V₅S₈–graphite hybrid nanosheets as a high rate-capacity and stable anode material for sodium-ion batteries

Abstract

Here we report chemically-exfoliated V₅S₈ and graphite hybrid nanosheets (ce-V₅S₈–C) as a novel anode material for sodium-ion batteries (SIBs). It exhibits much improved sodiation capacity, rate capability, reversibility and stability compared to other major SIB anode materials.

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Broader context

Sodium ion batteries (SIBs) are an appealing alternative to lithium ion batteries (LIBs) for large-scale energy storage as sodium is earth abundant and widely accessible. The development of high-performance anode materials is a bottleneck for the success of SIBs. In the present work, we report that monoclinic structured V₅S₈ nanosheets, when combined with graphite, are a promising anode material for high-performance SIBs. The synthesized V₅S₈ possesses a unique crystal structure consisting of VS₂ monolayer building blocks, between which one-quarter of the available crystallographic sites are occupied by V atoms. Within each VS₂ monolayer, VS₆ octahedra are face-shared with the adjacent VS₆ octahedra centered on the V atoms in the V-partially depleted interlayer. Such a unique structure facilitates the production and transport of electronic/ionic charge carriers, giving an increased electronic/ionic conductivity, and thus promoting a fast three-dimensional (3D) electron and Na⁺-diffusion transport for reversible charge/discharge cycles. Electrochemical performance results indeed suggest that the V₅S₈–C hybrid is a promising anode material for SIBs with a good reversible capacity, cycling stability and rate capacity.

Rechargeable lithium-ion batteries (LIBs) have been widely used in consumer electronics, and are expected to be a major propulsion power for all electric vehicles (AEVs) and/or hybrid electric vehicles (HEVs) soon, and for large-scale energy storage devices for the utility grid in the near future.^1,2 However, the unevenly distributed and limited lithium resource on the earth could hinder the full deployment of LIB technology in these areas. Sodium-ion batteries (SIBs) working with the same intercalation chemistry, on the other hand, are considered to be a more sustainable option than LIBs, simply due to the abundance of Na on the earth.^3,4 The technical challenge for intercalation chemistry SIBs is, however, the difficulty of accommodating larger Na⁺ (0.102 nm for Na⁺vs. 0.076 nm for Li⁺) in the appropriate hosting electrode materials, as well as the energetic instability of Na⁺ within the graphene layers of graphite.^5,6 This reality has severely limited the use of graphite, an excellent LIB anode, as a practical anode material for SIBs.

To search for better SIB anode materials, a wide range of compounds including hard carbons,^7–10 metals and alloys (Sb, Sn, Se and SnSb),^11–14 metal oxides (SnO₂ and TiO₂)^15,16 and transition metal dichalcogenides (TMDs: MoS₂, WS₂, NbS₂, and VS₂)^17–20 have been extensively investigated in the past. In particular, TMDs have garnered much attention because their layered structure possesses a suitable interlayer spacing to host Na⁺, thus enabling a better cycle capacity and reversibility.^17,20 VS₂ is one member of the TMDs family, exhibiting a two-dimensional graphene-like layered structure as shown in Fig. 1A and C, with an interlayer spacing of 0.576 nm, much greater than that of graphite.^21–23 The larger interlayer spacing in VS₂ offers not only a broad range of electronic properties, but also sufficient room to host Na⁺. Studies have shown that the VS₂ monolayer in a spin-polarized ground state exhibits a superior metallic property and a lower or similar ionic diffusion energy barrier as compared to graphite,^21–23 making it a very attractive anode material for SIBs.

Fig. 1 Schematic illustration of VS₂ and V₅S₈ crystal structures. (A) The top-view and (C) side-view of the atom arrangements in VS₂; (B) the top-view and (D) side-view of the atom arrangements in V₅S₈. The gray and hollow hatched gray balls represent V atoms, and the yellow balls represent S atoms.

However, one of the drawbacks of using VS₂ as an SIB anode is the anisotropy in diffusion because of its two-dimensional layered structure, requiring significant engineering effort to ensure good alignment of the crystallographic planes with a diffusional direction in order to maximize the intercalation efficiency.^24,25 Here, we report a new three-dimensional compound V₅S₈ with multiple electronic/ionic pathways as a promising SIB anode material. To the best of our knowledge, this is the first report of testing V₅S₈ as an SIB anode. The electrochemical performance of SIB using V₅S₈-based anodes has been extensively characterized, and in situ X-ray diffraction (XRD) along with ex situ high resolution transmission microscopy (HRTEM), X-ray photoemission spectroscopy (XPS) and XRD have also been utilized to assist in the understanding of the underlying enhancement mechanisms.

The crystal structure of V₅S₈ is illustrated in Fig. 1B and D, which can be characterized as a superstructure consisting of ordered V vacancies in a NiAs-type structure according to early superconductors studies.^26–28 Specifically, the structure contains VS₂ monolayer building blocks alternating with extra V-atoms partially occupying one-quarter of the available sites between the monolayers (vacant V and occupied V are denoted as V_I and V_II, respectively). From this structural feature, the formula of V₅S₈ can, therefore, be written as V_0.25VS₂. Furthermore, as illustrated in Fig. 1D, the V_IIS₆ octahedra have the common edges in the V-atoms fully occupied monolayers, while the V_I atoms in the V-atoms partially occupied interlayers have the same local symmetry as the V_II atoms in the V-atoms fully occupied monolayers. The octahedra in the V-atoms fully occupied monolayers are linked to the V_IS₆ octahedra in the V-atoms partially filled interlayers via common faces. Since the V_IIS₆ octahedra linking the two adjacent VS₂ monolayers can provide additional pathways for electronic/ionic charge carriers, an enhanced electronic/ionic conductivity along the c-axis (perpendicular to the VS₂ monolayers) is expected.^29,30

There are three types of V₅S₈ based materials evaluated in this study as SIB anodes in addition to the baseline chemically exfoliated VS₂ (denoted as ce-VS₂): bulk V₅S₈ (denoted as b-V₅S₈), chemically exfoliated V₅S₈ (denoted as ce-V₅S₈) and ce-V₅S₈/graphite hybrid (denoted as ce-V₅S₈–C). The synthesis details for these materials are given in the ESI.† The phase compositions of these materials are shown in Fig. S5 of the ESI.† The morphological features of the ce-V₅S₈ and ce-V₅S₈–C hybrids are shown in Fig. 2, whereas those of the less interesting b-V₅S₈ are given in Fig. S8 (ESI†) for comparison. From Fig. 2A and C, ce-V₅S₈ is clearly seen to consist of flake-like nanosheets. Meanwhile, Fig. 2B and D show that the ce-V₅S₈–C hybrid exhibits a similar morphology consisting of nanoscaled sheets. The HRTEM images in Fig. 2E and F near the edges of the V₅S₈ nanosheets reveal that the number and thickness of nanosheets in ce-V₅S₈ and ce-V₅S₈–C are 12–15 and 6.79–8.49 nm, respectively, as indicated by the alternating dark and bright patterns. Moreover, the interlayer spacing in the edge area is found to be 0.57 nm, corresponding to the (002) plane of V₅S₈ (JCPDS: 81-1596, d = 0.566 nm). The selected area electron diffraction (SAED) patterns of ce-V₅S₈ shown in Fig. 2G and for ce-V₅S₈–C shown in Fig. 2H can be indexed to the monoclinic structure of V₅S₈, where the diffraction rings can be indexed to the (002), (220), (222), (2̄24) and (513) planes of the structure, respectively. For comparison, the microstructure of chemically exfoliated VS₂ (ce-VS₂) has also been shown in Fig. S10 (ESI†), where the ce-VS₂ exhibits a flower-like morphology (Fig. S10A, ESI†), and a high magnification SEM image indicates that the ce-VS₂ microflowers consist of VS₂ nanosheets with a thickness of ∼10–15 nm.

Fig. 2 SEM, TEM, HRTEM images and SAED patterns of ce-V₅S₈ nanosheets (A, C, E and G) and ce-V₅S₈–C hybrids (B, D, F and H), respectively.

It is worth pointing out that the graphite in the ce-V₅S₈–C hybrid overcoats the outer surface of ce-V₅S₈. This is indirectly evidenced by the lower transparency shown in Fig. 2D than in Fig. 2C, and directly evidenced by the XPS analysis shown in Fig. S21 (ESI†), the HRTEM image shown in Fig. S22 (ESI†), and the EDS mapping shown in Fig. S9 (ESI†), where V, S and C are homogeneously distributed in the ce-V₅S₈–C nanosheets. The thin C-layer is deemed multifunctional, which will be further discussed in the following.

The electrochemical performances of ce-VS₂, ce-V₅S₈ and ce-V₅S₈–C as SIB anodes are compared in Fig. 3. It is evident from Fig. 3A that ce-V₅S₈ displays an initial discharge capacity of 701 mA h g⁻¹ at a current density of 0.1 A g⁻¹, compared to 241 mA h g⁻¹ of ce-VS₂ at the same current density, as shown in Fig. 3C. In the subsequent cycles, the charge–discharge profiles of ce-V₅S₈ do not change significantly, still delivering a discharge capacity of 385 mA h g⁻¹ at 1.0 A g⁻¹ after 100 cycles. This is compared with ce-VS₂ that has a discharge capacity of 109 mA h g⁻¹ at 1.0 A g⁻¹ after 100 cycles (Fig. 3F). Ce-V₅S₈ also exhibited a better rate performance. Fig. 3D shows that ce-V₅S₈ can deliver a rate capacity of 591, 548, 508, 464, 407, 304 and 190 mA h g⁻¹ at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g⁻¹, respectively. The performance of ce-V₅S₈–C as the anode in a full battery, with Na₃V₂(PO₄)₃ as the cathode, is provided in Fig. S14C and D of the ESI.†

Fig. 3 Electrochemical performance of ce-V₅S₈ based SIB anodes: the first five-cycle charge–discharge profiles, rate capability and cycling performance at 1.0 A g⁻¹ for ce-V₅S₈ (A, D and G) and ce-V₅S₈–C (B, E and G), respectively. Charge–discharge profiles of ce-VS₂ nanosheets as the anode material for SIBs at 0.1 A g⁻¹ and 1.0 A g⁻¹, respectively (C), and comparison of the cycling performance of ce-VS₂ and ce-V₅S₈ nanosheets as the anode materials for SIBs at 1.0 A g⁻¹ (F). Mass loadings of all the anode materials tested were controlled at 0.77 mg for easy comparison.

After being coated with graphite, the formed ce-V₅S₈–C hybrid exhibits an increase in initial discharge capacity to 902 mA h g⁻¹ at 0.1 A g⁻¹ (Fig. 3B), followed by an appreciable capacity loss in the following cycles. The large irreversible capacity results from the irreversible reaction between sodium and the surface functional group(s) of the graphite thin layer, the solid-electrolyte interphase (SEI) formed on the surface of the ce-V₅S₈–C nanosheets and sulfur dissolution into the electrolyte during the first sodiation process.³¹ The rate capacity of ce-V₅S₈–C shown in Fig. 3E is also enhanced by the presence of graphite, demonstrating a rate capacity of 682, 643, 616, 584, 485, 389 and 344 mA h g⁻¹ at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g⁻¹, respectively. Last, from Fig. 3G, it is clear that ce-V₅S₈–C can maintain a higher discharge capacity of 496 mA h g⁻¹ at 1.0 A g⁻¹ after 500 cycles with a Coulombic efficiency >99.5%. A similar graphite enhancement has also been observed in other studies, where a thin gel like film was observed to form at the electrode/electrolyte interface.^32–33 The thin film formed is believed to play a crucial role in improving the capacity retention. Compared to other TMD based SIB anodes shown in Table S1 (ESI†),^34–49 the ce-V₅S₈–C anode clearly shows a better rate capability, reversibility and stability. Overall, the graphite-enhanced performance arises from the improved electronic conductivity, suppressed volume expansion, stabilization of electrode microstructure, formation of a stable SEI and more importantly, mitigated sulfur dissolution into the electrolyte; the latter has been confirmed using ICP analysis as shown in Table S4 of the ESI.†

To understand the enhancement mechanisms observed above, we also performed cyclic voltammetry (CV) on the SIB with ce-V₅S₈–C as the anode. Fig. 4 shows the results when cycled between 0.01 and 3.00 V at a scanning rate of 0.1 mV s⁻¹. The reduction peaks observed at 1.01, 0.53 and 0.11 V, respectively, are related to the initial sodiation of the V₅S₈ nanosheets. A weak reduction peak appearing at 1.01 V during the first cathodic scan is related to the insertion of Na⁺ into V₅S₈ accompanied by the formation of SEI. The following two broad reduction peaks centred at 0.53 and 0.11 V correspond to the partial/full sodiation by forming Na_xV₅S₈ and the decomposition of V₅S₈ to form amorphous Na₂S and V, respectively. The oxidation peaks centred at 1.35 and 2.1 V, on the other hand, can be assigned to the conversion reaction between Na₂S and V, further desodiation and generation of V₅S₈, respectively. From the 2nd to 5th cycles, the CV curves are almost overlapping, indicating a good reversibility of the Na⁺ storage reactions.

Fig. 4 Cyclic voltammograms (CV) of ce-V₅S₈–C nanosheets as the anode materials for SIBs for the first five cycles at a scan rate of 0.1 mV s⁻¹.

To support the above hypothesized potential-dependent sodiation/desodiation reaction mechanisms, in situ XRD was performed on a live SIB containing a ce-V₅S₈–C anode operated in a voltage window of 0.01–3.00 V; the diffraction patterns are shown in Fig. 5B and D. The corresponding desodiation/sodiation processes to each phase composition are color-marked in Fig. 5A–E. It needs to be pointed out that the peak at 45.2° is derived from the Be window; the peaks at 38.7°, 41.4° and 44.1° are derived from BeO and the peak at 26.5° corresponds to the carbon black paper. The peaks at 15.7°, 17.4°, 35.2°, 44.7° and 45.3° are from the active ce-V₅S₈–C anode, corresponding to the (002), (111), (222), (2̄24) and (224) planes of the ce-V₅S₈ nanosheets, respectively. Based on the reaction sequence revealed in the CV curves in Fig. 4, we propose a 5-stage scheme in Fig. 5E to illustrate the structural evolution and reaction mechanisms undertaken in ce-V₅S₈–C containing SIBs.

Fig. 5 The charge–discharge profiles (A and C), the corresponding contour plots (B) and selected 2θ region plot (D) of the in situ XRD results at different discharge/charge states of the ce-V₅S₈–C hybrid anodes, and a schematic illustration of the energy storage mechanism of the ce-V₅S₈–C hybrid anode at different stages (E).

In stage (I) (1.5–0.4 V), the rapid decrease in potential corresponds well with the peak shifts towards the lower 2θ of the (002) and (222) planes of ce-V₅S₈ in Fig. 5B and D. These shifts can be explained by the lattice expansion of the V₅S₈ nanosheets due to Na⁺ intercalation. In stage (II) (0.4–0.25 V), the peaks located at 15.7° (002), 34.9° (4̄02) and 35.2° (222) continue to shift towards lower 2θ, suggesting further lattice expansion of the ce-V₅S₈ nanosheets caused by more Na⁺ intercalation. Combined with the ex situ TEM results shown in Fig. S15 (ESI†), the observation of Na₂S in the products suggests a partial concurrent conversion reaction taking place. In contrast, in stage (III) (0.25–0.01 V), no noticeable shifts in the diffraction peaks of the ce-V₅S₈ nanosheets can be observed, implying that lattice expansion, thus Na⁺ intercalation, has stopped. When it was fully discharged to 0.01 V, a new diffraction peak centered at 38.8° appeared. This new peak is identified to be associated with Na₂S, a product of the Na⁺ conversion reaction. As the ce-V₅S₈–C anode was recharged to 1.5 V in stage (IV), the peak of Na₂S gradually disappeared, suggesting dissociation of the conversion reaction product. When it was fully recharged to 3.0 V in stage (V), the (002) and (222) characteristic peaks of the V₅S₈ nanosheets reappeared again.

To recap, the proposed five stages are: (I) the V₅S₈ intercalation reaction (1.5–0.4 V), (II) the combination of the intercalation and conversion reaction (0.4–0.25 V), and (III) the conversion reaction, which accounts for the conversion capacity (0.25–0.01 V). Conversely, stage (IV) and (V) correspond to the dissociation reaction of Na₂S to Na⁺, V and Na_xV₅S₈, and the deintercalation of Na_xV₅S₈ to form V₅S₈ nanosheets, respectively. The (de)intercalation and conversion processes appear to be rather reversible, while their irreversible capacity losses mainly arise from the inactive V residue, S dissolution and SEI formation in the initial cycle. Combining electrochemical performance and the in situ XRD results, the Na⁺ storage mechanism in the ce-V₅S₈–C anode can be characterized by intercalation and conversion reactions as follows:

Na⁺ intercalation: V₅S₈ + xNa⁺ + xe⁻ → Na_xV₅S₈ (>0.4 V) (1)

Na⁺ intercalation: V₅S₈ + xNa⁺ + xe⁻ → Na_xV₅S₈ and partial Na⁺ conversion: Na_xV₅S₈ + (16 − x)Na⁺ + (16 − x)e⁻ → 8Na₂S + 5V (0.4–0.25 V) (2)

Full Na⁺ conversion: Na_xV₅S₈ + (16 − x)Na⁺ + (16 − x)e⁻ → 8Na₂S + 5V (<0.25 V) (3)

The reverse reactions (4) and (5) take place at <1.35 V and >1.35 V, respectively, during the charging process.

8Na₂S + 5V → Na_xV₅S₈ + (16 − x)Na⁺ + (16 − x)e⁻ (<1.35 V) (4)

Na_xV₅S₈ → V₅S₈ + xNa⁺ + xe⁻ (>1.35 V) (5)

Moreover, for the first five-cycle charge–discharge profiles of the ce-V₅S₈–C anode, Fig. 3B shows that the first discharge voltage profile differs appreciably from all others. For example, there are three distinct discharge plateaus at 1.0, 0.5 and 0.25 V, respectively, for the first discharge profile. They correspond to the three reduction peaks at 1.01, 0.53 and 0.11 V shown in Fig. 4, respectively, in the initial sodiation process of the V₅S₈ nanosheets. But, in the following cycles, ce-V₅S₈–C undergoes an activation process like in the case of CoS₂ and CoSe₂,^50,51 where the CV peak at 0.53 V becomes broader and moves towards a higher voltage; this observation is consistent with the discharge profiles shown in Fig. 3B. However, the intensity of the CV peak at 0.11 V, which is associated with the conversion reaction, decreases and the voltage plateau at 0.25 V, which is also related to the conversion reaction, becomes shorter. As indicated by the in situ XRD results in Fig. 5A and D, poorly crystallized or partially amorphous V₅S₈ were formed during the desodiation process in the ce-V₅S₈–C electrode, which is responsible for the weakened conversion reaction in the following cycles. They are responsible for the difference observed between the first discharge voltage profile and all others of ce-V₅S₈–C. The phenomenon observed in ce-V₅S₈ is similar to that of ce-V₅S₈–C.

In summary, monoclinic structured ce-V₅S₈ and graphite hybrid nanosheets were fabricated using a chemical exfoliation method, and investigated as anode materials for SIBs. Electrochemical evaluations indicate that ce-V₅S₈–C is a better SIB anode than ce-V₅S₈ and ce-VS₂, demonstrating a high reversible discharge capacity (682 mA h g⁻¹ at 0.1 A g⁻¹) and a reasonable cycle life (496 mA h g⁻¹ at 1.0 A g⁻¹ after 500 cycles). It particularly exhibits high rate capacities of 389 and 344 mA h g⁻¹ at 5.0 and 10.0 A g⁻¹, respectively. This good performance is mainly attributed to the unique three-dimensional monoclinic structure of V₅S₈, which facilitates electronic/ionic diffusion and conversion reactions, as well as graphite. The in situ XRD study revealed that the high capacity of ce-V₅S₈–C originates from a combined Na⁺ intercalation and conversion reaction. Overall, this work demonstrates that the ce-V₅S₈–C hybrid is a promising anode material for SIBs.

Acknowledgements

We gratefully acknowledge the financial support from the Natural Science Foundation of China (51402109), the Project of Public Interest Research and Capacity Building of Guangdong Province (2014A010106007), the Pearl River S&T Nova Program of Guangzhou (201506010030), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200) and the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2016A030306010).

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, additional XRD, SEM, and sodium storage performance of ce-V₅S₈, ce-V₅S₈–C and ce-VS₂ nanosheets. See DOI: 10.1039/c6ee03173k

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