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Superior lithium-ion storage performances of SnO2 powders consisting of hollow nanoplates
Journal of Alloys and Compounds 797 (2019) 380e389
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Superior lithium-ion storage performances of SnO2 powders consisting of hollow nanoplates Jae Hun Choi a, 1, Seung-Keun Park b, 1, Yun Chan Kang a, * a b
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea Department of Chemical Engineering, Kongju National University, Budae-Dong 275, Chungnam 314-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2019 Received in revised form 27 April 2019 Accepted 11 May 2019 Available online 13 May 2019
Hierarchical structured transition metal oxides have attracted considerable attention as anode materials for lithium-ion batteries because they possess large surface area that can provide large contact area with the electrolyte and short diffusion distance for Li ions. Here, a hierarchical structured assembly of hollow SnO2 nanoplates is synthesized by one-step oxidation of SnS2 powders. The SnS2 powders comprising of dense nanoplates synthesized by the hydrothermal method transform into SnO2 powders comprising of hollow nanoplates by nanoscale Kirkendall diffusion at the oxidation temperature of 500 C. After the transformation of SnS2 into SnO2 powders, the Brunauer-Emmett-Teller surface area of the powders increases from 22.8 to 82.7 m2 g1. The hierarchical structured SnO2 powders show superior lithium-ion storage performances compared to SnS2 powders with the same structure. The discharge capacities of SnS2 and SnO2 powders at a current density of 1 A g1 for the 300th cycle are 273 and 754 mA h g1, respectively. The SnO2 powders show a high reversible capacity of 169 mA h g1 even at an extremely high current density of 30 A g1. The outstanding electrochemical properties of the SnO2 powders can be attributed to their unique morphological structure having hollow nanoplates and optimum crystallite size, which increases the contact area between the active materials and the electrolyte and the buffered stress caused by the volume expansion during cycling. © 2019 Elsevier B.V. All rights reserved.
Keywords: Kierkendall diffusion Nanostructured materials Lithium-ion batteries Hydrothermal process Tin oxide
1. Introduction Owing to their high capacity and reasonable cycling performance, nanostructured materials of transition metal compounds such as oxides [1e6], sulfides [7e9], and selenides [10e12] have been used as anode materials for lithium-ion batteries (LIBs). Especially, hierarchical structured transition metal compounds have attracted considerable interest as anode materials with excellent rate performance owing to their large surface areas, which provide a large contact area with the electrolyte and a short diffusion distance for Li ions [13e20]. Hierarchical structured materials of transition metal chalcogenides have been extensively studied because of their directional crystal growth [21e25]. Metal sulfide and selenide materials grow mainly in the direction that yields plate- and rod-like crystals. SnS2 has been widely studied for LIBs because of its CdI2-type structure with stacked layers enabling
* Corresponding author. E-mail address: [email protected] (Y.C. Kang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.05.120 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the intercalation/deintercalation of alkali metal ions [26e30]. Zhai et al. fabricated ultrathin hexagonal SnS2 nanosheets via hydrothermal reaction as LIB anode materials [31]. These nanosheets exhibited a highly reversible lithium-ion storage and highly stable cyclic performance. Gao et al. synthesized SnS2 nanoplates that grew vertically on reduced graphene oxide nanoribbons for LIB anodes [32]. Graphene oxide nanoribbons were prepared by oxidative opening of multiwalled carbon nanotubes and the layered SnS2 nanoplates were fabricated by chemical vapor deposition. The assembly of these materials showed excellent stability and high capacity in both the half and full cells owing to the synergy between the layered SnS2 nanoplates and the high conductivity of the reduced graphene oxide nanoribbons. Wang et al. fabricated SnS2 nanoplates by a hydrothermal process and controlled their crystallinity and thickness by changing the temperature and pH [33]. The SnS2 nanoplates prepared at 200 C and pH of 10.5 showed excellent performance (high cycling stability and rate capability) as an anode material for LIBs. This is because temperature affected the thickness and crystallinity of these nanoplates, while pH adjusted the nucleation rate and the speed of transformation of SnS2 to SnO2,
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i.e., the pH value affected the internal crystal structure. However, metal chalcogenide materials show lower Li ion capacities than their corresponding oxides [34]. Recently, hollow structured metal oxide materials have been synthesized from metal chalcogenide materials using the Kirkendall effect for LIBs anode materials. Park et al. synthesized hollow SnO2 nanoplates via the oxidation of SnSe nanoplates [35]. The SnSe nanoplates synthesized via spray pyrolysis transformed into hollow structured SnO2 nanoplates through the Kirkendall effect. Park et al. prepared hierarchical structured Fe2O3 rod clusters using a hydrothermal process and subsequent oxidation [36]. During the oxidation process, FeSe2 rod clusters transformed into Fe2O3 rod clusters with numerous empty nanovoids through the Kirkendall effect. The Fe2O3 rod clusters showed excellent Li-ion storage performance. Despite the availability of a few reports, the transformation of metal chalcogenides into their corresponding hollow metal oxides via the Kirkendall effect is still in an early stage and is limited to a few materials. Furthermore, synthesizing diverse and unique structured Sn-based composite through spray pyrolysis has limits compared to fabricating through hydrothermal process in respect of controlling the shape and crystal growth. In this study, we synthesized a hierarchical structured assembly of hollow SnO2 nanoplates obtained from SnS2 by a hydrothermal method. During the one-step oxidation process, the SnS2 nanopowders converted into hollow structured SnO2 via the Kirkendall effect while maintaining their morphology. The prepared SnO2 powders were compared with SnS2 powders. The SnO2 powders showed better Li ion storage performance, better cycling stability, and higher capacity than SnS2.
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2.2. Characterization techniques To examine the morphology and structure of the hierarchical structured SnS2 and SnO2 powders, scanning electron microscopy (SEM, Tescan, VEGA3) and field-emission transmission electron microscopy (FE-TEM, JEOL, JEM-2100F) were used. The crystal phases of the samples were verified by X-ray diffraction (XRD, X'Pert PRO) using Cu Ka radiation at the Korea Basic Science Institute (Daegu). The chemical composition of the SnS2 and SnO2 nanoplates was examined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The pore size and surface areas of the nanoplatelets were measured by using the Brunauer-EmmettTeller (BET) method, using pure N2 as the adsorbate gas. The thermal behavior of the nanoparticles was investigated using a Pyris 1 thermogravimetric (TG) analyzer (Perkin Elmer) at a heating rate of 10 C min1 in air. 2.3. Electrochemical measurements The electrochemical properties of the hierarchical structured SnS2 and SnO2 powders were evaluated using a standard 2032-type coin cell. The LIB anodes were prepared by casting a mixture of active materials, Super P, and sodium carboxymethyl cellulose (mass ratio of 7:2:1) in DI water onto a Cu foil. The coin cell was composed of a Li metal foil as the counter electrode, a porous polymer membrane as the separator, and 1 M LiPF6 dissolved in fluoroethylene carbonate/dimethyl carbonate at a volumetric ratio of 1:1 as the electrolyte. Galvanostatic charge/discharge measurements were performed using a WBCS-3000s cycler (WonATech, Korea) over the potential range of 0.001e3.0 V at various current densities. The diameter of the electrode was 1.4 cm and the mass loading of the electrode was 0.9 mg cm2.
2. Experimental section 3. Results and discussion 2.1. Sample preparation The hierarchical structured assembly of hollow SnO2 nanoplates was synthesized using a hydrothermal process followed by onestep thermal oxidation. First, the hierarchical structured assembly of dense SnS2 nanoplates was synthesized by a one-pot hydrothermal process as reported in the previous literature [37]. Tin(Ⅳ) chloride pentahydrate (0.701 g; Sigma, 98%), thiourea (0.457 g; Junsei, 98%), and 20 mL of poly(ethylene glycol) (400) were dissolved in 35 mL of deionized water. After stirring for 30 min, this solution was transferred into a Teflon-lined autoclave and was then heat-treated at 180 C for 15 h in an oven. The resulting product was collected by centrifugation. To prepare the hierarchical structured assembly of hollow SnO2 nanoplates, the obtained dense SnS2 powders were post-treated at temperatures of 400, 500, and 600 C under air atmosphere for 3 h, and the resulting samples were denoted as SnO2-A400, SnO2-A500, and SnO2-A600, respectively.
The formation mechanism of hierarchical structured assembly of hollow SnO2 nanoplates from SnS2 is shown in Scheme 1. First, hierarchical SnS2 powders consisting of self-assembled nanoplates were synthesized by a facile hydrothermal process (Scheme 1-a) [37]. After the one-step oxidation process, the SnS2 powders transformed into hollow SnO2. Partial oxidation of SnS2 to form a SnS2-x-SnO composite and the conversion of the SnS2-x-SnO composite into SnO2 occurred during the oxidation process. Through the oxidation process, dense SnS2 nanoplates converted into hollow SnO2 nanoplates by nanoscale Kirkendall diffusion (Scheme 1-b). The morphological and compositional changes of the powders after the oxidation process were investigated. The SnS2 precursors synthesized through a hydrothermal method showed a hierarchical structure with dense nanoplates having a size of approximately 1 mm, as shown in the SEM and low resolution TEM images (Fig. 1a and b). The TEM image shown in Fig. 1c shows the well faceted SnS2
Scheme 1. Schematic 1 illustration of formation mechanism of hierarchical structured assembly of hollow SnO2 nanoplates.
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crystals. The high-resolution (HR) TEM image (Fig. 1d) shows distinct 0.39 nm-spaced lattice fringes that correspond to the (101) crystal plane of the hexagonal SnS2 phase (JCPDS card:#23-0677) [38,39]. The selected area electron diffraction (SAED) and XRD patterns of the SnS2 powders shown in Fig. 1e and Fig. S1, respectively, match well to those of hexagonal SnS2. The sharp XRD peaks indicate that the SnS2 nanoplates were highly crystalline. However, the presence of several broad peaks indicates that the product contained ultrafine SnO2 nanocrystals because the SnS2 nanoplates partially oxidized to form SnO2 thin layer on the surface. The elemental mapping images (Fig. 1f) and energy dispersive X-ray
(EDX) (Fig. S2a) results confirm that the SnS2 nanoplates consisted mostly of Sn and S with a little O. The morphologies and crystal structures of the SnO2 powders obtained at the oxidation temperatures of 400, 500, and 600 C were investigated. The SnS2 powders transformed into SnO2 powders even at a low oxidation temperature of 400 C, as confirmed by the XRD patterns shown in Fig. S1. As shown in Fig. 2, the overall morphologies of the SnO2 powders observed using SEM and low resolution TEM images was similar to that of the SnS2 powders irrespective of oxidation temperature. However, the high magnification TEM images shown in Fig. 3 revealed that the morphology of
Fig. 1. Characteristics of hierarchical structured assembly of dense SnS2 nanoplates obtained by hydrothermal process: (a) SEM, (b, c) TEM, (d) HR-TEM images, (e) SAED pattern, and (f) elemental mapping images.
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Fig. 2. Morphologies of hierarchical structured assembly of SnO2 nanoplates obtained by post-treatment in air: (a, c, e) SEM and (b, d, f) TEM images of hollow (a, b) SnO2-A400, (c, d) SnO2-A500, and (e, f) SnO2-A600 powders.
the nanoplates was affected by the oxidation temperature. The dense SnS2 nanoplates transformed into hollow structured SnO2 nanoplates at oxidation temperatures in the range of 400e500 C. The TEM images shown in Fig. 3a and b show a clear inner space within the SnO2 nanoplates as indicated by arrows. The faster diffusion rate of Sn and S into the outer surface of the dense SnS2 nanoplates than that of oxygen gas into the interior of the nanoplates resulted in the formation of hollow SnO2 nanoplates [40,41]. The transformation of the dense SnS2 nanoplates into the hollow SnO2 nanoplates occurred via the Kirkendall effect, as illustrated in Scheme 1. On the other hand, the SnO2 nanoplates oxidized at
600 C did not show any hollow space (Fig. 3c). Hollow structured SnO2 nanoplates were formed by the Kirkendall effect when the temperature was increased up to 600 C. However, sintering at a high oxidation temperature of 600 C resulted in the transformation of the hollow SnO2 nanoplates into filled ones. The SnO2 nanoplates consisted of a large number of ultrafine nanocrystals with different sizes, as shown by the high-resolution TEM (HRTEM) images (Fig. 3def). The SnO2 nanoplates showed 0.27 and 0.34 nm-spaced lattice fringes corresponding to the (101) and (110) crystal planes of the tetragonal SnO2 phase (JCPDS card: #41-1455), respectively [42]. The SAED patterns shown in Fig. S3 also
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Fig. 3. Morphologies and elemental mapping images of (a, d, g) SnO2eA400, (b, e, h) SnO2-A500, and (c, f, i) SnO2-A600 powders: (aec) TEM images, (def) HR-TEM images, and (gei) elemental mapping images of hollow SnO2 powders.
confirmed the formation of a pure SnO2 phase irrespective of the oxidation temperature. The mean crystallite sizes of SnO2-A400, SnO2-A500, and SnO2-A600 powders as measured from the HRTEM images were 8, 8, and 10 nm, respectively. Scherrer's equation was also used to estimate the mean crystallite sizes of the SnS2 (using the peak at 32.1 ) and SnO2 (using the peak at 26.5 ) powders. The mean crystallite sizes of SnS2, SnO2-A400, SnO2-A500, and SnO2-A600 powders were found to be 13, 5, 6, and 7 nm, respectively. These results suggest that more crystal growth was observed as heat treatment temperature increased. The elemental mapping images (Fig. 3) and EDX data (Figs. S2bed) also showed that the SnO2 nanoplates were mostly composed of Sn and O. The S content of the nanoplates decreased from 1.6 to 0.6 wt% as the oxidation temperature was increased from 400 to 600 C. To investigate the mechanism underlying the formation of hierarchical SnO2 via the Kirkendall effect, an additional experiment was carried out to identify the intermediates formed during the oxidation process. The oxidation of the SnS2 powders was carried out at 400 C for 10 and 30 min. We selected SnO2-A400 powders for further investigation because at the temperature of 500 C, complete conversion of dense structure to hollow structure occurred too fast to identify the intermediate phase. The sample treated for 10 min showed both SnS2 and SnO peaks, while that treated for 30 min showed only SnO2 peaks (Fig. S4). Through the additional experiment, we found that a SnS2-x-SnO composite was
formed as the intermediate during the surface oxidation of SnS2. Further oxidation transformed SnO into SnO2 while SnS2-x transformed into hollow SnO2 nanoplates via the Kirkendall effect. Through this oxidation mechanism, the dense SnS2 nanoplates transformed into hollow SnO2 nanoplates by nanoscale Kirkendall diffusion. XPS analysis was performed for analyzing the chemical composition and electron states of SnS2 and SnO2-A500 powders (Fig. 4). The XPS survey spectra shown in Fig. S5 confirm the presence of Sn, S, and O in the powders. The Sn 3d peak of SnO2 shifted to higher energies as compared to that of SnS2 (Fig. 4a and b). This can be attributed to the replacement of sulfur (low electronegativity) by oxygen (high electronegativity) in SnO2 [43,44]. In addition, the additional SeO peak with a binding energy of 169.5 eV and the upshift of the S 2p peak of SnO2 can be attributed to the remaining oxidized S atoms (Fig. 4c and d) [43,44]. The O 1s spectrum of SnS2 (Fig. 4e) shows that partial oxidation occurred on the surface of the SnS2 nanoplates. The SnO2-A500 powders showed a small ratio of the oxygen ions generated from PEG and other organic molecules to the lattice oxygen, as revealed by the O 1s spectrum of SnS2 (Fig. 4f) [45]. This indicates that as the oxidation of the SnS2 nanoplates progressed, the amount of the lattice oxygen produced increased, resulting in the formation of the SnO2 nanoplates. The TG curves of SnS2 and SnO2-A500 were obtained under air atmosphere (Fig. S6). The SnS2 powders showed an initial
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Fig. 4. High resolution XPS spectra of (a, c, e) SnS2 and (b, d, f) SnO2-A500 powders of (a, b) Sn 3d, (c, d) S 2p, and (e, f) O 1s.
weight loss at 250e300 C because of the decomposition of PEG 400 deposited during the hydrothermal process [46e48]. As shown by the XRD patterns, the second-step weight loss at around 400 C was caused by the partial oxidation of SnS2 to form the SnS2-x-SnO composite and its subsequent conversion into SnO2 [49,50]. The negligible weight loss of SnO2-A500 powders indicates that SnO2 powders with trace amount of sulfur were formed at this temperature. The pore structures of the SnS2 and SnO2 powders were investigated by carrying out N2 adsorption-desorption measurements (Fig. S7). SnO2 powders exhibited type IV isotherms with a distinct
hysteresis loop at the relative pressures P P1 0 of 0.4e0.8, 0.4e0.9, and 0.4e0.9 for SnO2-A400, SnO2-A500, and SnO2-A600, respectively (Fig. S7a). As per the IUPAC nomenclature, this curve can be assigned to the different processes occurring between the adsorption and desorption of N2 gas from the mesopores of the SnO2 powders [51e53]. The BJH pore size distribution results showed that SnO2-A400 showed mesopores with sizes in the range of 3e5 nm (Fig. S7b). The pore volume of the mesopores with a size of 5e9 nm increased with an increase in the oxidation temperature. Crystal growth at the oxidation temperature of 600 C resulted in the formation of large-sized mesopores. The BET surface areas of
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Fig. 5. CV curves of (a) SnS2, (b) SnO2-A400, (c) SnO2-A500, and (d) SnO2-A600 powders at scan rate of 0.1 mV s1.
SnS2, SnO2-A400, SnO2-A500, and SnO2-A600 powders were 22.8, 84.3, 82.7, and 48.5 m2 g1, respectively. The electrochemical performances of SnS2 and SnO2 powders, used as the LIB anodes, were compared through cyclic voltammetric (CV) and galvanostatic charge/discharge measurements. The CV curves for the 1st, 2nd, and 5th cycles of the powders obtained at 0.1 mV s1 are shown in Fig. 5. In the first cathodic scan, the SnS2 powders exhibited three reduction peaks at 1.8, 1.2, and 0.1 V and two small peaks at around 1.5 and 0.9 V (Fig. 5a). The reduction peak at around 1.8 V, which was not observed during the subsequent cycles, can be attributed to the intercalation of lithium into the SnS2 layers without causing the phase decomposition [54e56]. The peaks at approximately 1.5, 1.2, and 0.9 V in the first cathodic sweep can be assigned to the formation of the solid electrolyte interface (SEI) layer and the decomposition of the SnS2 nanocrystals into metallic Sn and Li2S that could divide into three steps [54e56]. The peaks at 0.1 and 0.5 V observed in cathodic and anodic scans, respectively, suggest that Li ions subsequently reacted with Sn metal to form a LixSn alloy and that a reverse reaction occurred [54e56]. The anodic peak at 1.8 V can be attributed to the oxygenation of Sn nanocrystals in the charged state at higher potentials. The cathodic peak shifted from 1.2 to 1.4 V after the first cycle as a result of the formation of ultrafine nanocrystals during the first cycle [57]. The CV curves of the SnO2 powders, showed reduction peaks at 0.8 and 0.2 V during the first cathodic scan. The sharp peak at about 0.8 V may contribute to formation of SEI layer and reduction of SnO2 and the formation of metallic SneLi2O composite [58]. After the first cycle, the peak shifted to higher potential as the formation of ultrafine nanocrystals that improved
the electrochemical reaction kinetics. The peak at around 0.2 V was associated with the alloying of Sn to form LixSn [35,59]. The oxidation peak at approximately 0.5 V observed for all the cycles can be attributed to the dealloying of LixSn [35,59]. The broad peaks at about 1.2 and 1.8 V during the charge cycles represented the conversion reactions of Sn to SnO and SnO2, respectively [60]. After the first cycle, a couple of broad oxidation peaks appeared at around 1.3 V attributing to the transformation of metallic tin into tin oxide. However, the number of oxidation peaks reduced as the cycling progressed, indicating that the formation of tin oxide was partially reversible [61]. The initial discharge and charge curves of the SnS2 and SnO2 powders at a current density of 1.0 A g1 are shown in Fig. 6. The SnS2 powders showed clear plateaus at 0.1 and 1.2 V in the discharge curve. On the other hand, the SnO2 powders showed plateau at about 0.85 V that attributed to formation of metallic SneLi2O composite and SEI layer that related to high irreversible capacity of first discharge process. These results are consistent with those observed from the CV curves shown in Fig. 5. The initial discharge capacities of SnS2, SnO2-A400, SnO2-A500, and SnO2A600 powders were 1439, 1808, 1803, and 1583 mA h g1, respectively, and their initial Coulombic efficiencies were 70, 60, 59, and 60%, respectively. A reason for the low initial Coulombic efficiency of SnO2 was the enhanced electrolyte decomposition and the formation of the SEI layer on the surface of the microspheres with a large surface area [62,63]. The other reason was the higher reversible formation of Li2S than that of Li2O [64,65]. The cycling performances of the samples at a current density of 1.0 A g1 are shown in Fig. 6b. The discharge and charge capacities of the SnS2
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Fig. 6. Electrochemical properties of hierarchical structured assembly of SnS2 and SnO2 nanoplates: (a) initial discharge-charge profiles, (b) cycling performances at a current density of 1.0 A g1, and (c) rate performances.
powders decreased considerably during the first 20 cycles and showed a steady decrease to 273 mA h g1 after 300 cycles. After 20 cycles, the SnO2 powders showed better cycling stability than the SnS2 powders. The discharge capacities of SnO2-A400, SnO2-A500, and SnO2-A600 powders for the 300th cycle were 577, 754, and 855 mA h g1, respectively. SnO2-A500 showed better cycling stability than the other SnO2 powders After 200 cycles, the capacities of SnO2-A600 gradually increased that might be caused by pulverization of electrode that also caused collapse of the SEI layer and subsequently, a number of lithium ions were consumed to form new SEI layer during the reaction between active materials and lithium ions [66,67]. However, in the case of SnO2-A400, the formation of ultrafine SnO2 nanocrystals resulted in slight capacity fading after 200 cycles. The cycling performances of the samples at the lower and higher current density were also measured that shown in Fig. S8. At the current density of 0.5 A g1, the discharge and charge capacities of SnO2 powders were declined during the
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first 20 cycles, however, they showed stable cycle ability after 100 cycles that correspond to the result at the current density of 1.0 A g1. However, at the current density of 3.0 A g1, SnO2 powders showed continuous decreased capacities during the cycles. Particularly, contrary to the SnO2-A400 and -A500 samples, SnO2A600 increased its capacity at around 300th cycle at the current density of 3.0 A g1. In addition, after the capacity increased, the capacity of the sample became unstable compared to other samples. This tendency also might be caused by pulverization of electrode as shown in Fig. S9. The SnO2 powders also showed considerable rate performances irrespective of their inner morphological structure (Fig. 6c). The SnO2 powders showed high stability even at an extremely high current density of 30.0 A g1. In the case of SnO2-A500, the average discharge capacities of 1099, 804, 698, 585, 468, 372, 277, 213, and 169 mA h g1 were obtained at the current densities of 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0 A g1, respectively. Notably, a high discharge capacity of 755 mA h g1 was attained when the current density returned to 0.5 A g1. SnO2-A500 showed excellent cycling stability even at high current densities considering the capacity fading of the SnO2 powders during the first 20 cycles. The electrochemical impedance spectroscopy (EIS) results of the SnO2 electrodes before cycling and after 1 and 50 cycles are shown in Fig. 7. The EIS data could be well matched with a Randle-type equivalent circuit model that the semicircles in the highmedium-frequency region and the straight line in the lowfrequency range that compose the Nyquist plots are consistent with the degree of the charge transfer resistance (Rct) and the solidstate diffusion of lithium ions (Zw), respectively as shown in Fig. S10 [68e73]. The Nyquist plots of the fresh SnO2 electrodes showed similar Rct values. Compared to the Nyquist plots of the fresh cells, the Rct values of the electrodes decreased significantly after the formation of ultrafine nanocrystals during the initial cycle. The Rct values of SnO2-A400 and eA500 powders increased to 100.7 and 132.5 U, however, that of SnO2-A600 powders increased to 215.2 U after 50 cycles, indicating that the unstable cycling performance of SnO2-A600 powders compared to the other samples. The morphology of SnO2-A500 was well maintained even after 50 cycles (Fig. S11). The outstanding electrochemical properties of SnO2A500 can be attributed to their unique morphological structure consisting of hollow nanoplates and the optimum crystallite size, which increased the contact area between the active materials and the electrolyte and the buffer stress caused by volume expansion during cycling. The first charge/discharge curve and cycling performance of the SnO2-A500/LiMn2O4 full cell with a cut-off voltage of 3.0e4.1 V are shown in Fig. 8 [74]. The information of LiMn2O4 cathode material was described in the previous literature and Fig. S12 [75]. The current densities were calculated by measuring the weight of the SnO2-A500 anode. As shown in Fig. 8a, the SnO2-A500 anode
Fig. 7. Nyquist plots of SnO2 powders: (a) before cycling, (b) after 1st cycle, and (c) after 50th cycle.
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between the active materials and the electrolyte and the buffer stress caused by the volume expansion during cycling. We believe that the strategy used in this work can be extended to synthesizing hierarchical structured hollow metal oxide materials from the corresponding sulfides through the Kirkendall effect. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03034473 and NRF2017R1A4A1014806). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.05.120. References
Fig. 8. (a) Initial charge/discharge curves, (b) cycling performance, and (c) rate performance of SnO2eA500/LiMn2O4 full cell, and (d) an LED powered by a full cell.
showed charge and discharge capacities of about 710 and 612 mA h g1, respectively, in a full cell with a LiMn2O4 counterpart. This anode showed a low initial Coulombic efficiency because of the formation of the SEI layer [62,63]. However, the Coulombic efficiency and cycling stability increased after 100 cycles (Fig. 8b). The rate performance of the SnO2-A500/LiMn2O4 full cell was measured at various current densities while maintaining a constant cut-off voltage range. The SnO2-A500/LiMn2O4 full cell showed the final discharge capacities of 543, 494, 435, 394, 361, and 333 mA h g1 at the current densities of 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 A g1, respectively (Fig. 8c). The photo-image of LED powered by a full cell is shown in Fig. 8d. 4. Conclusions In summary, a hierarchical structured assembly of hollow SnO2 nanoplates was successfully prepared by using a hydrothermal method followed by one-step oxidation. During the early stage of oxidation, the outer diffusion of tin cations and sulfur in SnS2 was faster than the inner diffusion of oxygen gas, which resulted in the formation of hollow SnO2 nanoplates. The hierarchical assembly of the hollow SnO2 nanoplates exhibited excellent Li-ion storage performance with high stability and good rate capability because of their substantial inner voids which increased the contact area
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Superior lithium-ion storage performances of SnO2 powders consisting of hollow nanoplates Jae Hun Choi a, 1, Seung-Keun Park b, 1, Yun Chan Kang a, * a b
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea Department of Chemical Engineering, Kongju National University, Budae-Dong 275, Chungnam 314-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2019 Received in revised form 27 April 2019 Accepted 11 May 2019 Available online 13 May 2019
Hierarchical structured transition metal oxides have attracted considerable attention as anode materials for lithium-ion batteries because they possess large surface area that can provide large contact area with the electrolyte and short diffusion distance for Li ions. Here, a hierarchical structured assembly of hollow SnO2 nanoplates is synthesized by one-step oxidation of SnS2 powders. The SnS2 powders comprising of dense nanoplates synthesized by the hydrothermal method transform into SnO2 powders comprising of hollow nanoplates by nanoscale Kirkendall diffusion at the oxidation temperature of 500 C. After the transformation of SnS2 into SnO2 powders, the Brunauer-Emmett-Teller surface area of the powders increases from 22.8 to 82.7 m2 g1. The hierarchical structured SnO2 powders show superior lithium-ion storage performances compared to SnS2 powders with the same structure. The discharge capacities of SnS2 and SnO2 powders at a current density of 1 A g1 for the 300th cycle are 273 and 754 mA h g1, respectively. The SnO2 powders show a high reversible capacity of 169 mA h g1 even at an extremely high current density of 30 A g1. The outstanding electrochemical properties of the SnO2 powders can be attributed to their unique morphological structure having hollow nanoplates and optimum crystallite size, which increases the contact area between the active materials and the electrolyte and the buffered stress caused by the volume expansion during cycling. © 2019 Elsevier B.V. All rights reserved.
Keywords: Kierkendall diffusion Nanostructured materials Lithium-ion batteries Hydrothermal process Tin oxide
1. Introduction Owing to their high capacity and reasonable cycling performance, nanostructured materials of transition metal compounds such as oxides [1e6], sulfides [7e9], and selenides [10e12] have been used as anode materials for lithium-ion batteries (LIBs). Especially, hierarchical structured transition metal compounds have attracted considerable interest as anode materials with excellent rate performance owing to their large surface areas, which provide a large contact area with the electrolyte and a short diffusion distance for Li ions [13e20]. Hierarchical structured materials of transition metal chalcogenides have been extensively studied because of their directional crystal growth [21e25]. Metal sulfide and selenide materials grow mainly in the direction that yields plate- and rod-like crystals. SnS2 has been widely studied for LIBs because of its CdI2-type structure with stacked layers enabling
* Corresponding author. E-mail address: [email protected] (Y.C. Kang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.05.120 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the intercalation/deintercalation of alkali metal ions [26e30]. Zhai et al. fabricated ultrathin hexagonal SnS2 nanosheets via hydrothermal reaction as LIB anode materials [31]. These nanosheets exhibited a highly reversible lithium-ion storage and highly stable cyclic performance. Gao et al. synthesized SnS2 nanoplates that grew vertically on reduced graphene oxide nanoribbons for LIB anodes [32]. Graphene oxide nanoribbons were prepared by oxidative opening of multiwalled carbon nanotubes and the layered SnS2 nanoplates were fabricated by chemical vapor deposition. The assembly of these materials showed excellent stability and high capacity in both the half and full cells owing to the synergy between the layered SnS2 nanoplates and the high conductivity of the reduced graphene oxide nanoribbons. Wang et al. fabricated SnS2 nanoplates by a hydrothermal process and controlled their crystallinity and thickness by changing the temperature and pH [33]. The SnS2 nanoplates prepared at 200 C and pH of 10.5 showed excellent performance (high cycling stability and rate capability) as an anode material for LIBs. This is because temperature affected the thickness and crystallinity of these nanoplates, while pH adjusted the nucleation rate and the speed of transformation of SnS2 to SnO2,
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i.e., the pH value affected the internal crystal structure. However, metal chalcogenide materials show lower Li ion capacities than their corresponding oxides [34]. Recently, hollow structured metal oxide materials have been synthesized from metal chalcogenide materials using the Kirkendall effect for LIBs anode materials. Park et al. synthesized hollow SnO2 nanoplates via the oxidation of SnSe nanoplates [35]. The SnSe nanoplates synthesized via spray pyrolysis transformed into hollow structured SnO2 nanoplates through the Kirkendall effect. Park et al. prepared hierarchical structured Fe2O3 rod clusters using a hydrothermal process and subsequent oxidation [36]. During the oxidation process, FeSe2 rod clusters transformed into Fe2O3 rod clusters with numerous empty nanovoids through the Kirkendall effect. The Fe2O3 rod clusters showed excellent Li-ion storage performance. Despite the availability of a few reports, the transformation of metal chalcogenides into their corresponding hollow metal oxides via the Kirkendall effect is still in an early stage and is limited to a few materials. Furthermore, synthesizing diverse and unique structured Sn-based composite through spray pyrolysis has limits compared to fabricating through hydrothermal process in respect of controlling the shape and crystal growth. In this study, we synthesized a hierarchical structured assembly of hollow SnO2 nanoplates obtained from SnS2 by a hydrothermal method. During the one-step oxidation process, the SnS2 nanopowders converted into hollow structured SnO2 via the Kirkendall effect while maintaining their morphology. The prepared SnO2 powders were compared with SnS2 powders. The SnO2 powders showed better Li ion storage performance, better cycling stability, and higher capacity than SnS2.
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2.2. Characterization techniques To examine the morphology and structure of the hierarchical structured SnS2 and SnO2 powders, scanning electron microscopy (SEM, Tescan, VEGA3) and field-emission transmission electron microscopy (FE-TEM, JEOL, JEM-2100F) were used. The crystal phases of the samples were verified by X-ray diffraction (XRD, X'Pert PRO) using Cu Ka radiation at the Korea Basic Science Institute (Daegu). The chemical composition of the SnS2 and SnO2 nanoplates was examined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The pore size and surface areas of the nanoplatelets were measured by using the Brunauer-EmmettTeller (BET) method, using pure N2 as the adsorbate gas. The thermal behavior of the nanoparticles was investigated using a Pyris 1 thermogravimetric (TG) analyzer (Perkin Elmer) at a heating rate of 10 C min1 in air. 2.3. Electrochemical measurements The electrochemical properties of the hierarchical structured SnS2 and SnO2 powders were evaluated using a standard 2032-type coin cell. The LIB anodes were prepared by casting a mixture of active materials, Super P, and sodium carboxymethyl cellulose (mass ratio of 7:2:1) in DI water onto a Cu foil. The coin cell was composed of a Li metal foil as the counter electrode, a porous polymer membrane as the separator, and 1 M LiPF6 dissolved in fluoroethylene carbonate/dimethyl carbonate at a volumetric ratio of 1:1 as the electrolyte. Galvanostatic charge/discharge measurements were performed using a WBCS-3000s cycler (WonATech, Korea) over the potential range of 0.001e3.0 V at various current densities. The diameter of the electrode was 1.4 cm and the mass loading of the electrode was 0.9 mg cm2.
2. Experimental section 3. Results and discussion 2.1. Sample preparation The hierarchical structured assembly of hollow SnO2 nanoplates was synthesized using a hydrothermal process followed by onestep thermal oxidation. First, the hierarchical structured assembly of dense SnS2 nanoplates was synthesized by a one-pot hydrothermal process as reported in the previous literature [37]. Tin(Ⅳ) chloride pentahydrate (0.701 g; Sigma, 98%), thiourea (0.457 g; Junsei, 98%), and 20 mL of poly(ethylene glycol) (400) were dissolved in 35 mL of deionized water. After stirring for 30 min, this solution was transferred into a Teflon-lined autoclave and was then heat-treated at 180 C for 15 h in an oven. The resulting product was collected by centrifugation. To prepare the hierarchical structured assembly of hollow SnO2 nanoplates, the obtained dense SnS2 powders were post-treated at temperatures of 400, 500, and 600 C under air atmosphere for 3 h, and the resulting samples were denoted as SnO2-A400, SnO2-A500, and SnO2-A600, respectively.
The formation mechanism of hierarchical structured assembly of hollow SnO2 nanoplates from SnS2 is shown in Scheme 1. First, hierarchical SnS2 powders consisting of self-assembled nanoplates were synthesized by a facile hydrothermal process (Scheme 1-a) [37]. After the one-step oxidation process, the SnS2 powders transformed into hollow SnO2. Partial oxidation of SnS2 to form a SnS2-x-SnO composite and the conversion of the SnS2-x-SnO composite into SnO2 occurred during the oxidation process. Through the oxidation process, dense SnS2 nanoplates converted into hollow SnO2 nanoplates by nanoscale Kirkendall diffusion (Scheme 1-b). The morphological and compositional changes of the powders after the oxidation process were investigated. The SnS2 precursors synthesized through a hydrothermal method showed a hierarchical structure with dense nanoplates having a size of approximately 1 mm, as shown in the SEM and low resolution TEM images (Fig. 1a and b). The TEM image shown in Fig. 1c shows the well faceted SnS2
Scheme 1. Schematic 1 illustration of formation mechanism of hierarchical structured assembly of hollow SnO2 nanoplates.
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crystals. The high-resolution (HR) TEM image (Fig. 1d) shows distinct 0.39 nm-spaced lattice fringes that correspond to the (101) crystal plane of the hexagonal SnS2 phase (JCPDS card:#23-0677) [38,39]. The selected area electron diffraction (SAED) and XRD patterns of the SnS2 powders shown in Fig. 1e and Fig. S1, respectively, match well to those of hexagonal SnS2. The sharp XRD peaks indicate that the SnS2 nanoplates were highly crystalline. However, the presence of several broad peaks indicates that the product contained ultrafine SnO2 nanocrystals because the SnS2 nanoplates partially oxidized to form SnO2 thin layer on the surface. The elemental mapping images (Fig. 1f) and energy dispersive X-ray
(EDX) (Fig. S2a) results confirm that the SnS2 nanoplates consisted mostly of Sn and S with a little O. The morphologies and crystal structures of the SnO2 powders obtained at the oxidation temperatures of 400, 500, and 600 C were investigated. The SnS2 powders transformed into SnO2 powders even at a low oxidation temperature of 400 C, as confirmed by the XRD patterns shown in Fig. S1. As shown in Fig. 2, the overall morphologies of the SnO2 powders observed using SEM and low resolution TEM images was similar to that of the SnS2 powders irrespective of oxidation temperature. However, the high magnification TEM images shown in Fig. 3 revealed that the morphology of
Fig. 1. Characteristics of hierarchical structured assembly of dense SnS2 nanoplates obtained by hydrothermal process: (a) SEM, (b, c) TEM, (d) HR-TEM images, (e) SAED pattern, and (f) elemental mapping images.
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Fig. 2. Morphologies of hierarchical structured assembly of SnO2 nanoplates obtained by post-treatment in air: (a, c, e) SEM and (b, d, f) TEM images of hollow (a, b) SnO2-A400, (c, d) SnO2-A500, and (e, f) SnO2-A600 powders.
the nanoplates was affected by the oxidation temperature. The dense SnS2 nanoplates transformed into hollow structured SnO2 nanoplates at oxidation temperatures in the range of 400e500 C. The TEM images shown in Fig. 3a and b show a clear inner space within the SnO2 nanoplates as indicated by arrows. The faster diffusion rate of Sn and S into the outer surface of the dense SnS2 nanoplates than that of oxygen gas into the interior of the nanoplates resulted in the formation of hollow SnO2 nanoplates [40,41]. The transformation of the dense SnS2 nanoplates into the hollow SnO2 nanoplates occurred via the Kirkendall effect, as illustrated in Scheme 1. On the other hand, the SnO2 nanoplates oxidized at
600 C did not show any hollow space (Fig. 3c). Hollow structured SnO2 nanoplates were formed by the Kirkendall effect when the temperature was increased up to 600 C. However, sintering at a high oxidation temperature of 600 C resulted in the transformation of the hollow SnO2 nanoplates into filled ones. The SnO2 nanoplates consisted of a large number of ultrafine nanocrystals with different sizes, as shown by the high-resolution TEM (HRTEM) images (Fig. 3def). The SnO2 nanoplates showed 0.27 and 0.34 nm-spaced lattice fringes corresponding to the (101) and (110) crystal planes of the tetragonal SnO2 phase (JCPDS card: #41-1455), respectively [42]. The SAED patterns shown in Fig. S3 also
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Fig. 3. Morphologies and elemental mapping images of (a, d, g) SnO2eA400, (b, e, h) SnO2-A500, and (c, f, i) SnO2-A600 powders: (aec) TEM images, (def) HR-TEM images, and (gei) elemental mapping images of hollow SnO2 powders.
confirmed the formation of a pure SnO2 phase irrespective of the oxidation temperature. The mean crystallite sizes of SnO2-A400, SnO2-A500, and SnO2-A600 powders as measured from the HRTEM images were 8, 8, and 10 nm, respectively. Scherrer's equation was also used to estimate the mean crystallite sizes of the SnS2 (using the peak at 32.1 ) and SnO2 (using the peak at 26.5 ) powders. The mean crystallite sizes of SnS2, SnO2-A400, SnO2-A500, and SnO2-A600 powders were found to be 13, 5, 6, and 7 nm, respectively. These results suggest that more crystal growth was observed as heat treatment temperature increased. The elemental mapping images (Fig. 3) and EDX data (Figs. S2bed) also showed that the SnO2 nanoplates were mostly composed of Sn and O. The S content of the nanoplates decreased from 1.6 to 0.6 wt% as the oxidation temperature was increased from 400 to 600 C. To investigate the mechanism underlying the formation of hierarchical SnO2 via the Kirkendall effect, an additional experiment was carried out to identify the intermediates formed during the oxidation process. The oxidation of the SnS2 powders was carried out at 400 C for 10 and 30 min. We selected SnO2-A400 powders for further investigation because at the temperature of 500 C, complete conversion of dense structure to hollow structure occurred too fast to identify the intermediate phase. The sample treated for 10 min showed both SnS2 and SnO peaks, while that treated for 30 min showed only SnO2 peaks (Fig. S4). Through the additional experiment, we found that a SnS2-x-SnO composite was
formed as the intermediate during the surface oxidation of SnS2. Further oxidation transformed SnO into SnO2 while SnS2-x transformed into hollow SnO2 nanoplates via the Kirkendall effect. Through this oxidation mechanism, the dense SnS2 nanoplates transformed into hollow SnO2 nanoplates by nanoscale Kirkendall diffusion. XPS analysis was performed for analyzing the chemical composition and electron states of SnS2 and SnO2-A500 powders (Fig. 4). The XPS survey spectra shown in Fig. S5 confirm the presence of Sn, S, and O in the powders. The Sn 3d peak of SnO2 shifted to higher energies as compared to that of SnS2 (Fig. 4a and b). This can be attributed to the replacement of sulfur (low electronegativity) by oxygen (high electronegativity) in SnO2 [43,44]. In addition, the additional SeO peak with a binding energy of 169.5 eV and the upshift of the S 2p peak of SnO2 can be attributed to the remaining oxidized S atoms (Fig. 4c and d) [43,44]. The O 1s spectrum of SnS2 (Fig. 4e) shows that partial oxidation occurred on the surface of the SnS2 nanoplates. The SnO2-A500 powders showed a small ratio of the oxygen ions generated from PEG and other organic molecules to the lattice oxygen, as revealed by the O 1s spectrum of SnS2 (Fig. 4f) [45]. This indicates that as the oxidation of the SnS2 nanoplates progressed, the amount of the lattice oxygen produced increased, resulting in the formation of the SnO2 nanoplates. The TG curves of SnS2 and SnO2-A500 were obtained under air atmosphere (Fig. S6). The SnS2 powders showed an initial
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Fig. 4. High resolution XPS spectra of (a, c, e) SnS2 and (b, d, f) SnO2-A500 powders of (a, b) Sn 3d, (c, d) S 2p, and (e, f) O 1s.
weight loss at 250e300 C because of the decomposition of PEG 400 deposited during the hydrothermal process [46e48]. As shown by the XRD patterns, the second-step weight loss at around 400 C was caused by the partial oxidation of SnS2 to form the SnS2-x-SnO composite and its subsequent conversion into SnO2 [49,50]. The negligible weight loss of SnO2-A500 powders indicates that SnO2 powders with trace amount of sulfur were formed at this temperature. The pore structures of the SnS2 and SnO2 powders were investigated by carrying out N2 adsorption-desorption measurements (Fig. S7). SnO2 powders exhibited type IV isotherms with a distinct
hysteresis loop at the relative pressures P P1 0 of 0.4e0.8, 0.4e0.9, and 0.4e0.9 for SnO2-A400, SnO2-A500, and SnO2-A600, respectively (Fig. S7a). As per the IUPAC nomenclature, this curve can be assigned to the different processes occurring between the adsorption and desorption of N2 gas from the mesopores of the SnO2 powders [51e53]. The BJH pore size distribution results showed that SnO2-A400 showed mesopores with sizes in the range of 3e5 nm (Fig. S7b). The pore volume of the mesopores with a size of 5e9 nm increased with an increase in the oxidation temperature. Crystal growth at the oxidation temperature of 600 C resulted in the formation of large-sized mesopores. The BET surface areas of
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Fig. 5. CV curves of (a) SnS2, (b) SnO2-A400, (c) SnO2-A500, and (d) SnO2-A600 powders at scan rate of 0.1 mV s1.
SnS2, SnO2-A400, SnO2-A500, and SnO2-A600 powders were 22.8, 84.3, 82.7, and 48.5 m2 g1, respectively. The electrochemical performances of SnS2 and SnO2 powders, used as the LIB anodes, were compared through cyclic voltammetric (CV) and galvanostatic charge/discharge measurements. The CV curves for the 1st, 2nd, and 5th cycles of the powders obtained at 0.1 mV s1 are shown in Fig. 5. In the first cathodic scan, the SnS2 powders exhibited three reduction peaks at 1.8, 1.2, and 0.1 V and two small peaks at around 1.5 and 0.9 V (Fig. 5a). The reduction peak at around 1.8 V, which was not observed during the subsequent cycles, can be attributed to the intercalation of lithium into the SnS2 layers without causing the phase decomposition [54e56]. The peaks at approximately 1.5, 1.2, and 0.9 V in the first cathodic sweep can be assigned to the formation of the solid electrolyte interface (SEI) layer and the decomposition of the SnS2 nanocrystals into metallic Sn and Li2S that could divide into three steps [54e56]. The peaks at 0.1 and 0.5 V observed in cathodic and anodic scans, respectively, suggest that Li ions subsequently reacted with Sn metal to form a LixSn alloy and that a reverse reaction occurred [54e56]. The anodic peak at 1.8 V can be attributed to the oxygenation of Sn nanocrystals in the charged state at higher potentials. The cathodic peak shifted from 1.2 to 1.4 V after the first cycle as a result of the formation of ultrafine nanocrystals during the first cycle [57]. The CV curves of the SnO2 powders, showed reduction peaks at 0.8 and 0.2 V during the first cathodic scan. The sharp peak at about 0.8 V may contribute to formation of SEI layer and reduction of SnO2 and the formation of metallic SneLi2O composite [58]. After the first cycle, the peak shifted to higher potential as the formation of ultrafine nanocrystals that improved
the electrochemical reaction kinetics. The peak at around 0.2 V was associated with the alloying of Sn to form LixSn [35,59]. The oxidation peak at approximately 0.5 V observed for all the cycles can be attributed to the dealloying of LixSn [35,59]. The broad peaks at about 1.2 and 1.8 V during the charge cycles represented the conversion reactions of Sn to SnO and SnO2, respectively [60]. After the first cycle, a couple of broad oxidation peaks appeared at around 1.3 V attributing to the transformation of metallic tin into tin oxide. However, the number of oxidation peaks reduced as the cycling progressed, indicating that the formation of tin oxide was partially reversible [61]. The initial discharge and charge curves of the SnS2 and SnO2 powders at a current density of 1.0 A g1 are shown in Fig. 6. The SnS2 powders showed clear plateaus at 0.1 and 1.2 V in the discharge curve. On the other hand, the SnO2 powders showed plateau at about 0.85 V that attributed to formation of metallic SneLi2O composite and SEI layer that related to high irreversible capacity of first discharge process. These results are consistent with those observed from the CV curves shown in Fig. 5. The initial discharge capacities of SnS2, SnO2-A400, SnO2-A500, and SnO2A600 powders were 1439, 1808, 1803, and 1583 mA h g1, respectively, and their initial Coulombic efficiencies were 70, 60, 59, and 60%, respectively. A reason for the low initial Coulombic efficiency of SnO2 was the enhanced electrolyte decomposition and the formation of the SEI layer on the surface of the microspheres with a large surface area [62,63]. The other reason was the higher reversible formation of Li2S than that of Li2O [64,65]. The cycling performances of the samples at a current density of 1.0 A g1 are shown in Fig. 6b. The discharge and charge capacities of the SnS2
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Fig. 6. Electrochemical properties of hierarchical structured assembly of SnS2 and SnO2 nanoplates: (a) initial discharge-charge profiles, (b) cycling performances at a current density of 1.0 A g1, and (c) rate performances.
powders decreased considerably during the first 20 cycles and showed a steady decrease to 273 mA h g1 after 300 cycles. After 20 cycles, the SnO2 powders showed better cycling stability than the SnS2 powders. The discharge capacities of SnO2-A400, SnO2-A500, and SnO2-A600 powders for the 300th cycle were 577, 754, and 855 mA h g1, respectively. SnO2-A500 showed better cycling stability than the other SnO2 powders After 200 cycles, the capacities of SnO2-A600 gradually increased that might be caused by pulverization of electrode that also caused collapse of the SEI layer and subsequently, a number of lithium ions were consumed to form new SEI layer during the reaction between active materials and lithium ions [66,67]. However, in the case of SnO2-A400, the formation of ultrafine SnO2 nanocrystals resulted in slight capacity fading after 200 cycles. The cycling performances of the samples at the lower and higher current density were also measured that shown in Fig. S8. At the current density of 0.5 A g1, the discharge and charge capacities of SnO2 powders were declined during the
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first 20 cycles, however, they showed stable cycle ability after 100 cycles that correspond to the result at the current density of 1.0 A g1. However, at the current density of 3.0 A g1, SnO2 powders showed continuous decreased capacities during the cycles. Particularly, contrary to the SnO2-A400 and -A500 samples, SnO2A600 increased its capacity at around 300th cycle at the current density of 3.0 A g1. In addition, after the capacity increased, the capacity of the sample became unstable compared to other samples. This tendency also might be caused by pulverization of electrode as shown in Fig. S9. The SnO2 powders also showed considerable rate performances irrespective of their inner morphological structure (Fig. 6c). The SnO2 powders showed high stability even at an extremely high current density of 30.0 A g1. In the case of SnO2-A500, the average discharge capacities of 1099, 804, 698, 585, 468, 372, 277, 213, and 169 mA h g1 were obtained at the current densities of 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0 A g1, respectively. Notably, a high discharge capacity of 755 mA h g1 was attained when the current density returned to 0.5 A g1. SnO2-A500 showed excellent cycling stability even at high current densities considering the capacity fading of the SnO2 powders during the first 20 cycles. The electrochemical impedance spectroscopy (EIS) results of the SnO2 electrodes before cycling and after 1 and 50 cycles are shown in Fig. 7. The EIS data could be well matched with a Randle-type equivalent circuit model that the semicircles in the highmedium-frequency region and the straight line in the lowfrequency range that compose the Nyquist plots are consistent with the degree of the charge transfer resistance (Rct) and the solidstate diffusion of lithium ions (Zw), respectively as shown in Fig. S10 [68e73]. The Nyquist plots of the fresh SnO2 electrodes showed similar Rct values. Compared to the Nyquist plots of the fresh cells, the Rct values of the electrodes decreased significantly after the formation of ultrafine nanocrystals during the initial cycle. The Rct values of SnO2-A400 and eA500 powders increased to 100.7 and 132.5 U, however, that of SnO2-A600 powders increased to 215.2 U after 50 cycles, indicating that the unstable cycling performance of SnO2-A600 powders compared to the other samples. The morphology of SnO2-A500 was well maintained even after 50 cycles (Fig. S11). The outstanding electrochemical properties of SnO2A500 can be attributed to their unique morphological structure consisting of hollow nanoplates and the optimum crystallite size, which increased the contact area between the active materials and the electrolyte and the buffer stress caused by volume expansion during cycling. The first charge/discharge curve and cycling performance of the SnO2-A500/LiMn2O4 full cell with a cut-off voltage of 3.0e4.1 V are shown in Fig. 8 [74]. The information of LiMn2O4 cathode material was described in the previous literature and Fig. S12 [75]. The current densities were calculated by measuring the weight of the SnO2-A500 anode. As shown in Fig. 8a, the SnO2-A500 anode
Fig. 7. Nyquist plots of SnO2 powders: (a) before cycling, (b) after 1st cycle, and (c) after 50th cycle.
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between the active materials and the electrolyte and the buffer stress caused by the volume expansion during cycling. We believe that the strategy used in this work can be extended to synthesizing hierarchical structured hollow metal oxide materials from the corresponding sulfides through the Kirkendall effect. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03034473 and NRF2017R1A4A1014806). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.05.120. References
Fig. 8. (a) Initial charge/discharge curves, (b) cycling performance, and (c) rate performance of SnO2eA500/LiMn2O4 full cell, and (d) an LED powered by a full cell.
showed charge and discharge capacities of about 710 and 612 mA h g1, respectively, in a full cell with a LiMn2O4 counterpart. This anode showed a low initial Coulombic efficiency because of the formation of the SEI layer [62,63]. However, the Coulombic efficiency and cycling stability increased after 100 cycles (Fig. 8b). The rate performance of the SnO2-A500/LiMn2O4 full cell was measured at various current densities while maintaining a constant cut-off voltage range. The SnO2-A500/LiMn2O4 full cell showed the final discharge capacities of 543, 494, 435, 394, 361, and 333 mA h g1 at the current densities of 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 A g1, respectively (Fig. 8c). The photo-image of LED powered by a full cell is shown in Fig. 8d. 4. Conclusions In summary, a hierarchical structured assembly of hollow SnO2 nanoplates was successfully prepared by using a hydrothermal method followed by one-step oxidation. During the early stage of oxidation, the outer diffusion of tin cations and sulfur in SnS2 was faster than the inner diffusion of oxygen gas, which resulted in the formation of hollow SnO2 nanoplates. The hierarchical assembly of the hollow SnO2 nanoplates exhibited excellent Li-ion storage performance with high stability and good rate capability because of their substantial inner voids which increased the contact area
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