Recently, sodium ion battery (SIB) has attracted considerable attentions and been regarded as the promising alternative to Li ion battery (LIB) for electric energy storage (EES), owing to the nature abundance of sodium sources [1]. Graphite, the typical LIBs anode has been studied as the SIBs anode, but the specific capacity is low as it cannot store sufficient Na⁺ between its layers. Therefore, searching for appropriate anode material is a hot topic and vital for developing high performance SIBs. The 2D layer structured TMDCs (MoS₂, WS₂, and VS₂ et al.), have attracted great interests [2-4], due to their sandwiched framework can facilitate the electrons and Li⁺/Na⁺ ions transportation. However, the (de)sodiation process might induce a phase transition of the TMDs and fast degradation of electrochemical performance over cycling [2].

In this study, NbS₂ nanosheets have been synthesized by the combination of solid state sulfuration and chemical exfoliation method (ce-NbS₂). The ce-NbS₂ flakes seem to be diminished and homogeneous dispersed, they are much thinner and smaller than the bulk NbS₂ (b-NbS₂) flakes (Fig. 1A). HRTEM in the edge areas of ce-NbS₂ is shown in Fig. 1B, it suggests that the NbS₂ flakes after chemical exfoliation were stacked up with only 14 to 15 single layers. An inter-planar spacing of 6.65 nm has been found for the ce-NbS₂ flakes.

In situ XRD of ce-NbS₂ has been performed at selected charged/discharged states in the first two-cycle between 0.01-3.0 V at 0.1 A g^-1 (Fig. 1C and D). On the sodiation process, the main peak of CE-NbS₂ located at 15° is gradually shifted towards left, indicating the crystal lattice is expanded as a consequence of Na⁺ ions insertion. A new remarkable peak has been observed at 13.5°, which is perceived to be the Na_xNbS₂, a solid evidence of Na⁺ ions insertion into the NbS₂ framework. After discharging to 0.01 V, the peak at 15° almost disappears, while the intensity of the peak located at 13.5^o becomes stronger, demonstrating the further insertion of Na⁺ ions. This process is reversed in the followed charging process. After charging to 3.0 V, the peaks of NbS₂ recover. In the second cycle, similar phenomena have been observed, demonstrating a high reversible Na⁺ ions intercalation reaction in NbS₂ nanosheets. Therefore, the possible reaction can be expressed as follows,

NbS₂+ x Na⁺ + xe^- ⇔ Na_xNbS₂ (1)

Therefore, the ce-NbS₂ nanosheets are expected to maintain the same configurational phase upon sodiation.

The initial discharge capacity of ce-NbS₂ nanosheets and b-NbS₂ reach to 377 and 324 mAh g^-1 (Fig. 1E and F), respectively. Followed by the first cycle, ce-NbS₂ delivers a high reversible capacity of 205 mAh g^-1 at 0.1 A g^-1. But that of b-NbS₂ drops rapidly to 181 mAh g^-1 after five cycles at 0.1 A g^-1. After 100 cycles test at 0.5 A g^-1, ce-NbS₂ still can deliver a capacity of 157 mAh g^-1, while the b-NbS₂ exhibits a fast capacity drop from 173.3 to 81.6 mAh g^-1 (Fig. 3G).

Fig. 1. SEM (A) and HRTEM (B) images of ce-NbS₂. First two-cycle charge-discharge curves (C) and the corresponding in situ XRD patterns collected at the selected charged/discharged states between 0.01 and 3.0 V at 0.1 A·g^-1 (D) of ce-Nb₂S nanosheets based anode. Initial charge/discharge profiles from 0.01 to 3.0 V vs. Na⁺/Na at 0.1 A g^-1, long-term cycling performance at a cycling rate of 0.5 A g^-1 of b-NbS₂ (E and G) and ce-NbS₂ nanosheets (F and G), respectively.

In summary, the ce-NbS₂ nanosheets deliver a reversible specific capacity (205 mAh g^-1 at 100 mA g^-1), and excellent cycling stability (a high capacity of 157 mAh g^-1 at 0.5 A g^-1 after 100 cycles). In situ X-ray diffraction test demonstrates that ce-NbS₂ nanosheets can maintain its configuration upon soidation/desodiation, which is a meaningful character of long cycle life anode for SIBs.

References

[1] X. H. Rui, W. P. Sun, C. Wu, Y. Yu, Q. Y. Yan, Adv. Mater., 27 (2015) 6670.

[2] Z. Hu, L. X. Wang, K. Zhang, J. B. Wang, F. Y. Cheng, Z. L. Tao, J. Chen, Angew. Chem. Int. Ed., 126 (2014) 13008.

[3] J. Y. Liao, A. Manthiram, Nano Energy, 18 (2015) 20.

[4] D. W. Su, S. X. Dou, G.X. Wang, Chem. Commun., 50 (2014) 4192.