 0.091 to 0.494 eV, underscoring their potential to support high-rate charge/discharge processes. Additionally, B/C-doped WS2/G heterostructures exhibit inconsiderably structural deformation during sodium intercalation, making them suitable candidates as anode materials in batteries with high cycling stability. Our findings provide valuable insights into the effect of the dopants within the sodium intercalation mechanisms of WS2/G heterostructures, paving the way for the rational design of next-generation anode materials for high-performance sodium-ion batteries.
Developing and optimizing advanced anode materials for SIBs has been an area of growing interest. Previous research indicates that WS2 monolayer (ML) possesses several attractive properties for SIBs, including high stability, reversibility, and a suitable adsorption environment for sodium-ions, thanks to the tuneable dimensions of interfacial spacing and the precise nanostructure.6 Despite these advantages for SIBs, WS2 ML exhibits low-rate capability, low electronic conductivity, slow diffusion of Na ions, and considerable volume expansion during sodiation/desodiation processes, leading to quick capacity fading.7
Vertically stacking on graphene to form heterostructures has been proposed as a promising strategy to enhance the performance of transition metal dichalcogenide (TMD) ML as anode materials in SIBs.7,8 This approach enhances electrical conductivity without requiring a metallic substrate, conducting additives, or polymeric binders of heterostructures.8–18 Moreover, the graphene matrix of the heterostructures can effectively buffer the volume change during the Na insertion/extraction process and increase the anode's overall electrochemical function.10,12 In recent studies, WS2/G heterostructures have been successfully synthesized through ultrasonication, chemical vapor deposition, and hydrothermal techniques, achieving uniform growth of WS2 nanocrystals on graphene nanosheets.8,12,14 It was used as an anode material for both lithium-ion batteries and SIBs due to its high surface area and good in-plane conductivity, which facilitates fast electron transfer during electrochemical reactions of the material.12,13,16–18 As electrodes in SIBs, the WS2/G exhibits a good reversible sodium storage capacity of about 590 mA h g−1.12 It also shows excellent high-rate performance and cyclability.
With the aim of developing and optimizing advanced anode materials based on WS2/G composites for SIBs, several rational strategies have been proposed from both experimental and theoretical perspectives. One effective approach involves rolling graphene into hollow nanotubes to encapsulate WS2 nanostructures, thereby constructing a highly conductive and electrolyte-accessible framework. This architecture not only facilitates electron and ion transport but also effectively mitigates volume changes during cycling.19 Another promising design introduces a three-layer shell structure composed of a stable porous carbon shell, WS2 nanosheets, and nitrogen-doped graphene, which significantly enhances lithium and sodium storage performance. This configuration delivers a high discharge capacity of 205 mA h g−1 after 900 cycles at 0.5 A g−1 for SIBs.20 It was reported that co-doping with nitrogen and oxygen modifies the electronic structure of WS2, reducing its bandgap from 1.6 eV to 0 eV and increasing the interlayer spacing in WS2/G composites. These modifications significantly improve electron and ion transport, resulting in exceptional electrochemical performance with an ultrafast Na+ storage capability and remarkable cycling stability over 3000 cycles.21 Additionally, substituting the sulfur atoms in WS2 or MoS2 ML, another two-dimensional material in the TMD family, with boron/carbon (B/C) has been proposed as an effective pathway to increase electronic conductivity, enhance sodium intercalation strength, and accelerate diffusion of electrons and ions.22–25 Similarly, dopants on the sulfur layer of the WS2/G heterostructure are expected to modify the surface properties by creating additional active sites for sodium intercalation, thereby improving the storage capacity of SIBs.
A good understanding of the dopant effect on the sodium intercalation mechanism and sodium migration within WS2/G heterostructures is critical to boost their application as new nanoscale anodes for high-performance SIBs. Hence, we perform a systematic first-principles study to explore the impact of boron (B) and carbon (C) doping on Na intercalation mechanism and diffusion in WS2/G heterostructures for potential use as anode materials in SIBs. We focus on analysing the effect of B/C dopants on sodium intercalation ability, electronic conductivity, structural stability, and Na-ionic migration characteristics of the WS2/G heterostructures. Our findings provide a meaningful and promising strategy for designing advanced SIB anodes.
To achieve a heterostructure with a minor lattice mismatch, we constructed a bilayer based on a 4 × 4 × 1 supercell of WS2 monolayer stacked above a 5 × 5 × 1 supercell of graphene, to investigate the Na intercalation mechanism of the C/B-doped WS2/G heterostructures. A Monkhorst–Pack scheme30 with 6 × 6 × 1 and 9 × 9 × 1 k-mesh is required to converge the plane-wave basis set for the optimization and electronic calculations of the doped WS2/G heterostructures, respectively. A convergence energy of 540 eV was applied to all investigated samples. The change in the total energy between the two consecutive steps is set to be 10−5 eV. Atomic positions are relaxed until the maximum residual force acting on each atom is smaller than 0.01 eV Å−1. A vacuum of 25 Å thickness was introduced between two neighbouring heterostructures to eliminate the atomic interactions between the imaged periodic systems. We utilized the VASPKIT package31 to post-process the electronic properties.
We computed the Na binding energy (Eads) for each explored configuration using the following relation:
Ebind = Ecomplex − Edop − ENa, | (1) |
Δρ = ρcomplex − ρdop − ρNa, | (2) |
![]() | (3) |
Isolated WS2 | Isolated graphene | Pristine heterostructure | B-doped heterostructure | C-doped heterostructure | |
---|---|---|---|---|---|
Lateral lattice constant | a = b = 12.66 Å | a = b = 12.34 Å | a = b = 12.46 Å | a = b = 12.46 Å | a = b = 12.46 Å |
W–S bond length (Å) | 2.42 | — | 2.41 | 2.41 | 2.41 |
C–W bond length (Å) | — | — | — | — | 2.02 |
B–W bond length (Å) | — | — | — | 2.12 | — |
C–C bond length (Å) | — | 1.43 | 1.44 | 1.44 | 1.44 |
Strain sustained by WS2 layer (%) | — | — | 1.57 | 1.57 | 1.57 |
Strain sustained by graphene layer (%) | — | — | −0.97 | −0.97 | −0.97 |
Interlayer distance (Å) | — | — | 3.39 | 3.40 | 3.40 |
Interlayer binding energy (meV per atom) | — | — | −33.6 | −33.1 | −30.5 |
Doping with elements of lower electronegativity has been reported to be beneficial in expanding the local interfacial space, creating additional active sites for Na+ intercalation, and reducing the energy barrier for Na+ migration by altering the electronic properties.48,49 Fig. S2† presents the optimized geometric structure of 4:
5 WS2/G heterostructures with a single B/C-doped atom. The lattice parameters of the bilayers upon doping remain unchanged, as shown in Table 1. Both the C–C and W–S bond lengths are also unaffected, while the calculated binding energies are −31.3 for B doping and −30.5 meV for C doping. The unchanged in lattice parameters and the negative binding energies indicate that the doped WS2/G heterostructures maintain high structural stability, making them suitable for use as anode materials. The interlayer spacing between the graphene and WS2 layers around the dopants slightly expanses by 0.01 Å as doping, which contributes to enhance sodium storage capability and facilitate Na+ diffusion kinetics.48,50 Additionally, the W–B and W–C bond lengths around the dopants are 2.12 Å and 2.02 Å, respectively, both considerably shorter than the W–S bond length. These bond contractions induce local lattice distortions, breaking the structural symmetry of the heterostructure and potentially modifying the charge carrier distribution.
To investigate the impact of B and C dopants on the sodiation behaviours of WS2/G heterostructures, a single Na atom is sequentially placed at various active sites, which include the top of the dopant atom, the top of a W atom, the W–C/B bridge site, and the hollow site at the centre of a WS2 hexagon near the dopant on the WS2/G surface. For comparison, Na intercalation on the surface of pristine WS2/G heterostructure are also exanimated. Pristine WS2/G exhibits a weak sensitivity to Na, in which the binding energies for Na intercalation on the surface and within the interfacial region are −0.229 eV and −0.283 eV, respectively. The intercalated distance (the shortest distance from the intercalated Na and the WS2 surface) is 2.11 Å for the surface site and 1.82 Å for the interfacial site. Fig. 1(a and c) shows the most stable configurations of Na intercalated on the surface of B- and C-doped WS2/graphene heterostructures with intercalation distances of 1.73 Å and 1.43 Å, respectively. These distances are reduced by 0.38 Å and 0.68 Å, respectively, compared to that in the undoped WS2/G structure. Corresponding to this reduction in intercalation distance, the binding energies for Na on the B-doped and C-doped heterostructures are −1.461 eV and −1.702 eV, as listed in Table 2, indicating a remarkably enhanced Na intercalation upon doping. These findings suggest that B and C dopants act as anchoring centers for Na+ ions, enhancing local intercalation. Thus, a rational increase in dopant concentration can create more favorable adsorption sites and boost the Na+ storage capacity of WS2/G anodes. We also sequentially inserted a single sodium atom into the interfacial space at four different positions, which include the bottom of an W atom, the W–S bridge, and the hollow site below the center of a WS2 hexagon near the bottom of the dopant. The results indicate that Na ions prefer to anchor at the W-bottom site near the doped-B/C atoms as illustrated in Fig. 1(b and d). The Na intercalation energy for the B/C-doped system is −1.234/–0.834 eV, which is significantly more negative than that without dopants and aligns well with the observed 0.05 Å reduction in intercalation distance upon doping. The results suggest that B and C doping on the surface of the WS2/G heterostructure not only facilitates Na intercalation above the surface but also enhances intercalation within the interfacial region. The enhanced sodium intercalation induced by doping is a crucial factor, enabling a uniform Na+ distribution as shown in Fig. S3† with the intercalation of six sodium atoms and thereby effectively suppressing dendrite formation on the anode during cycling process.51
![]() | ||
Fig. 1 The most stable configurations of the B/C-doped WS2/G as adding single Na atom either above the surface (a/c) or within the interfacial space (b/d) with top view. |
Dopant | On surface | Within interfacial region | |||||
---|---|---|---|---|---|---|---|
W top | Dopant top | Hollow | Bridge | W bottom | Hollow | Bridge | |
B | −1.461 | −1.453 | −1.450 | −1.451 | −1.234 | −1.139 | −1.219 |
C | −1.581 | −1.591 | −1.702 | −1.577 | −0.834 | −0.726 | −0.813 |
Upon Na-ion intercalation, structural deformation caused by large volume changes can result in stress accumulation, leading to mechanical failure, disrupted electrical pathways, and diminished active surface area of the electrode.52 This degradation not only compromises electrode integrity but also hampers Na+ transport kinetics, adversely impacting the battery's rate capability and long-term cycling.53 Therefore, identifying anode materials with small volumetric changes during cycling is crucial to ensuring long cycle life of electrodes in SIBs. To evaluate the volumetric changes of B/C-doped WS2/G heterostructures induced by Na addition with respect to the unintercalated case, we use the following equation:
![]() | (4) |
Parameter | Dopant | Na number above the surface | Na number in the interfacial space | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | 1 | 2 | 3 | 4 | 5 | 6 | ||
Volume change (%) | B | −0.9 | 0 | 0 | −0.4 | −0.5 | −0.3 | −1.0 | 0 | −0.2 | −0.2 | −0.5 | −0.5 |
C | 0 | 0 | −0.7 | −1.1 | −0.5 | −0.4 | −1.0 | 0 | −0.1 | −0.2 | −0.4 | −0.5 | |
Interfacial spacing (Å) | B | 3.38 | 3.40 | 3.39 | 3.38 | 3.37 | 3.36 | 3.87 | 4.18 | 4.31 | 4.38 | 4.40 | 4.47 |
C | 3.39 | 3.39 | 3.39 | 3.39 | 3.33 | 3.33 | 3.73 | 4.21 | 4.31 | 4.39 | 4.42 | 4.46 |
Change in interfacial distance during Na-ion intercalation can directly influence the ease of sodium insertion and migration within the anode's interfacial region during charge/discharge cycles. As presented in Table 3, when Na atoms are positioned on the surface, the interlayer spacing either remains nearly unchanged or slightly decreases, by less than 0.07 Å, even as the number of intercalated Na atoms increases up to six. This small variation in interfacial spacing suggests that the doped heterostructures provide a space, well-suited for efficient Na insertion and diffusion within the interfacial region. Unlike Na intercalation on the surface, the insertion of Na atoms within the interfacial region of the doped WS2/G heterostructures leads to a significant local expansion of the interfacial distance near the intercalation sites. This distance can increase by as much as 1.07 Å, reaching up to 4.47 Å. As the number of intercalated Na ions increases, the interfacial distance further expands to create a more accommodating reservoir for sodium intercalation and de-intercalation, which is beneficial for enhancing ion storage capacity, facilitating ion transport, and promote rapid charge-transfer reactions.57 Despite the interfacial space expansion, the doped WS2/G heterostructures exhibit only a small overall volume change, a retaining their van der Waals (vdW) layered geometry, and especially, and a uniform Na+ distribution even with the insertion of up to six Na ions, as shown in Fig. S3.† This promises a good cycling stability during the charge/discharge processes, making doped WS2/G heterostructures as potential candidates for durable anode materials in SIBs.
![]() | ||
Fig. 2 Projected band structure and projected density of states (DOS) of pristine WS2/G (a), B-doped WS2/G (b), and C-doped WS2/G (c). |
Pristine heterostructure | B-doped heterostructure | C-doped heterostructure | |
---|---|---|---|
WS2 total charge (e) | −0.046 | +0.957 | +2.543 |
Graphene total charge (e) | +0.046 | +0.046 | +0.189 |
Dopant total charge (e) | — | −1.003 | −2.732 |
Conducting layers | Graphene | Graphene WS2 | Graphene WS2 |
Origins of additional conducting mechanisms | WS2 top valance band B-dopant induced shallow accepter band | Partially occupied C-dopant-induced band |
High electrical conductivity of the anodic material allows fast ion movement within the electrode structure, leading to a high capacity and performance. Introducing dopants into WS2/G heterostructures is expected to enhance their electronic conductivity. As shown in Fig. 2(b and c), the doped WS2/G heterostructures have an electronic band structure with type-I band alignment of metallic composite materials. This indicates that introducing dopants enhance conductivity of the heterostructure, which is beneficial for their application as anodic materials in SIBs. B/C doping induces an isolated, weakly dispersive impurity state with a high density of states (DOS) near the Fermi level and within the electronic gap of the WS2. The impurity states can act as extra energy levels, affecting electronic properties of the systems by allowing more effortless movement of electrons through the material, boosting the charge transport properties, and thereby facilitating a fast electron transfer during electrochemical reactions of the material.63,64 Although doping induces local lattice distortions, the absence of significant changes in the dispersion of the bands near the Fermi level implies that mobility is not drastically affected by doping-induced structural distortion. Furthermore, upon introducing dopants, we can see an inconsiderable change in the individual electronic band structure of both graphene and WS2 around the Dirac cone, which indicates a weak influence of the dopants on the interlayer interaction within the heterostructure.
The redistribution of charge carriers on both the WS2 and graphene sides due to B/C doping could play a critical role in enhancing the electrical conductivity of the heterostructures as well as the local chemical affinity toward Na+ ions at both their surface and interface. A careful analysis of the dopant-induced charge redistribution can provide further insights into the mechanisms responsible for these enhancements. Within the B-doped heterostructure, the Fermi level is at the top of the doped WS2 valence band. This allows electrons to be excited from the valence band of the doped WS2 layer to the graphene layer, enabling significant participation of the doped WS2 layer in electrical conduction. Fig. 3(c) presents the EBDCD at the top of the WS2 valence band at a K-point near the M point, where a band crossing occurs between WS2 and graphene. The charge densities indicate that the carriers in the B-doped WS2 layer are delocalized throughout the WS2 layer, including the region around the dopant, which exhibits B px/y – W d bonding characteristic as highlighted by a red circle in Fig. 3(c). Moreover, the B dopant also induces a shallow acceptor band located between 1.32 eV and 1.42 eV above the Fermi level (see Fig. 2(b)), characterized by contributions from non-bonding B pz, WS2, and graphene pz states as the corresponding EBDCD represented in Fig. 3(d). The significant contribution of graphene pz states to the B-dopant-induced acceptor band, as well as the presence of B px/y – W d bonding states in the WS2 valence band, demonstrating a dopant-induced electron transfer to the B dopant like the Bader charge analysis (Table 4). Differing from B doping, introducing C dopant induces a smaller shift in the Fermi level as shown in Fig. 2(c). It can see that the Fermi level overlaps the dopant pz. The highest occupied orbitals of WS2 reside in the energy range below 0.613 eV from the Fermi level, retaining the characteristic shape of the EBDCD isosurface in case without dopants as depicted in Fig. 3(e), and do not participate in the conduction process. The possibility of an additional electrical conduction mechanism arises from the partially occupied band induced by the dopant. The corresponding EBDCD, shown in Fig. 3(e and g), demonstrates that the carriers associated with this impurity state are localized around the dopant site, particularly, in the pz shape regions, with no significant distribution in the graphene layer. The excited holes at this impurity state might transfer into the graphene, like that happens in pristine heterostructures due to type-I band alignment with the Fermi level crossing the impurity state, increasing the conductivity of graphene layer. As a result, the graphene layer becomes positive charged with value of +0.189e, which is greater 4.1 times than that as doping B as observed from Table 4. It implies that inducing isolated impurity state due to C dopant is predicted to boost vertical hole transfer from the WS2 side to the graphene side to enhance the electrical conductivity of the bilayers.
To evaluate the impact of Na intercalation on the electronic structure of doped WS2/G bilayers, we carried out electronic structure analyses on the Na-intercalated systems. These systems exhibit metallic electronic band structures similar to those of the systems without Na intercalation, as shown in Fig. 4, which is beneficial for electron and Na+ ion transport during charge/discharge processes. As summarized in Table 5, the Bader charge calculations reveal that Na atoms carry a formal charge ranging from +0.984 to +0.988 in both B- and C-doped heterostructures, presenting the reliability of the Na+/anode model. The charge transfer from Na to the doped heterostructure leads to a significant upward shift of the Fermi level, which lies around the Dirac point in Fig. 4. Upon Na intercalation, dopant orbitals become delocalized, occupying not only the impurity states as in the case without Na intercalation, but also the electronic energy states of WS2 around the Fermi level. This delocalization leads to an increase in the DOS near the Fermi level, thereby enhancing the electrical conductivity of the system. In the undoped system, the WS2 nanosheet receives 0.764e from the intercalated Na atom at the preferred surface site and 0.486 e when Na is located within the interfacial space. However, the B/C-doped WS2 nanosheets within doped WS2/G heterostructures gain more charge upon Na intercalation, 0.980/0.932 e from surface-intercalated Na and 0.875/0.611 e from Na positioned in the interfacial space, as summarized in Table 5. This indicates that introducing dopants can promote electron trapping during Na intercalation,65 thereby enhancing Na+ ion diffusion and improving the overall electrochemical performance of the material during sodium-ion battery cycling, cycling as observed in experiment works.63,64 The effect of Na intercalation on the electronic properties of doped heterostructures differs, depending on whether it occurs at the surface or the interface. Specifically, when Na is anchored at the surface sites of the bilayer, the doped WS2 monolayer receives the majority of the electron charge, 99.2% for B doping and 93.7% for C doping. This charge transfer primarily affects the electrical properties of the doped WS2 layer, while the electronic properties of the graphene layer are only weakly affected. In addition, the dopants act as electron trapping centers. Specifically, the B dopant gains 0.260 e and the C dopant receives 0.367 e, resulting in electron accumulation around the dopant sites, as shown in Fig. 5(a and c). When Na is intercalated at the interface, the graphene layer gains a significantly higher amount of charge compared to the case with Na intercalated at the surface: 0.113 e with B doping and 0.376 e with C doping. This more distributed charge transfer is attributed to the shorter distance between the intercalated Na and the graphene layer. Consequently, noticeable electron accumulation on the graphene surface is evident in Fig. 5(b and d). The charge transfer from interface-intercalated Na can enhance the electrical conductivity of both the doped WS2 and graphene layers.
Heterostructure | Individual part | Na above the surface | Na at interfacial space |
---|---|---|---|
Non dopant | WS2 (e) | +0.764 | +0.486 |
Graphene (e) | +0.211 | +0.582 | |
Na (e) | −0.975 | −0.988 | |
B-dopped WS2/G | WS2 (e) | +0.720 | +0.921 |
Graphene (e) | +0.008 | +0.113 | |
Doped B (e) | +0.260 | −0.046 | |
Na (e) | −0.988 | −0.988 | |
C-dopped WS2/G | WS2 (e) | +0.565 | +0.510 |
Graphene (e) | +0.062 | +0.376 | |
Doped C (e) | +0.367 | +0.101 | |
Na (e) | −0.984 | −0.987 |
Fig. 6 presents the activation energy profiles as a function of the migration coordinate for Na diffusion along the three diffusion paths in both B- and C-doped WS2/G heterostructures. The profiles reveal that the energy barrier for Na migration above the surface of the doped WS2/G strongly depends on the type of dopant. Specifically, the energy barrier for Na diffusion on the surface of C-doped WS2/G is 0.494 eV, which is moderate and comparable to the energy barrier for Na migration through the interface of 1T-MoS2/G11 and Li migration in the interlayer space of WS2/G without doping.17 However, for the B-doped WS2/G system, the energy barrier for Na migration on the surface is considerably smaller, 0.091 eV, which is comparable to that of pristine graphene sheets66 and much smaller than that of pristine TMD nanosheets (0.49–0.95 V).67 This remarkable difference is mainly attributed to the electronegativity of the dopant type. As illustrated in Fig. 3(c), B doping with lower electronegativity compared to S induces carrier delocalization across the WS2 layer within the heterostructure, thereby significantly accelerating Na+ ion transport, as previously reported.68 Conversely, C doping with higher electronegativity results in carrier localization as shown in Fig. 3(e), which leads to a higher of the energy carrier for Na migration. This finding highlights the critical role of dopant selection as a strategic design approach for optimizing Na-ion mobility in WS2/graphene-based anode materials. For Na+ migration through the interfacial region along the a- and b-directions of the B- and C-doped WS2/graphene heterostructures, a slight difference in the activation energy profiles is observed between the two directions, as shown in Fig. 6. The energy barriers fall within a narrow range of 0.274 to 0.335 eV. These values are notably lower than those of pristine TMD nanosheets67 and similar heterostructure such as 1T-MoS2/G (∼0.7 eV),11 facilitating favorable Na+ diffusion within the interfacial space. The low migration barrier of Na ions in the interface region is attributed to two main factors. First, introducing dopants enhances the electrical conductivity of the system with good electrical conductivity of metallic materials, thereby facilitating charge transport.69 Second, introducing dopants breaks the local intrinsic lattice symmetry of heterostructures, which lead to a charge inhomogeneity, creating low potential energy landscape for Na ion immigration at interface region.21 These findings demonstrates that B/C doping in WS2/G heterostructures can create energetically favourable pathway for sodium ions, similar to that pointed out in previous reports.70
By analyzing idealized theoretical models, our work uncovers the role of dopants in tailoring sodium intercalation and diffusion processes in WS2/G heterostructures, offering guidance for the design of advanced anode materials. However, translating these theoretical insights into real electrochemical performance under practical cycling conditions requires a further consideration of several key factors such as dopant distribution uniformity, defect formation, and potential Na clustering at high concentrations, which may significantly influence Na intercalation behavior and alter ion diffusion kinetics. This study opens new avenues for future research to bridge the gap between theory and experiment. Promising directions include evaluating the long-term cycling stability of doped WS2/G heterostructures under realistic battery operating conditions, assessing thermal stability and structural integrity during Na intercalation via Ab initio Molecular Dynamics simulations, and investigating long-term ion diffusion and storage kinetics through kinetic Monte Carlo (KMC) methods.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra04616e |
This journal is © The Royal Society of Chemistry 2025 |