Cubic Crystal-Structured SnTe for Superior Li- and Na-Ion Battery

May 9, 2017 - Cubic Crystal-Structured SnTe for Superior Li- and Na-Ion Battery Anodes. Ah-Ram Park and Cheol-Min Park. School of Materials Science an...
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Cubic Crystal-Structured SnTe for Superior Liand Na-Ion Battery Anodes Ah-Ram Park and Cheol-Min Park* School of Materials Science and Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea S Supporting Information *

ABSTRACT: A cubic crystal-structured Sn-based compound, SnTe, was easily synthesized using a solid-state synthetic process to produce a better rechargeable battery, and its possible application as a Sn-based high-capacity anode material for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) was investigated. The electrochemically driven phase change mechanisms of the SnTe electrodes during Li and Na insertion/extraction were thoroughly examined utilizing various ex situ analytical techniques. During Li insertion, SnTe was converted to Li4.25Sn and Li2Te; meanwhile, during Na insertion, SnTe experienced a sequential topotactic transition to NaxSnTe (x ≤ 1.5) and conversion to Na3.75Sn and Na2Te, which recombined into the original SnTe phase after full Li and Na extraction. The distinctive phase change mechanisms provided remarkable electrochemical Li- and Na-ion storage performances, such as large reversible capacities with high Coulombic efficiencies and stable cyclabilities with fast C-rate characteristics, by preparing amorphous-C-decorated nanostructured SnTe-based composites. Therefore, SnTe, with its interesting phase change mechanisms, will be a promising alternative for the oncoming generation of anode materials for LIBs and NIBs. KEYWORDS: lithium-ion batteries, sodium-ion batteries, tin telluride, Sn-based compound anodes, Te-based compound anodes

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with appreciable quantities of the Na ions, they also exhibit large capacity fading, similar to that observed for Li-alloy-based materials, because the active materials are pulverized by the large volume variation caused by expansion/contraction during repeated Na insertion/extraction.14−18,20−25 Sn-based materials have been enthusiastically considered as high-capacity anodes in both LIBs and NIBs because Sn has a high theoretical capacity and forms large portions of Li- and Na-rich alloy phases (Li4.25Sn: 959.5 mAh g−1 and Na3.75Sn: 846.6 mAh g−1).26−31 Sn-based materials, such as SnO, SnO2, SnS2, and SnP3, have been extensively investigated to complement the capacity fading that occurs during repeated cycling because their interesting crystal structures produce distinctive electrochemical reactions during Li or Na insertion/ extraction.9,32−36 Te, a chalcogen group element, can also alloy with Li and Na by forming Li2Te and Na2Te (theoretical capacity for LIBs and NIBs: 420 mAh g−1), respectively. Although Te exhibits a smaller gravimetric capacity than other chalcogen group elements (e.g., S and Se), it has a large volumetric capacity of approximately 2621 mAh cm−3 because of its high density (6.24 g cm−3). Its large volumetric capacity

s representative energy storage systems, Li-ion batteries (LIBs) are used in most portable electronics and are promising for the oncoming generation of electric vehicles.1−5 However, the need for high-performance materials and cutting-edge technologies to produce better secondary batteries is continuously increasing to meet consumers’ desires for a more convenient lifestyle. The most attractive materials for better LIBs are Si- and Sn-based anodes because they can form higher Li-containing alloys (e.g., Li3.75Si and Li4.25Sn, at room temperature) than graphite (LiC6, 372 mAh g−1) for commercialized LIBs.1−8 However, Si- and Sn-based materials show undesirable capacity fading during cycling because of their pulverization by the large volume variation caused by expansion/contraction during repeated Li insertion/extraction.6−12 Na-ion batteries (NIBs) constitute an expected secondary battery system because of the abundant reserves and environmental harmlessness of Na metal.13−17 However, the interlayer distance of graphite is too narrow to accept Na ions; thus, hard carbon (or nongraphitizing carbon) is used as an effective NIB anode, which has a relatively small reversible capacity (approximately 250 mAh g−1) with a poor initial Coulombic efficiency (ICE).14−19 Therefore, Na-alloy-based materials, Sn, P, Sb, etc., are suggested as alternative NIB anode materials.11,20−25 Although Na-alloy-based materials can alloy © 2017 American Chemical Society

Received: March 24, 2017 Accepted: May 9, 2017 Published: May 9, 2017 6074

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Figure 1. Synthesis and electrochemical performance evaluation of a cubic crystal-structured SnTe. (a) Synthesis of a cubic crystal-structured SnTe that combines tetragonal Sn with cubic Te. (b) XRD data of synthesized SnTe and its standard (JCPDS # 46-1210). (c) EXAFS spectra of synthesized SnTe and Sn metal. (d) Voltage profile of the SnTe electrode for LIBs at 100 mA g−1. (e) Voltage profile of the SnTe electrode for NIBs at 50 mA g−1.

has led to its use in various Te-based electrodes for possible applications to rechargeable LIBs and NIBs.37−42 SnTe, an intermetallic compound of Sn and Te, is a compound semiconductor with a IV−VI narrow band gap (0.18 eV) and is utilized in diverse electronic devices, such as infrared detectors, and insulators.43−46 The density of SnTe (6.445 g cm−3) is much higher than that of other Sn-based compounds; thus, SnTe shows a large volumetric capacity as electrodes for LIBs and NIBs. As mentioned above, each of Sn and Te can alloy with large amounts of Li and Na. Considering these advantageous features of SnTe, it can be utilized as a highcapacity electrode for LIBs and NIBs. Nevertheless, SnTe has not been reported as an electrode material for rechargeable LIBs or NIBs.

In this study, we successfully synthesized SnTe with a cubic crystal structure using a solid-state ball milling (BM) process and tested it as an electrode material in LIBs and NIBs to circumvent the problems associated with Sn and Te when they are used solely as electrodes. Additionally, the distinctive phase change mechanisms of SnTe during Li and Na insertion/ extraction were evaluated based on ex situ X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) analyses, and electrochemical performance data, including cyclic voltammetry (CV) and differential capacity (dQ/dV) plots. Furthermore, we also examined the use of a SnTe-based nanocomposite modified with amorphous-C and evaluated its suitability in the oncoming generation of high-capacity and high-performance LIBs and NIBs. 6075

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Figure 2. Electrochemical characterization and phase change mechanism of the SnTe electrode for LIBs. (a) dQ/dV plots for the first and second cycles of the SnTe electrode. (b) CV plots for the first and second cycles of the SnTe electrode. (c) Ex situ XRD data of the SnTe electrode during the first lithiation/delithiation. (d) EXAFS spectra of the SnTe electrode during the first lithiation/delithiation. (e) Schematic representation of a two-step crystallographic phase change mechanism of SnTe during lithiation/delithiation.

RESULTS AND DISCUSSION As shown in Figure 1a, a cubic crystal-structured SnTe (space group: Fm3m, a = 6.327 Å) was synthesized by combining tetragonal-structured Sn and puckered layer cubic-structured Te using a facile solid-state BM technique. All XRD peaks of the synthesized SnTe are well matched to the standard SnTe phase (JCPDS # 46-1210), and no other phases or impurities are observed (Figure 1b). The average crystallite size of SnTe was approximately 29.5 nm (calculated from the full width at half-maximum of the main plane (111) using the Scherrer formula). Additionally, particle size analysis (PSA) and scanning electron microscopy (SEM) results for the synthesized SnTe are shown in Figure S1a,b, respectively, which confirms that the average particle size of SnTe was approximately 10.08 μm. Bright-field transmission electron microscopy (TEM, Figure S1c) and high-resolution TEM (Figure S1d) images containing diffraction patterns (DPs) demonstrate that the SnTe particles were composed of small and well developed nanocrystallites. The Sn K-edge EXAFS

spectrum of SnTe is shown in Figure 1c, and the main Sn−Te bond (2.86 Å) of SnTe agreed well with that previously reported for SnTe.29 The XRD and EXAFS data demonstrate that the cubic crystal-structured SnTe can be synthesized easily using a facile solid-state BM technique. To confirm its electrochemical properties, galvanostatically driven charge− discharge tests of the SnTe electrodes for LIB and NIB were performed, and their voltage profiles are displayed in Figure 1d,e, respectively. When the SnTe electrode was electrochemically tested for LIB applications (current density: 100 mA g−1, voltage range: 0−2.5 V vs Li+/Li, Figure 1d), its first discharge/ charge capacity was 666.9/591.6 mAh g−1, showing a very high ICE of 88.7%. Additionally, when the SnTe electrode was electrochemically tested for NIB applications (current density: 50 mA g−1, voltage range: 0−2.5 V vs Na+/Na, Figure 1e), the first discharge/charge capacity of SnTe was 632.2/542.5 mAh g−1, showing a very high ICE of 85.8%. Considering the theoretical capacities for LIBs (680 mAh g−1; calculated based on the stabilized final phases, Li4.25Sn and Li2Te, at room 6076

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Figure 3. Electrochemical characterization and phase change mechanism of the SnTe electrode for NIBs. (a) dQ/dV plots for the first and second cycles of the SnTe electrode. (b) CV plots for the first and second cycles of the SnTe electrode. (c) Ex situ XRD data of the SnTe electrode during the first sodiation/desodiation. (d) EXAFS spectra of the SnTe electrode during the first sodiation/desodiation. (e) Schematic representation of a three-step crystallographic phase change mechanism of SnTe during sodiation/desodiation.

temperature) and NIBs (626 mAh g−1; calculated based on the stabilized final phases, Na3.75Sn and Na2Te, at room temperature),26−31 the electrodes completely reacted and displayed highly reversible reactions. This suggests that the open cubic crystal-structured SnTe enables easy diffusion and accommodation of Li and Na ions. Although the capacity retentions of the SnTe electrode were greatly enhanced compared to that of the Sn and Te electrodes (Figure S2 for LIBs and Figure S3 for NIBs), the capacity after 30 cycles drastically decreased to 318 mAh g−1 (for LIBs) and 119 mAh g−1 (for NIBs), owing to pulverized active materials and their isolated and broken electrical connections from the current collector, which was caused by repeated volume expansion/contraction during the formation/release of Li-rich (Li4.25Sn and Li2Te) or Na-rich (Na3.75Sn and Na2Te) phases. The pulverized and agglomerated active materials and their isolated and broken electrical connections were confirmed using SEM results after 30 cycling of the SnTe electrodes for LIBs and NIBs (Figure S4). The dQ/dV plots show the results for the first and second cycles using the SnTe electrode in LIBs. Figure 2a displays two

distinctive voltage ranges during lithiation and delithiation, comprising a sharp peak around 1.2 V and several tiny peaks around 0.3−0.7 V during the first lithiation, which correspond to lithiation peaks for the Li−Te and Li−Sn alloys, respectively. However, several tiny peaks near 0.3−0.8 V and 1.5−2.0 V were also observed during the first delithiation, which correspond to the delithiation peaks for the Li−Sn and Li−Te alloys, respectively.1,11,12,28,37−40 Figure 2b shows a CV with peaks located at a similar potential as that of the dQ/dV plots, although they are slightly shifted to the left by the overpotential. The dQ/dV and CV plots indicate that the SnTe electrode underwent a two-step electrochemical reaction during lithiation/delithiation. To verify this reaction, ex situ XRD analyses (Figure 2c) and Sn K-edge EXAFS (Figure 2d) were performed in reference to the dQ/dV plots in Figure 2a. When the SnTe electrode was lithiated from an open-circuit voltage (OCV, t0 in Figure 2c,d) to 0.9 V (t1 in Figure 2c,d), SnTe converted to Li2Te and Sn (t1 in Figure 2c). Sn was also demonstrated in the EXAFS result (t1 in Figure 2d) because the main Sn−Te bond (2.86 Å) of SnTe was transformed into the 6077

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ACS Nano main Sn−Sn bond (2.80 Å) of the Sn metal.28,29,46 At the completely lithiated state of 0 V, the ex situ XRD results show the formation of Li4.25Sn and Li2Te phases (t2 in Figure 2c). Additionally, the EXAFS result (t2 in Figure 2d) also supports Li4.25Sn (∼2.6 Å, main Sn−Li bond of Li4.25Sn) as the final lithiated phase at room temperature, which was demonstrated by the results of electrochemical phase change mechanism of Sn electrode for LIBs in Figure S5. Conversely, when the voltage was increased to 1.0 V during the delithiation reaction (t3 in Figure 2c,d), Li4.25Sn dealloyed into the Sn phase (t3 in Figure 2c), and the main EXAFS peak of the Sn metal reappeared at this potential (t3 in Figure 2d). At the completely delithiated state of 2.5 V (t4 in Figure 2c,d), the ex situ XRD and EXAFS results show that the SnTe phase reemerged because of the recombination of the Sn and Te phases. Based on the ex situ analyses, the SnTe electrode experienced conversion and alloying reactions to the Li4.25Sn and Li2Te phases and then recombined into the original SnTe phase. This interesting phase change mechanism for the SnTe electrode during lithiation/delithiation is summarized briefly by the following chemical formulas:

of Na in SnTe. The corresponding sodiated capacity was approximately 160 mAh g−1 (except for the capacity related to solid-electrolyte interface formation), which means that approximately 1.5 mol of Na was inserted into the cubic crystal-structured SnTe. When the voltage was lowered to 0.2 V, NaxSnTe (x ≤ 1.5) converted to Na2Te and amorphous Sn (a-Sn, t2 in Figure 3c). The presence of a-Sn was demonstrated by the EXAFS spectrum, corresponding to the main Sn−Sn bond of Sn metal (2.80 Å, t2 in Figure 3d).28 At the fully sodiated-state of 0.0 V, the ex situ XRD spectrum showed crystalline Na3.75Sn and Na2Te peaks and the ex situ EXAFS spectra also showed the main Sn−Na bond (∼2.7 Å) corresponding to the Na3.75Sn phase (t3 in Figure 3d), which was demonstrated by the results of electrochemical phase change mechanism of Sn electrode for NIBs in Figure S6. These results clearly demonstrate that the converted a-Sn alloyed with Na, forming the Na3.75Sn phase during sodiation. However, during desodiation, the crystalline Na3.75Sn phase transformed to a-Sn, which was confirmed by the ex situ XRD (t4 in Figure 3c) and EXAFS spectra (t4 in Figure 3d). At the further sodiated state of 1.0 V, NaxSnTe (x ≤ 1.5) reappeared, which was confirmed by ex situ XRD and EXAFS measurements (t5 in Figure 3c,d). Finally, at the fully desodiated state of 2.5 V, the main Sn−Te EXAFS bond (2.86 Å) of the SnTe phase reappeared (t 6 in Figure 3d), which definitely demonstrates a recombination of the SnTe phase. According to the ex situ analyses, the following interesting phase change mechanism of the SnTe electrode during sodiation/desodiation is proposed and briefly summarized.

During lithiation: SnTe → Sn + Li 2Te → Li4.25Sn + Li 2Te During delithiation: Li4.25Sn + Li 2Te → Sn + Li 2Te → SnTe

The two-step crystallographic phase change mechanism for SnTe during lithiation/delithiation is represented in detail in Figure 2e. First, the cubic crystal-structured SnTe transformed into Sn and cubic Li2Te by conversion; then, Sn additionally alloyed with Li to form a cubic Li4.25Sn phase. Conversely, during delithiation, cubic Li4.25Sn dealloyed to Sn and then recombined to the original phase, cubic SnTe, after full delithiation. This two-step reaction mechanism of alloying/ conversion during lithiation and dealloying/recombination during delithiation is a quite distinctive and advantageous feature because the recombined compounds generally show better electrochemical performances.47−50 The dQ/dV and CV plots are displayed in Figure 3a,b for electrochemical analyses of the sodiation/desodiation mechanism of the SnTe electrode for NIBs. In Figure 3a, the dQ/dV plot displays three distinctive voltage ranges during sodiation and desodiation, respectively, comprising a large peak at ∼1.0 V, some small peaks around 0.2−0.4 V, and some large peaks around 0−0.2 V during the first sodiation. However, during the first desodiation, some larger peaks around 0−0.3 V, some small peaks around 0.5−0.8 V, and a large peak at ∼1.5 V were observed. Additionally, the CV plot in Figure 3b also shows similar peak positions as that in the dQ/dV plot, but the peaks are slightly shifted to the left by the overpotential, as seen for the CV results for LIBs. The dQ/dV and CV plots imply that the SnTe electrode undergoes a three-step electrochemical reaction during sodiation/desodiation. Ex situ analyses using XRD (Figure 3c) and Sn K-edge EXAFS (Figure 3d) were performed (referenced on the dQ/dV plot in Figure 3a) to reveal this three-step reaction. The ex situ XRD peaks (t1 in Figure 3c) of the SnTe phase did not change when the sodiation proceeded in SnTe from OCV (t0 in Figure 3c,d) to 0.8 V (t1 in Figure 3c,d). However, the main bond (2.86 Å) in the EXAFS spectrum for SnTe slightly shifted to 2.84 Å (t1 in Figure 3d), which demonstrates that a topotactic transition reaction occurred between SnTe and NaxSnTe. Considering the relatively large Na-ion, the topotactic transition reaction may be associated with the substitutional solid-solution reaction

During sodiation: SnTe → NaxSnTe(x ≤ 1.5) → a − Sn + Na 2Te → Na3.75Sn + Na 2Te During desodiation: Na3.75Sn + Na 2Te → a − Sn + Na 2Te → Na xSnTe(x ≤ 1.5) → SnTe

The interesting three-step crystallographic phase change mechanism of SnTe during sodiation/desodiation is represented in detail in Figure 3e. First, the cubic crystal-structured SnTe transformed topotactically into NaxSnTe (x ≤ 1.5), and converted to a-Sn and cubic Na2Te. Then, a-Sn alloyed with Na, forming a cubic Na3.75Sn phase. In contrast, during desodiation, cubic Na3.75Sn dealloyed into a-Sn, and then recombined into NaxSnTe (x ≤ 1.5). After full desodiation, the cubic SnTe phase was fully recovered. The three-step topotactic transition/conversion/alloying during sodiation and the dealloying/recombination/topotactic transition during desodiation of SnTe, respectively, are fully addressed. Recently, carbon-decorated nanostructured composites have been considered as promising alternative solutions to realize high-capacity Li- and Na-alloy-based anodes.1,6−8,10−12,32,36−42,47−54 The carbon-decorated nanocomposites afford enhanced Li- and Na-ion storage characteristics and enhanced diffusion kinetics because of their large surface areas and short diffusion paths; the stable cyclability enhanced the accommodation ability of the strain generated during cycling.1,49−54 Therefore, the SnTe-based nanostructured composite, SnTe/C, decorated with amorphous-C (Super P) was produced by an additional solid-state BM process for 6 h to enhance the electrochemical performance of SnTe. The solidstate BM process involves repeated welding, fracturing, and rewelding of particles by the repeated ball-powder-ball collisions,55 therefore, the SnTe crystallites were fragmented 6078

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Figure 4. Morphological characteristics of the amorphous-C-decorated SnTe-based nanocomposite. (a) Bright-field TEM image. (b) Highresolution TEM image with its corresponding DP and FT patterns. (c) Scanning TEM image with its corresponding EDS elemental (Sn, Te, and C) mappings. (d) Voltage profile (current density: 100 mA g−1) of the SnTe/C nanocomposite electrode for LIBs. (e) Voltage profile (current density: 50 mA g−1) of the SnTe/C nanocomposite electrode for NIBs.

impurities were produced and the average crystallite size of SnTe was approximately 9.5 nm (calculated from the full width at half-maximum of the main plane (111) using the Scherrer formula). The calculated average crystallite size matched well with the high-resolution TEM result. The prepared SnTe/C nanocomposite was tested as LIB and NIB electrodes; their galvanostatically driven charge−discharge profiles are displayed in Figure 4d,e, respectively. The dQ/dV plots of the SnTe/C nanocomposite electrodes for LIBs and NIBs are shown in Figure S9a,b, respectively. The dQ/dV plots of the SnTe/C nanocomposite electrodes for LIBs and NIBs are almost the same as those of the SnTe electrode, although the peaks were slightly smoothed by the incorporation of amorphous carbon in the SnTe/C nanocomposites, which demonstrates that SnTe/C nanocomposite electrodes for LIBs and NIBs have same phase change mechanism with those of SnTe electrodes for LIBs and NIBs. Interestingly, the SnTe/C nanocomposite electrodes showed superior electrochemical performances, such as high lithiation/

and refined uniformly within the amorphous-C matrix. To identify the nanostructured morphology of the SnTe/C nanocomposite, SEM, PSA, and TEM analyses were performed, and these results are shown in Figures S7a,b and 4, respectively. The SEM and PSA results confirm that the average particle size of SnTe/C nanocomposite was approximately 6.67 μm. Brightfield TEM (Figure 4a) and high-resolution TEM (Figure 4b) images containing DPs and Fourier-transformed (FT) patterns corresponding to selected SnTe crystallites in the SnTe/C nanocomposite show that the SnTe nanocrystallites were approximately 4−10 nm and well dispersed within the amorphous-C matrix. The d-spacings of the FT patterns for each nanocrystallite corresponded to the respective cubic SnTe phases. Additionally, dark-field TEM images arranged with the energy-dispersive spectroscopy (EDS) elemental (Sn, Te, and C) mappings of the SnTe/C nanocomposite (Figure 4c) demonstrate the even dispersion of SnTe nanocrystallites within the amorphous-C matrix. The powder XRD results for the SnTe/C nanocomposite (Figure S8) indicate that no 6079

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Figure 5. Investigations of enhanced electrochemical performances for the amorphous-C-decorated SnTe-based nanocomposite electrodes. (a) Electrochemical impedance results of the SnTe and SnTe/C nanocomposite electrodes for LIBs before cycling and after the 10th cycle. (b) Electrochemical impedance results of the SnTe and SnTe/C nanocomposite electrodes for NIBs before cycling and after the 10th cycle. (c,d) Ex situ high-resolution TEM images with diffraction patterns of the SnTe/C nanocomposite electrode for LIBs after the 10th and 50th cycles. (e,f) Ex situ high-resolution TEM images with diffraction patterns of the SnTe/C nanocomposite electrode for NIBs after the 10th and 50th cycles.

delithiation capacities of 800/654 mAh g−1 (or 1616/1321 mAh cm−3) with an ICE of 81.8% for LIBs and sodiation/ desodiation capacities of 541/339 mAh g−1 (or 1093/685 mAh cm−3) with an ICE of 62.7% for NIBs. To compare the contributed capacity of C within the SnTe/C nanocomposite, galvanostatically driven charge−discharge profiles of the BMtreated C vs Li+ and Na+ are displayed in Figure S10a,b, respectively. Although the BM-treated C had stable capacity retentions, they showed poor ICEs (Figure S10c,d). The contributed irreversible capacities of the BM-treated C content (40 wt%) were 145 mAh g−1 for LIBs and 116 mAh g−1 for NIBs, respectively. Considering the irreversible capacities of BM-treated C, SnTe in the SnTe/C nanocomposite almost fully reacted with Li. Although SnTe in the SnTe/C nanocomposite did not fully react with Na, its reversibility was enhanced compared to that of the SnTe electrode. Remarkably, the large reversible capacities were maintained with essentially no deterioration, even after 50 cycles. The superior Li and Na storage characteristics may be attributed to the enhanced electrical conductivity by provision of the conducting amorphous-C-embedded SnTe-based nanocomposites. Therefore, the electrochemical impedance results for the SnTe and SnTe/C nanocomposite electrodes for LIBs and NIBs are compared in Figure 5a,b, respectively, to confirm the enhanced electrochemical conductivity of the SnTe/C nanocomposite; the SnTe/C nanocomposite electrodes for the LIBs

and NIBs have even smaller semicircles, indicating a higher electrical conductivity than that for the pristine SnTe electrode, which retained even after 10 cycle. Additionally, ex situ highresolution TEM analyses were performed to compare the variation of the SnTe nanocrystallites before and after cycling (Figure 5c−f). Notably, the 4−10 nm SnTe crystallites before cycling (Figure 4b) were reduced to 2−3 nm after 10 cycles (Figure 5c for LIBs and Figure 5e for NIBs) and maintained their size, even after 50 cycles (Figure 5d for LIBs and Figure 5f for NIBs). These results were achieved by repeated conversion/ recombination reactions through ongoing cycling and are attributed to the excellent reversibility of the stabilized and small (approximately 2−3 nm) nanocrystallites. The gravimetric and volumetric capacity vs cycling number (current density: 100 mA g−1 for LIBs and 50 mA g−1 for NIBs) of the SnTe, SnTe/C nanocomposite, and commercial carbon-based (graphite for LIBs and hard carbon for NIBs) electrodes for LIBs and NIBs and current-rate (C-rate) characteristics (0.1C, 0.2C, 0.5C, 1C, 2C, and 3C rates) of the SnTe/C nanocomposite electrode for LIBs and NIBs were examined, as shown in Figure 6. Although the SnTe electrode exhibited poor cycling behavior for LIBs because of the large volume expansion/contraction by the formation/release of the Li4.25Sn and Li2Te phases, the SnTe/C nanocomposite electrode for the LIBs indicated large gravimetric and volumetric capacities of 647 mAh g−1 and 1308 mAh cm−3, 6080

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Figure 6. Electrochemical performances of the amorphous-C-decorated SnTe-based nanocomposite electrodes for LIBs and NIBs. (a) Gravimetric capacity vs cycling number (cycling rate: 100 mA g−1) of the SnTe, SnTe/C nanocomposite, and commercial carbon-based (MCMB graphite) electrodes for LIBs. (b) Volumetric capacity vs cycling number (cycling rate: 100 mA g−1) of the SnTe/C nanocomposite and graphite electrodes for LIBs. (c) C-rate characteristics of the SnTe/C nanocomposite (1C-rate: 650 mA g−1) and graphite (1C-rate: 320 mA g−1) electrodes for LIBs. (d) Gravimetric capacity vs cycling number (cycling rate: 50 mA g−1) of the SnTe, SnTe/C nanocomposite, and commercial carbon-based (hard carbon) electrodes for NIBs. (e) Volumetric capacity vs cycling number (cycling rate: 50 mA g−1) of the SnTe/C nanocomposite for NIBs and hard-carbon electrodes for NIBs. (f) C-rate characteristics of the SnTe/C nanocomposite (1C-rate: 320 mA g−1) and hard-carbon (1C-rate: 200 mA g−1) electrodes for NIBs.

conducting amorphous-C matrix, which may alleviate the large volume expansion/contraction of active SnTe during the Li- and Na-insertion/extraction and may also contribute to shorter Li-ion diffusion paths for small nanocrystallites and enhanced Li-ion conductivity by employing conductive C. Additionally, repeated conversion/recombination reactions during the ongoing Li- and Na-insertion/extraction reactions created stabilized nanocrystallites and extremely small (2−3 nm) SnTe nanocrystallites.

respectively, after 100 cycles, which correspond to an extremely stable capacity retention of 99.0% (Figure 6a,b). The volumetric capacity of the SnTe/C nanocomposite electrode for LIBs was more than three-times higher than that for the commercial graphite (meso-carbon microbead, MCMB) electrode. Furthermore, the SnTe/C nanocomposite electrode had large volumetric capacities at fast C-rates: 1150 mAh cm−3 at 1C-rate, 1100 mAh cm−3 at 2C-rate, and 1048 mAh cm−3 at 3C-rate (Figures 6c and S11a). As for the electrochemical performance results for the LIB tests, the SnTe/C nanocomposite electrode for the NIBs also showed superior cycling behaviors and fast C-rate characteristics (Figures 6d,e,f). This provides stable gravimetric and volumetric capacities of 316 mAh g−1 and 639 mAh cm−3, respectively, after 100 cycles (capacity retention of 99.1%). Notably, the volumetric capacity of the SnTe/C nanocomposite electrode for NIBs was more than three-times higher than that for the commercial hardcarbon electrode (Figure 6e). Furthermore, the SnTe/C nanocomposite electrode showed fast C-rate characteristics with large volumetric capacities at various C-rates: 490 mAh cm−3 at 1C-rate, 455 mAh cm−3 at 2C-rate, and 430 mAh cm−3 at 3C-rate (Figures 6f and S11b). To use the topotactic transition, the SnTe/C nanocomposite electrode was electrochemically cycled between the SnTe and NaxSnTe (x ≤ 1.5) phases (potential window: 0.75−2.5 V, cycling rate: 50 mA g−1), which is shown in Figure S12a,b. Although the reversible capacity was relatively small (approximately 253 mAh cm−3 and 125 mAh g−1), it was retained after 300 cycles without any capacity fading during the topotactic transition reactions, which was better than those of hard carbon electrode. The superior cycling performance with highly reversible capacities and extremely fast C-rate characteristics for the SnTe/C nanocomposite electrode for LIBs and NIBs could be attributed to the nanosized (4−10 nm) SnTe crystallites within the

CONCLUSIONS We proposed an easily synthesized cubic crystal-structured Snbased compound, SnTe, and we tested its possible applications as a Sn-based high-capacity anode material for LIBs and NIBs. The electrochemically driven two-step conversion/alloying during lithiation and dealloying/recombination during delithiation and the three-step topotactic transition/conversion/ alloying during sodiation and dealloying/recombination/topotactic transition during desodiation phase change mechanisms for the SnTe electrodes, respectively, were revealed utilizing various ex situ analytical techniques. Based on its distinctive phase-change mechanism, the remarkable electrochemical Liand Na-ion storage performances were attained by preparing amorphous-C-decorated nanostructured SnTe-based composites. The SnTe/C nanocomposite showed highly reversible lithiation/delithiation capacities of 800/654 mAh g−1 (or 1616/ 1321 mAh cm−3) with an ICE of 81.8% for LIBs and sodiation/ desodiation capacities of 541/339 mAh g−1 (or 1093/685 mAh cm−3) with an ICE of 62.7% for NIBs. Further, it showed extremely stable capacity retentions of 99.0% for LIBs and 99.1% for NIBs after 100 cycles and fast C-rate characteristics of 1048 mAh cm−3 for LIBs and 430 mAh cm−3 for NIBs at a 3C-rate. Particularly, when the SnTe/C nanocomposite electrode was tested within the topotactic transition range for 6081

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ACS Nano NIBs (voltage window: 0.75−2.5 V vs Na+/Na), it showed greatly enhanced capacity retentions, which were attained by the topotactic transition from SnTe to NaxSnTe (x ≤ 1.5) with almost negligible structural variation. In summary, a cubic crystal-structured SnTe and its amorphous-C-decorated nanocomposite were successfully synthesized and tested as anodes for LIBs and NIBs. The distinctive electrochemical phase change mechanisms, comprised of two-step (for LIBs) and three-step (for NIBs) phase change mechanisms, were fully demonstrated. The amorphousC-decorated nanostructured SnTe-based composite electrodes showed excellent electrochemical performances when tested as LIB and NIB anodes, including large reversible capacities, excellent capacity retentions, and rapid C-rate characteristics, demonstrating that they could be easily used for applications requiring high-capacity and high-performance anodes. We anticipate that the cubic crystal structured SnTe will be a promising alternative for the upcoming generation of anode materials for LIBs and NIBs.

approximately 3.5 mg of active materials on the electrode. CV profiles were obtained by measuring the current−voltage (I−V) response at a scanning rate of 0.10 mV s−1 (potential window: 0.0−2.5 V vs Li+/Li and Na+/Na) using a ZIVE-MP2A electrochemical workstation (ZIVELAB). The Li- and Na-ion electrochemical cells were tested galvanostatically (potential window: 0.0−2.5 V vs Li+/Li and Na+/Na) at current densities of 100 mA g−1 (LIBs) and 50 mA g−1 (NIBs), respectively, using a Maccor-4000 (Maccor) battery cycling tester. The gravimetric capacity was calculated using the weight of the active materials, whereas the volumetric capacity was calculated by multiplying the weight of the active materials by their tap densities (SnTe: 3.86 g cm−3, SnTe/C: 2.02 g cm−3, graphite (MCMB): 1.27 g cm−3, hard carbon (Aekyung Petrochemical): 0.86 g cm−3). Additionally, the C-rate characteristics were measured at various C-rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 3C (1C-rate: 650 mA g−1 for LIBs and 320 mA g−1 for NIBs).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02039. PSA, SEM, and TEM results of synthesized SnTe; additional voltage profiles of Sn and Te electrodes for LIBs and NIBs; SEM results of before and after cycling for SnTe electrodes; voltage profiles; dQ/dV results; ex situ XRD; and Sn K-edge EXAFS results for Sn electrodes for LIBs and NIBs, PSA, SEM, and XRD of the amorphous-C-decorated SnTe-based nanocomposite; electrochemical performances of BM-treated C (Super P); electrochemical performances of the amorphous-Cdecorated SnTe-based nanocomposite in the topotactic transition region; dQ/dV results for the SnTe, ball-milled amorphous C, and SnTe-based nanocomposite; and voltage profiles at various C-rates for amorphous-Cdecorated SnTe-based nanocomposites (PDF)

METHODS Material Synthesis. The cubic crystal-structured SnTe was synthesized by a simple solid-state synthetic route. Commercial Sn (99.9%, DaeJung Chemicals and Metals), Te (99.98%, Sigma-Aldrich) powders, and stainless-steel balls were placed in an 80 mL Ar-filled hardened-steel vial, and the BM process was conducted for 6 h using a SPEX-8000 M Mixer/Mill at room temperature. The powder:ball weight ratio was 1:20. Then, the same solid-state BM process was performed again using the fabricated SnTe and carbon black (Super-P, Timcal) powders for 6 h to obtain an amorphous-C-modified SnTebased nanocomposite. Preliminary electrochemical studies indicated that the SnTe:C weight ratio was 60:40, and it exhibited optimized electrochemical performances based on the ICE, reversible capacity, and cycling stability results. Material Characterization. The crystallinity and morphology of SnTe and its amorphous-C-modified nanocomposites were characterized using XRD (X-Max/2000-PC with a Cu Kα target source), high-resolution TEM (JEM ARM 200F, JEOL, operating at an accelerating voltage of 200 kV), and EDS (attached to the TEM). To observe the phase change mechanism in the electrodes during the Liand Na-ion reactions, ex situ analyses, such as XRD, high-resolution TEM, and EXAFS, were performed. Sn K-edge EXAFS measurements for SnTe and its amorphous-C-modified nanocomposite electrodes were performed on the 7D-XAFS beamline (storage ring: 3.0 GeV) at the Pohang Light Source (PLS, Republic of Korea). Electrochemical Measurements. To evaluate the electrochemical properties of SnTe and its amorphous-C-modified nanocomposite, galvanostatically driven charge−discharge measurements were performed using coin-typed electrochemical cells that were assembled in an Ar-filled glovebox. In each LIB and NIB test, Li and Na foils, respectively, were used as the counter and reference electrodes. The LIB electrolyte was comprised of a 1-M LiPF6 salt in an ethylene carbonate/diethyl carbonate (1:1 by volume) solvent (Panax STARLYTE), and the NIB electrolyte was comprised of a 1 M NaClO4 salt in ethylene carbonate/propylene carbonate (1:1 by volume) solvent with 5% fluoroethylene carbonate additives (Panax STARLYTE). A Celgard 2400 polypropylene membrane (Celgard) and glass microfiber filter (GF/D, Whatman) were used as separators for the LIBs and NIBs, respectively. The electrodes were prepared using SnTe or its amorphous-C-modified nanocomposite powder as the Li- and Na-active materials, carbon black (Denka) as a conducting additive, and a polyvinylidene fluoride binder at a weight ratio of 70:15:15. The mixtures were dissolved in N-methyl-2-pyrrolidone as a slurry. Then, each slurry was coated onto a Cu foil using a doctor blade and dried at 120 °C in a vacuum oven for 3 h. The average electrode loading was 4.43 mg cm−2, comprising an electrode area of 0.79 cm2, thickness of approximately 0.07 mm, and average weight of

AUTHOR INFORMATION Corresponding Author

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

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

C.-M.P. initiated the study and outlined the experiments. A.R.P. synthesized the samples and performed various material and electrochemical analyses. C.-M.P. supervised the research and wrote the manuscript. All authors discussed the results reported in the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2014R1A2A1A11053057). This work was also supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2017-2014-0-00639) supervised by the IITP (Institute for Information & communications Technology Promotion). 6082

DOI: 10.1021/acsnano.7b02039 ACS Nano 2017, 11, 6074−6084

Article

ACS Nano

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