SnSe alloy as a promising anode material for Na-ion batteries

Abstract

SnSe alloy is examined for the first time as an anode for Na-ion batteries, and shows excellent electrochemical performance including a high reversible capacity of 707 mA h g⁻¹ and stable cycle performance over 50 cycles. Upon sodiation, SnSe is changed into amorphous Na_xSn nanodomains dispersed in crystalline Na₂Se, and SnSe is reversibly restored after desodiation.

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In recent years, the demand for large scale energy storage devices has increased with the development of electric vehicles (EVs) and the smart grid, and Li-ion batteries are considered promising candidates because of their high energy density.¹ However, there is growing concern that lithium resources are insufficient to meet the demands of large scale applications. Moreover, the recent price of lithium raw materials has shown sharp increases. Consequently, Na-ion batteries are an alternative to Li-ion batteries because sodium resources are much more abundant and inexpensive than lithium.² However, there is a critical obstacle to their development. The energy density of Na-ion batteries is slightly lower than that of Li-ion batteries because the reversible capacity and operating voltage of currently reported electrode materials in Na-ion batteries are lower in comparison. This implies that it is not easy to replace Li-ion batteries with Na-ion batteries because the cost per energy stored ($/Wh) of Na-ion batteries does not provide much advantage.³ Therefore, new electrode materials having higher reversible capacities are necessary if one is to increase the energy density of Na-ion batteries.

Recently, there have been notable achievements in the development of high-capacity anode materials such as phosphorus,⁴ phosphides,⁵ oxides,⁶ sulfides,⁷ selenides⁸ and alloy-based materials such as Sn⁹ and Sb.¹⁰ In particular, Sn-based materials have been a focus as promising anode materials. Sn easily reacts with group VA elements (pnictogens including N, P, and Sb) and group VIA elements (chalcogens including O, S, and Se) to form binary compounds such as pnictogenides and chalcogenides. Among these binary compounds, SnSb,¹¹ SnO₂,¹² SnS₂,¹³ and Sn₄P₃^5a,b have been examined, and show promising electrochemical performance via conversion reactions forming Na_αSn and Na_βX (X = pnictogens or chalcogens) during sodiation. In the case of tin nitride, it was calculated that the conversion reaction with Na is not thermodynamically allowed.¹⁴ Tin selenide, however, has not been examined as an anode for Na-ion batteries. SnSe is a p-type semiconductor (band gap of 0.9–1.3 eV) with an orthorhombic crystal structure. In SnSe, Se is more electronegative than Sn whose oxidation state is +2.¹⁵

In this study, SnSe was for the first time examined as an anode material for Na-ion batteries, although a few studies have reported the electrochemical reaction mechanism of SnSe for Li-ion batteries.¹⁶ SnSe showed excellent electrochemical performance for Na-ion batteries, including a high reversible capacity of 707 mA h g⁻¹ and very stable cycle performance with negligible capacity fading over 50 cycles.

SnSe/C composite powders were obtained via facile ball milling in an Ar atmosphere using tin selenide and Super P carbon with a weight ratio of 7 : 3. Fig. 1a shows the X-ray diffraction (XRD) pattern of as-prepared SnSe/C powders; no impurities were observed. The space group of the SnSe crystal structure is Pnma, and SnSe crystallizes in the orthorhombic lattice. A Raman spectrum of the SnSe/C composite shows that Super P carbon in the composite is a disordered carbon (Fig. 1b), where the two peaks around 1350 and 1580 cm⁻¹ are designated as D and G bands, respectively. Fig. 1c shows a field emission scanning electron microscopy (FE-SEM) image of the SnSe/C composite powders. Primary particles with sizes tens of nm were aggregated to form secondary particles with sizes hundreds of nm. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) mapping images of the SnSe/C composite indicate that SnSe and carbon are homogeneously mixed at the nanoscale level (Fig. 1d).

Fig. 1 (a) X-ray diffraction pattern of SnSe/C; (b) Raman spectra of SnSe/C (red line) and bare SnSe (black line); (c) FE-SEM image and (d) STEM image and EDS mapping images of carbon, tin, and selenium.

The electrochemical performance of the SnSe/C composite was evaluated using a half cell with a sodium metal electrode at a current density of 143 mA g⁻¹ (∼0.2C). Fig. 2a shows the cycle performance of the SnSe/C composite, exhibiting negligible capacity fading over 50 cycles. Its reversible capacity is 707 mA h g⁻¹, delivering 91% of the theoretical capacity of SnSe (780 mA h g⁻¹). This calculation assumes that SnSe is changed into Na₂Se and Na₁₅Sn₄ during sodiation. The coulombic efficiency in the first cycle was 75.3%. Fig. 2b shows galvanostatic voltage profiles of the SnSe/C electrode. The sodiation profile is characterized by two regions including a plateau located at about 0.9 V and a sloping profile between 0.6 and 0 V, which is clearly shown in the differential capacity (dQ/dV) plots (Fig. 2c). The peak area (corresponding to capacity) around 0.9 V for the first cycle decreased after subsequent cycling. This indicates that the plateau at 0.9 V for the first cycle is attributable to irreversible electrolyte decomposition such as that occurring through the formation of a solid electrolyte interphase (SEI). However, the small hump around 1 V was still observed in the 10th cycle, suggesting that the plateau at 0.9 V for the first cycle includes not only irreversible electrolyte decomposition but also reversible (de)sodiation. Furthermore, the rate performance of the composite was examined, and it delivered about 350 mA h g⁻¹ at 1C rate (Fig. S1, ESI†).

Fig. 2 (a) Cycle performance, (b) voltage profiles, (c) differential capacity (dQ/dV) plots, and (d) GITT profiles of SnSe/C.

In order to elucidate the (de)sodiation mechanism of SnSe/C during charge–discharge, ex situ XRD analysis of the SnSe/C was performed at various degrees of state of charge (SOC) in the first cycle (Fig. 3). It revealed that sodiation of the SnSe/C proceeds in two consecutive reactions. Upon approaching a redox potential of 0.6 V vs. Na/Na⁺, the formation of crystalline Na₂Se and Sn was observed with the disappearance of SnSe, as shown in Fig. 3 (ii). This indicates that the plateau at 0.9 V is attributed to the two-phase reaction for conversion of SnSe into Na₂Se and Sn. After full sodiation (at 0 V), the XRD peaks of Sn disappeared, and only those of Na₂Se were observed (Fig. 3 (iii)). This indicates that the amorphous Na_xSn phase was formed via the sodiation between 0.6 and 0 V. This is the one-phase reaction, because a sloping voltage profile was observed in this region, as shown in the quasi-open-circuit voltage (QOCV) profile obtained via a galvanostatic intermittent titration technique (GITT) (Fig. 2d). After full desodiation (at 2 V), a small broad XRD peak near 31° corresponding to SnSe was observed, indicating that SnSe was reversibly restored from Na_xSn and Na₂Se (Fig. 3 (v)). The (de)sodiation mechanism of SnSe is summarized, as follows:

Fig. 3 Ex situ XRD patterns of the SnSe/C composite electrode collected at various points as indicated in the corresponding voltage profile: (i) pristine, (ii) 0.6 V, (iii) 0 V, (iv) 0.5 V, (v) 1 V, and (vi) 2 V.

Sodiation:

Step 1: SnSe + 2Na⁺ + 2e⁻ → c-Sn + c-Na₂Se (1)

Step 2: c-Sn + xNa⁺ + xe⁻ + c-Na₂Se → a-Na_xSn + c-Na₂Se (2)

Desodiation:

a-Na_xSn + c-Na₂Se → SnSe + (2 + x)Na⁺ + (2 + x)e⁻ (3)

(a: amorphous, c: crystalline)

Further support for the (de)sodiation mechanism of SnSe/C was given by the ex situ TEM analysis combined with EDS mapping images. Fig. 4a shows the bright-field TEM and EDS mapping images of the fully sodiated SnSe/C electrode. After full sodiation (at 0 V), Na_xSn domains (dark region) with sizes between tens of and a few hundred nm dispersed in the Na₂Se matrix (bright region) were observed, which is clearly supported by the EDS mapping images for the elements of Sn, Se, and Na. After full desodiation, the segregated Sn domains disappeared, and homogenously-overlapped EDS maps for Sn and Se were observed (Fig. 4b). This indicates that SnSe was reversibly formed after full desodiation. A relatively weak EDS intensity from Na was also observed even after desodiation; this is attributed to SEI layers containing Na. In addition, it is considered that the morphology change showing the confinement of Na_xSn domains in the Na₂Se matrix was critical in the better cycle performance of SnSe compared to previous reports for Sn.⁹ It is well-known that the pulverization or cracking of Sn is caused by the agglomeration of Sn particles during cycling, resulting in capacity fading of Sn electrodes. However, in the case of SnSe, Sn or Na_xSn domains were embedded in the Na₂Se matrix during cycling, and thus agglomeration of Sn is inhibited, leading to enhanced cycle performance. Moreover, the cycle performance of pure Sn is dependent on its operating voltage range because the electrolyte decomposition is accelerated at >0.8 V vs. Na/Na⁺.^9a Therefore, pure Sn showed poor cycle performance in the voltage range of 0–1.5 V vs. Na/Na⁺. However, pure Se exhibited excellent cycle performance better than pure Sn, indicating that the electrolyte decomposition on the surface of Se is not as severe as that of Sn.¹⁷ In the case of SnSe, the surface of Sn is not exposed to electrolytes because of Sn or Na_xSn domains embedded in the Na₂Se matrix, and this results in the alleviated electrolyte decomposition of SnSe. Therefore, the better cycle performance of SnSe than pure Sn is partially attributed to that the surface of Se is exposed to electrolytes.

Fig. 4 The STEM images and EDS elemental mapping of tin, selenium, carbon, and sodium of fully (a) sodiated and (b) desodiated SnSe/C.

In summary, a SnSe/C composite was obtained through facile ball milling, and we demonstrated, for the first time, its electrochemical performance as an anode material for Na-ion batteries. SnSe/C is remarkable as a promising anode candidate for Na-ion batteries because of its reversible specific capacity. In this study, SnSe/C showed excellent electrochemical performance including a high reversible capacity of 707 mA h g⁻¹ and excellent cycle performance with negligible capacity fading over 50 cycles. The improved cycle performance of SnSe/C was attributed to the confinement effect of Na_xSn domains in the Na₂Se matrix during cycling. In addition, the reversible (de)sodiation mechanism of SnSe was established through ex situ XRD and TEM with EDS analyses. It is believed that understanding this reaction mechanism of SnSe will contribute to developing alloy-based materials for Na-ion batteries.

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP and MOE) (No. 2010-0019408 and NRF-2013R1A1A2013446), and by Korea Electrotechnology Research Institute (KERI) Primary research program through the National Research Council of Science & Technology funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. 14-12-N0101-69).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section. See DOI: 10.1039/c4cc06106c

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