Tin Selenides with Layered Crystal Structures for Li-Ion Batteries

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Tin Selenides with Layered Crystal Structures for Li-Ion Batteries: Interesting Phase Change Mechanisms and Outstanding Electrochemical Behaviors Dong-Hun Lee and Cheol-Min Park* School of Materials Science and Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea

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S Supporting Information *

ABSTRACT: Tin selenides with layered crystal structures, SnSe and SnSe2, were synthesized by a solid-state method and electrochemically tested for use as Li-ion battery anodes. The phase change mechanisms of these compounds were thoroughly evaluated by ex situ X-ray diffraction and Se K-edge extended X-ray absorption fine structure techniques. SnSe showed better electrochemical reversibility of Li insertion/extraction than SnSe2, which was attributed to remarkable conversion/recombination reactions of the former compound during lithiation/delithiation. Additionally, the electrochemical performance of SnSe was further enhanced by preparing carbon-modified nanocomposites using two different methods, that is, heat treatment (HT) for producing a carbon coating using polyvinyl chloride as a precursor and high-energy ball milling (BM) using carbon black powder. The SnSe/C electrode produced by BM showed a highly reversible initial capacity of 726 mA h g−1 with a good initial Coulombic efficiency of ∼82%, excellent cycling behavior (626 mA h g−1 after 200 cycles), and a fast C-rate performance (580 mA h g−1 at 2C rate). KEYWORDS: lithium-ion batteries, anode materials, tin selenides, reaction mechanism, nanocomposite electrodes

1. INTRODUCTION The accelerating development of portable electronic devices and electric vehicles increases the demand for better secondary batteries. The rechargeable Li-ion battery (LIB) is a representative secondary battery system, displaying a large energy density and high power. Commercial LIBs utilize graphite anodes, achieving stable cycling performance due to intercalation of Li into graphene layer gaps in graphite.1−7 However, these anodes exhibit a small theoretical capacity of 372 mA h g−1 due to the formation of LiC6 and poor rate capability. Therefore, materials based on Li alloys (e.g., those containing elements such as Si, Ge, Sn, P, and Sb) are highly promising for increasing the anode capacity, being able to bind/release a large amount of Li by alloying/dealloying.1,8−13 However, despite their high capacities, these alloys suffer from large volume changes accompanying Li insertion/extraction, which results in poor cycling behavior. Among the above materials, Sn−Li alloys hold great promise owing to the high theoretical capacity of Sn (959.5 mA h g−1), which forms Li4.25Sn at room temperature, exhibiting a low redox potential of ∼0.5 V vs Li+/Li.14−16 Although numerous studies have focused on circumventing the detrimental effect of Sn, proposing amorphous Sn-based composites, Sn-based intermetallics, and nanostructured Sn−C composites, the cycling performance of pulverized electrode materials still suffers from extreme volume changes (∼300%) during Li insertion/extraction.17−25 © 2017 American Chemical Society

Recently, Se has been used in electrode materials for rechargeable LIBs due to its ability to form an alloy with a large amount of Li, namely, Li2Se (theoretical capacity: 679 mA h g−1).26,27 However, the Se electrode suffers from low reversibility and poor cycling behavior, owing to its low electrical conductivity and large volume variations upon cycling. To overcome these disadvantages, the use of various carbon-modified Se-based nanocomposites has been suggested, where the poor electrical conductivity and large volume variations are compensated by a conducting carbon matrix.28−32 Sn and Se form two binary compounds, SnSe and SnSe2, which are both narrow band gap IV−VI semiconductors used in applications such as memory-switching devices, thermoelectric materials, and low-cost photovoltaics.33−37 Most interestingly, SnSe and SnSe2 exhibit typical layered crystal structures, containing Se2− anions and Sn2+ and Sn4+ cations, respectively. Recently, interesting 2D-layer- or 3D-frame-structured materials have attracted much attention as alternative LIB anodes, enabling easy Li-ion diffusion and storage within them.38−45 SnSe and SnSe2 exhibit high theoretical capacities of 847 and 800 mA h g−1, respectively, due to the formation of Li4.25Sn and Li2Se at room temperature and can therefore be utilized as Received: February 7, 2017 Accepted: April 12, 2017 Published: April 12, 2017 15439

DOI: 10.1021/acsami.7b01829 ACS Appl. Mater. Interfaces 2017, 9, 15439−15448

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ACS Applied Materials & Interfaces

Figure 1. Synthesis of layered SnSe and SnSe2. (a) XRD spectrum of manufactured SnSe compared to that of standard SnSe (JCPDS No. 48-1224) with a puckered-layer orthorhombic crystal structure. (b) XRD spectrum of manufactured SnSe2 compared to that of standard SnSe2 (JCPDS No. 23-0602) with a three-layer hexagonal crystal structure. (c) Sn K-edge EXAFS spectra of synthesized SnSe referenced to those of Sn metal and synthesized SnSe2. (d) Se K-edge EXAFS spectra of synthesized SnSe2 referenced to those of Se and synthesized SnSe.

Figure 2. Electrochemical performances of Sn, Se, SnSe, and SnSe2 for LIB electrodes. First to third cycle voltage profiles at a constant current density of 100 mA g−1 for a (a) Sn electrode, (b) Se electrode, (c) SnSe electrode, and (d) a SnSe2 electrode.

promising electrodes for rechargeable LIBs. However, only few studies on the utilization of SnSe in LIB electrodes have been

reported, demonstrating the poor reversibility with low initial Coulombic efficiency (CE) of these electrodes caused by the 15440


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Figure 3. Electrochemical characterization and electrochemically driven reaction mechanism of the SnSe electrode during lithiation/delithiation. (a) Plots of dQ/dV vs potential (Li+/Li) for the SnSe electrode during the first and second cycles. (b) Ex situ XRD pattern of the SnSe electrode during the first cycle. (c) Se K-edge EXAFS spectra of the SnSe electrode during the first lithiation. (d) Se K-edge EXAFS spectra of the SnSe electrode during the first delithiation. was prepared by heat treatment (HT) of a mixture of the synthesized SnSe and a polyvinyl chloride (PVC) as a carbon precursor at 700 °C for 3 h (heating rate: 3 °C min−1) in a quartz tube furnace under an atmosphere of high-purity Ar. PVC was added with reference to the result of thermogravimetric (TG) analysis (Figure S1 of the Supporting Information) to adjust the final contents of SnSe and C to 60 and 40 wt %, respectively. In the second route, an additional highenergy BM process was conducted for 6 h using powders of the synthesized SnSe and carbon black (Super P, Timcal). The preliminary content of SnSe and C in the obtained nanocomposites equaled 60 and 40 wt %, respectively, and was determined based on optimal electrochemical performance, for example, first reversible capacity, CE per cycle, and cyclability. 2.2. Material Characterization. The structures of SnSe and SnSe2 were probed by XRD (DMAX2500-PC, Rigaku, The Woodlands, TX) and Sn and Se K-edge EXAFS analyses. EXAFS spectra were recorded at the 7D (XAFS) beamline at the Pohang Light Source (PLS) in the Republic of Korea. The structure and morphology of nanostructured SnSe-based composites were analyzed by XRD, highresolution transmission electron microscopy (HRTEM; FEI F20, operating at 200 kV), and energy-dispersive X-ray spectroscopy (EDX) coupled with HRTEM. The structural changes of SnSe and SnSe2 electrodes during lithiation/delithiation were investigated by ex situ XRD and Se K-edge EXAFS. Additionally, electrochemical impedance (EI) measurements were conducted in a frequency range of 100 kHz to 10 mHz using a ZIVE-MP2A impedance analyzer to compare the electrical conductivities of SnSe and SnSe-based nanocomposites. 2.3. Electrochemical Measurements. The electrochemical evaluation of SnSe, SnSe2, and SnSe-based nanocomposites was performed by fabricating test electrodes consisting of the active powder materials (70 wt %), carbon black Denka (15 wt %) as a conducting agent, and polyvinylidene fluoride (15 wt %) dissolved in N-methyl-2pyrrolidone solution as a binder. Each mixture was coated on a 25-μmthick Cu foil as a current corrector, which was vacuum-dried at 120 °C

irreversible formation of low-conductive Li2Se during Li insertion.46−49 Additionally, the mechanism of the electrochemically driven phase change reaction during Li insertion/extraction has not been fully understood. In this study, we synthesized SnSe and SnSe2 exhibiting layered crystal structures (further referred to as layered SnSe and SnSe2) by a simple solid-state method and tested their electrochemical properties for use as LIB electrodes. Additionally, their electrochemically driven phase change reaction mechanism was thoroughly investigated using various ex situ analytical techniques such as X-ray diffraction (XRD) and Se K-edge extended X-ray absorption fine structure (EXAFS). On the basis of the observed electrochemical performances, the SnSe electrode was selected and further modified by preparing its nanostructured composites via two different synthetic routes. Furthermore, we compared the electrochemical performances of the above composites, suggesting an optimized solution for high-performance LIB electrodes based on nanostructured SnSe-based composites.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. SnSe and SnSe2 were synthesized by the following solid-state method. Stoichiometric amounts of Sn (99.9%, Dae-jung Chemicals & Metals, Shiheung City, Korea; average particle size: 45 μm) and Se (99.5%, Aldrich, St. Louis; average particle size: 100 μm) powders were placed in an 80 mL hardened steel vial together with hardened steel balls (diameter: 3/8 and 3/16 in.) inside a glovebox filled with high-purity Ar, with the ball-to-powder weight ratio equaling 20:1. Subsequently, high-energy ball milling (BM) was carried out for 6 h using a Spex-8000 machine (SPEX SamplePrep). Nanostructured SnSe-based composites were prepared via two different synthetic routes. In the first route, the SnSe/C nanocomposite 15441


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Figure 4. Electrochemical characterization and electrochemically driven reaction mechanism of the SnSe2 electrode during lithiation/delithiation. (a) Plots of dQ/dV vs potential (Li+/Li) for the SnSe2 electrode during the first and second cycles. (b) Ex situ XRD pattern of the SnSe2 electrode during the first cycle. (c) Se K-edge EXAFS spectra of the SnSe2 electrode during the first lithiation. (d) Se K-edge EXAFS spectra of the SnSe2 electrode during the first delithiation. for 12 h and pressed (electrode thickness: ∼10 μm). Electrodes were punched as circles with a diameter of 10 mm and an average material loading of ∼3.1 mg cm−2. Coin-type electrochemical cells were assembled and fabricated in a glovebox filled with high-purity Ar. Electrodes coated with active materials (SnSe, SnSe2, and SnSe-based nanocomposites; counter electrodes) and Li foil (reference electrode) were sandwiched using a Celgard 2400 separator. A 1 M solution of LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v, Panax STARLYTE) was used as an electrolyte. All cells were galvanostatically tested between 0.0 and 2.5 V (vs Li+/Li) at a constant current density of 100 mA g−1 using a battery cycling tester (Maccor 4000), with the exception of rate-capability tests. Differential capacity (dQ/dV) plots were obtained by differentiating the capacity with respect to the potential. Cyclic voltammetry (CV) tests were performed using a potentiostat (SP-240, Bio-Logic) in a voltage range of 0.0−2.5 V (vs Li+/Li) at a scan rate of 0.1 mV s−1.

SnSe2 (space group: P3m ̅ 1) with lattice parameters (a = 3.810 Å, c = 6.140 Å)52,53 are shown in the insets of part (a) and (b), respectively, of Figure 1, which reveal interesting layered crystal structures. Orthorhombic SnSe comprises puckered layers resembling those of black P,39 gray As,41 and GeS,54,55 whereas hexagonal SnSe2 features three-layer (Se−Sn−Se) packaged layers parallel to the (001) plane. These interesting layered structures result from the combination of layered cubic-structured Se and tetragonal-structured Sn (Figure S5). The structures of layered SnSe and SnSe2 were also confirmed by Sn and Se K-edge EXAFS measurements. In Figure 1c, the main Sn K-edge EXAFS peaks of SnSe and SnSe2 correspond to Sn−Se bond lengths of 2.30 and 2.23 Å, respectively. Additionally, the main Se K-edge EXAFS peaks in Figure 1d indicate that SnSe and SnSe2 exhibit bond lengths of 2.17 and 2.57 Å, respectively. The voltage profiles of Sn, Se, SnSe, and SnSe2 electrodes were recorded at a current density of 100 mA g−1 within a voltage range of 0−2.5 V, with results shown in Figure 2a−d, respectively. Despite exhibiting first lithiation/delithiation capacities of 977/770 mA h g−1 (Figure 2a) and showing high Li reversibility, the Sn electrode displayed a dramatic reversible capacity decrease to 21.2% after three cycles, which was caused by the large volume variation accompanying the formation/ release of Li4.25Sn during cycling. Conversely, the Se electrode showed poor Li reversibility, as indicated by its small first lithiation/delithiation capacity of 189/85 mA h g−1 (Figure 2b). Considering the high theoretical capacity (Li2Se: 679 mA h g−1) of the Se electrode, its poor Li reversibility was caused by the poor electrical conductivity of Se. The SnSe electrode exhibited first lithiation/delithiation capacities of 897/803 mA h g−1 and an excellent initial CE of 89.5% (Figure 2c). On the other hand,

3. RESULTS AND DISCUSSION On the basis of the Sn−Se binary phase diagram (Figure S2), SnSe and SnSe2 were synthesized using a simple high-energy BM route, and their identities were confirmed by XRD measurements (Figure 1a,b, respectively). All XRD peaks were assigned to standard crystalline SnSe (JCPDS No. 48-1224) and SnSe2 (JCPDS No. 23-0602) phases without any impurities. Additionally, particle size, SEM, and TEM analyses for the synthesized SnSe and SnSe2 were performed and the results are shown in Figures S3 and S4, respectively. These results confirm that they have well-developed crystallinity with micrometer-sized particles (average particle size of 7.1 μm for SnSe and 4.2 μm for SnSe2). The crystal structures of orthorhombic SnSe (space group: Pnma) with lattice parameters (a = 11.49 Å, b = 4.153 Å, and c = 4.440 Å)50,51 and hexagonal 15442


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Figure 5. Crystallographic phase change reaction mechanism of layered SnSe and SnSe2 during lithiation/delithiation. (a) Crystallographic conversion/recombination reaction mechanism of puckered-layer-structured SnSe during lithiation/delithiation. (b) Crystallographic conversion/ nonrecombination reaction mechanism of three-layer-packaged layer-structured SnSe2 during lithiation/delithiation.

as shown in Figure 2d, the first lithiation/delithiation capacities of SnSe2 equaled 850/557 mA h g−1, and a relatively poor initial CE of 65.5% was observed. The theoretical capacities of SnSe and SnSe2 (847 and 800 mA h g−1, respectively) demonstrated that both electrodes were fully lithitated, considering the capacity corresponding to the electrode−electrolyte interphase layer formation. However, only the SnSe electrode showed highly reversible delithiation behavior. To determine the cause of the enhanced Li reversibility of the SnSe electrode compared to that of the SnSe2 electrode, various ex situ analytical techniques were used, as evidenced by dQ/dV and CV plots. The above plots obtained for the SnSe electrode during the first and second cycles are shown in Figure 3a and Figure S6a. dQ/dV plots reveal a large peak at ∼1.25 V and several small peaks at 0.0−0.7 V during the first lithiation and several peaks at 0.3−0.8 and 1.7−2.0 V during the first delithiation, agreeing with the results of CV plots. The high-potential peaks at ∼1.25 and 1.7−2.0 V observed during the first lithiation and delithiation corresponded to Li−Se alloying and dealloying, respectively,26,27,29−31 whereas the lowpotential peaks at 0.0−0.7 and 0.3−0.8 V were attributed to Li−Sn alloying and dealloying, respectively.16−24 To rigorously confirm the assignments of peaks observed during electrochemical lithiation/delithiation, ex situ XRD and Se K-edge EXAFS analyses (Figure 3b−d) were performed at specific potentials indicated in dQ/dV plots. During lithiation, the XRD peaks of SnSe (Figure 3b-t0) disappeared, being replaced by those of Li2Se and Sn phases at 1.1 V (Figure 3b-t1). Similarly, the main EXAFS peak (∼2.2 Å) of SnSe was transformed by those of Li2Se (∼2.0 and ∼1.3 Å, Figure 3c-t1) at this potential. In the fully lithiated state (0.0 V), the XRD peaks of Sn were replaced by those of Li4.25Sn (Figure 3b-t2), with the main EXAFS peaks of Li2Se still being present.26,27 In contrast, during delithiation, the XRD peaks of Li4.25Sn disappeared, and

those of the Sn phase reappeared (1.2 V, Figure 3b-t3). Finally, in the fully delithiated state (2.5 V), XRD peaks of the SnSe phase (Figure 3b-t4) were observed. Additionally, the results of Se K-edge EXAFS analysis definitely confirm the full recombination of the SnSe phase after full delithiation (Figure 3d-t4). On the basis of these observations, the following reaction mechanism was proposed for the SnSe electrode. lithiation reaction: SnSe → Sn + Li 2Se → Li4.25Sn + Li 2Se delithiation reaction: Li4.25Sn + Li 2Se → Sn + Li 2Se → SnSe

The reaction mechanism of the SnSe2 electrode was also investigated using ex situ analyses based on dQ/dV and CV plots, as indicated in Figure 4 and Figure S6b, with the peaks observed in dQ/dV and CV plots being almost similar to those of the SnSe electrode (Figure 3a and Figure S6a). This observation implies that these peaks are involved in Li−Se alloying and dealloying in high-potential regions and in Li−Sn alloying and dealloying in low-potential regions. Ex situ XRD and EXAFS analyses (Figure 4b−d) were also performed at specific potentials indicated in dQ/dV plots. The results of these analyses obtained during lithiation demonstrated that the SnSe2 phase (Figure 4b-t0,c-t0) disappeared, being replaced by Li2Se and Sn phases at 1.1 V (Figure 4b-t1,c-t1), with Sn being converted into Li4.25Sn (Figure 4b-t2) in the fully lithiated state (0.0 V). In contrast, the XRD peaks of Li4.25Sn disappeared and those of the Sn phase reappeared (1.2 V, Figure 4b-t3) during delithiation, resembling the behavior of the SnSe electrode. However, in the fully delithiated state (2.5 V), XRD peaks of only the Sn phase were observed (Figure 4b-t4). The results of Se K-edge EXAFS confirm that Se and the unreacted Li2Se phase remained together (Figure 4d-t4). Thus, Li2Se was involved in a partially reversible reaction, with no recombination occurring, 15443


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ACS Applied Materials & Interfaces and the following reaction mechanism was suggested for the SnSe2 electrode. lithiation reaction: SnSe2 → Sn + Li 2Se → Li4.25Sn + Li 2Se

delithiation reaction: Li4.25Sn + Li 2Se → Sn + Li 2Se → Sn + Se (with irreversible Li 2Se)

Crystallographic representations of the phase change mechanism observed for layered orthorhombic SnSe and hexagonal SnSe2 during lithiation/delithiation are schematically shown in Figure 5, revealing that orthorhombic SnSe was converted into Li2Se and Li4.25Sn after full lithiation and was recombined after full delithiation (Figure 5a). Conversely, hexagonal SnSe2 underwent the same conversion reaction, forming Li2Se and Li4.25Sn; however, no recombination was observed for this compound during delithiation (Figure 5b). These results demonstrate that the high reversibility of the SnSe electrode is due to interesting conversion/recombination reactions of SnSe during lithiation/delithiation. The similar conversion/recombination reaction was also observed in several compound electrodes, which showed good electrochemical performances.56−60 Despite its high reversibility, the SnSe electrode still exhibited poor capacity retention. Recent research on Li alloys showed that nanostructured composites with embedded amorphous carbon exhibit enhanced electrochemical performance since amorphous carbon acts as a buffering matrix to hinder the aggregation of nanosized materials during cycling, also functioning as a conducting agent to increase the conductivity between the particles and the current collector. Therefore, to further enhance the electrochemical performance of SnSe, SnSe-based nanocomposites (SnSe/C) were prepared by two different synthetic methods. One of these methods utilized HT to produce a carbon coating using PVC as a carbon precursor, while the other one featured high-energy BM with amorphous carbon black powder. The prepared HT- and BMSnSe/C nanocomposites were characterized by XRD, as shown in (a) and (b), respectively, of Figure 6, showing peaks of the standard crystalline SnSe phase (JCPDS No. 48-1224). Interestingly, significant peak broadening was observed for BM-SnSe/C but not for HT-SnSe/C, confirming that nanocomposites with an effectively reduced crystallite size were obtained via BM. SnSe crystallite sizes of 19.9 and 5.5 nm were calculated for HT- and BM-SnSe/C nanocomposites, respectively, using the Scherrer equation. The results of EI measurements for SnSe, HT-, and BM-SnSe/C nanocomposite electrodes are compared in Figure 6c, highlighting their electrical conductivity differences. The semicircular region in this figure is related to the reaction at the electrode−electrolyte interface, reflecting charge-transfer impedance. HT- and BM-SnSe/C nanocomposites showed even smaller semicircles than SnSe, indicating that HT and BM processes can effectively enhance the electronic conductivity of nanocomposites. Additionally, the HT-SnSe/C nanocomposite showed a larger electrochemical resistance (21.7 Ω) than the BM-SnSe/C nanocomposite (15.5 Ω), demonstrating that BM technique is better suited for improving the electrical conductivity between particles than HT technique. The morphologies and microstructures of HT- and BMSnSe/C nanocomposites observed by HRTEM are depicted in Figures 7 and 8, respectively. For the HT-SnSe/C nanocomposite, bright-field (BF) (Figure 7a) and HRTEM images with corresponding Fourier transform (FT) patterns (Figure 7b)

Figure 6. Preparation and characterization of HT- and BM-SnSe/C nanocomposites. (a) XRD pattern of the HT-SnSe/C nanocomposite and that of the SnSe standard (JCPDS No. 48-1224). (b) XRD pattern of the BM-SnSe/C nanocomposite and that of the SnSe standard (JCPDS No. 48-1224). (c) Comparison of EI measurement results for SnSe, HT-SnSe/C nanocomposite, and BM-SnSe/C nanocomposite electrodes.

and selected-area electron diffraction (SAED) patterns (Figure 7c) were recorded. Nanocrystallites (NCs) of SnSe in HT-SnSe/C were uniformly embedded into the amorphous carbon matrix, having sizes of approximately 10−50 nm, as confirmed by EDX elemental (Sn, Se, and C) mappings (Figure 7d). BF and HRTEM images of the BM-SnSe/C nanocomposite with corresponding FT and SAED patterns are shown in Figure 8a−c, respectively, revealing that SnSe NCs in this nanocomposite were approximately 5−10 nm in size, being much smaller and more uniformly embedded in the amorphous carbon matrix than those of the HT-SnSe/C nanocomposite, as confirmed by EDX elemental (Sn, Se, and C) mappings (Figure 8d). The electrochemical performances of HT- and BM-SnSe/C nanocomposite electrodes are compared in Figure 9 and Table 1. Figure 9a shows voltage profiles of the HT-SnSe/C nanocomposite 15444


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Figure 7. Morphological characterization of the HT-SnSe/C nanocomposite. (a) TEM bright-field image. (b) HRTEM image with corresponding FT patterns. (c) SAED patterns with SnSe crystals. (d) Scanning TEM image with corresponding EDX mappings of Sn, Se, and C.

Figure 8. Morphological characterization of the BM-SnSe/C nanocomposite. (a) TEM bright field image. (b) HRTEM image with corresponding FT patterns. (c) SAED patterns with SnSe crystals. (d) Scanning TEM image with corresponding EDX mappings of Sn, Se, and C.

electrode, revealing a first lithiation/delithiation capacity of 722/581 mA h g−1 with an initial CE of 80.5% (Table 1), whereas the BM-SnSe/C nanocomposite electrode shows an even higher first lithiation/delithiation capacity of 885/726 mA h g−1 and an initial CE of 82.0% (Figure 9b and Table 1). The voltage profiles of HT- and BM-SnSe/C nanocomposites are a little smoothened compared with that of SnSe, which was attributed to the amorphous carbon in the nanocomposites. To quantitatively assess the contribution of amorphous carbon to the observed capacity, HT- and BM-treated carbons (obtained using PVC and Super-P carbon black, respectively) were electrochemically tested, with their voltage profiles and cycling behaviors shown in Figure S7a−d. As a result, the contributions of amorphous carbon (40 wt %) to the first irreversible capacities of HT- and BM-SnSe/C nanocomposites were estimated as 100 and 140 mA h g−1, respectively, demonstrating that the reactions of SnSe NCs present in the nanocomposites with Li were fully reversible. However, the reversible capacity of the HT-SnSe/C nanocomposite electrode dramatically decreased to 314 mA h g−1 after 50 cycles, whereas that of the BM-SnSe/C nanocomposite electrode retained a value of 655 mA h g−1 under the same conditions. These results demonstrate that the smaller the crystallite size of SnSe in the amorphous carbon matrix, the better the electrochemical performances. The cycling behaviors of commercial graphite (meso-carbon microbead, MCMB),

SnSe, SnSe2, HT-SnSe/C nanocomposite, and BM-SnSe/C nanocomposite electrodes are compared in Figure 9c. The BMSnSe/C nanocomposite electrode showed an excellent capacity retention of 626 mAh g−1 after 200 cycles, corresponding to capacity retention of 86% of the first reversible delithiation capacity. Figure 9d and Figure S8 show the voltage profiles and cycling behaviors, respectively, of the BM-SnSe/C nanocomposite electrode at various C rates (0.1, 0.2, 0.5, 1, and 2C), revealing very fast rate capability of 680 mA h g−1 at 1C (1C rate: 730 mA g−1) and 580 mA h g−1 at 2C (2C rate: 1460 mA g−1). The cycling behaviors and C-rate performances of the BM-SnSe/C nanocomposite electrode were much better than those of a commercial MCMB graphite electrode, which was attributed to the presence of SnSe NCs (5−10 nm in size) evenly embedded in the amorphous carbon matrix, enhanced electrical conductivity, and the recombination reaction of SnSe during repeated lithiation/delithiation.

4. CONCLUSION Layered tin selenides, SnSe and SnSe2, were synthesized and electrochemically tested for utilization as LIB anodes. Furthermore, their distinctive phase change reaction mechanisms featuring conversion/recombination (for SnSe) and conversion/ nonrecombination (for SnSe2) were carefully elucidated using ex situ analytical techniques. SnSe showed a much higher Li 15445


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Figure 9. Electrochemical performances of HT- and BM-SnSe/C nanocomposite electrodes. (a) Voltage profiles of the HT-SnSe/C nanocomposite electrode (potential window: 0−2.5 V vs Li+/Li, current rate: 100 mA g−1) for cycles 1−50. (b) Voltage profiles of the BM-SnSe/C nanocomposite electrode (potential window: 0−2.5 V vs Li+/Li, current rate: 100 mA g−1) for cycles 1−50. (c) Plot of capacity vs cycle number (cycling rate: 100 mA g−1) for MCMB graphite, SnSe, SnSe2, HT-SnSe/C nanocomposite, and BM-SnSe/C nanocomposite electrodes. (d) C-rate performances of the BM-SnSe/C nanocomposite electrode (1C rate: 730 mA g−1).

Table 1. Electrochemical Behaviors of SnSe, HT-SnSe/C, and BM-SnSe/C Nanocomposite Electrodes electrode (current: 100 mA g−1)

first lithiation capacity [mA h g−1]

first delithiation capacity [mA h g−1]

initial coulombic efficiency [%]

capacity retention (Xth/first delithiation capacity) [%]

SnSe HT-SnSe/C BM-SnSe/C

897 722 885

803 581 726

89.5 80.5 82.0

89.1 (X = 3) 46.8 (X = 100) 86.2 (X = 200)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-54-478-7746. Fax: +82-54-478-7769. E-mail: [email protected]. ORCID

Cheol-Min Park: 0000-0001-8204-5760

reversibility than SnSe2 owing to the occurrence of conversion/ recombination reactions. Additionally, to further enhance the electrochemical performance of SnSe, a SnSe-based nanocomposite decorated with amorphous carbon was prepared by BM and used in high-performance LIB anodes. The thus-obtained BM-SnSe/C nanocomposite electrode showed superior electrochemical performance, exhibiting a highly reversible initial capacity of 726 mA h g−1 with a good initial CE of 82%, excellent capacity of 626 mA h g−1 after 200 cycles, and a fast rate performance of 580 mA h g−1 at 2C.

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of the amorphous-C-decorated BM-SnSe/C nanocomposite at various C-rates (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2014R1A2A1A11053057).

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REFERENCES

(1) Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Li-Alloy Based Anode Materials for Li Secondary Batteries. Chem. Soc. Rev. 2010, 39, 3115−3141. (2) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novák, P. Insertion Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater. 1998, 10, 725−763. (3) Marom, R.; Amalraj, S. F.; Leifer, N.; Jacob, D.; Aurbach, D. A Review of Advanced and Practical Lithium Battery Materials. J. Mater. Chem. 2011, 21, 9938−9954. (4) Goriparti, S.; Miele, E.; De Angelis, F.; Di Fabrizio, E.; Zaccaria, R. P.; Capiglia, C. Review on Recent Progress of Nanostructured Anode Materials for Li-Ion Batteries. J. Power Sources 2014, 257, 421− 443.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01829. Additional TG analysis of PVC; Sn−Se binary phase diagram; particle size analyses for SnSe and SnSe2; SEM and TEM analyses for SnSe and SnSe2; crystal structures of tetragonal Sn, cubic Se, orthorhombic SnSe, and hexagonal SnSe2; CV results for SnSe- and SnSe2-based electrodes; electrochemical performances of HT-treated C (PVC) and BM-treated C (Super P); and cycling behaviors 15446


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Tin Selenides with Layered Crystal Structures for Li-Ion Batteries

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