Amorphous Tin Oxide Nanohelix Structure Based Electrode for Highly

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Amorphous Tin Oxide Nanohelix Structure Based Electrode for Highly Reversible Na-Ion Batteries Il Yong Choi,†,§ Changshin Jo,‡,||,§ Won-Gwang Lim,‡,⊥ Jong-Chan Han,† Byeong-Gyu Chae,† Chan Gyung Park,† Jinwoo Lee,*,⊥ and Jong Kyu Kim*,† Downloaded via BUFFALO STATE on July 22, 2019 at 20:56:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea ‡ Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea ⊥ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: An array of amorphous tin oxide (a-SnOx) nanohelixes (NHs) was fabricated on copper foil as an electrode for Na-ion batteries via the oblique angle deposition method, a solution- and surfactant-free process. The combination of the amorphous phase SnOx with a low oxidation number and its vertically aligned NH geometry with a large surface area and high porosity, which facilitate Na-ion dynamics and accommodate the volume changes, enabled a reversible capacity of up to 915 mA h g−1 after 50 cycles, fast rate capability with 48.1% retention at 2 A g−1, and high stability, which are superior to those of crystalline nanoparticle-based electrodes. KEYWORDS: amorphous tin oxide, nanohelix structures, oblique angle deposition, high reversibility, sodium-ion batteries

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reversibility of the electrochemical reaction between tin oxide and Na ions, which involves conversion and alloying reactions.10,11 Thus, tin oxides with low crystallinity, or even amorphous phase, would be beneficial for the insertion of Na ions into the electrode, preferentially through extended defects, resulting in an easy initiation of the breaking and rearrangement of tin (Sn) and oxygen (O) bonds.12−14 Despite such potential advantages of an amorphous anode, it is difficult to synthesize amorphous tin oxides by conventional solutionbased or surfactant-combined processes that require thermal annealing to remove remnant solvents and organic surfactants.15,16 Furthermore, high capacity anodes for SIBs suffer from severe volume expansion and hence structural degradation during repetitive Na-ion insertion/extraction reactions.9,17−20 Previous studies have reported improved cycle life and reversibility of anodes by applying nanostructured materials such as randomly distributed nanoparticles or nanowires; however, these structures require large amounts of a polymeric binder and carbon additive for maintaining the electrode integrity and electrical contact, which degrades both

odium-ion batteries (SIBs) have been identified as promising alternatives to lithium-ion batteries (LIBs) owing to the shortage and high price of lithium (Li) as well as the earth-abundance and low cost of sodium (Na).1,2 In order to demonstrate the economic and technological feasibility of SIBs, an improvement in the performance of both the anode and cathode is necessary because the energy and power densities of SIBs are not yet comparable with those of commercial LIBs. In particular, a lack of anode material with appropriate micro/nanostructure for sufficient Na storage, a large reversible capacity, and high structural stability remains an obstacle in developing high-performance SIBs. For example, graphite, a typical anode material used in LIBs, has shown insufficient Na insertion/extraction capacities when used in SIBs.3−6 Therefore, researchers have made substantial efforts to find new materials with appropriate micro/nanostructures for highly reversible and stable anodes for SIBs. Tin oxide (SnOx) has been considered as a promising anode material for SIBs owing to its earth-abundance, low cost, and a low operation potential.7,8 Theoretically, tin dioxide (SnO2) can deliver a high capacity up to 1378 mA h g−1, considering its conversion as well as alloying reactions with Na ions.9 Although many studies on tin oxides as anode materials for SIBs have been reported, there are several issues that should be considered and addressed. First, it is important to improve the © 2019 American Chemical Society

Received: December 28, 2018 Accepted: May 9, 2019 Published: May 9, 2019 6513

DOI: 10.1021/acsnano.8b09773 ACS Nano 2019, 13, 6513−6521

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Figure 1. (a) Schematic illustration of the oblique angle deposition method using an electron beam evaporator. (b) Nuclei of the deposited material form a self-shadowed region where subsequent incident vapor flux can no longer deposit, resulting in the formation of an array of slanted nanorods or vertically aligned NHs when the tilted substrate is rotated. (c) Schematic showing the changes occurring in the structure of SnOx NHs fabricated directly on a Cu substrate after discharging and charging processes. The empty spaces between the individual NHs that are laterally equidistant from each other enable great tolerance for the volume change during charging−discharging processes.

Figure 2. (a) SEM image of the array of SnOx NH structures. (b) HR-TEM image of the a-SnOx NH and SAED pattern acquired from the yellow square region in (b). (c) XRD patterns and (d) Raman spectra of the as-deposited SnOx NH, c-SnO2 NP, and c-SnO NP electrodes (scale bar: 1 μm for (a), 20 nm for (b)).

the volumetric and gravimetric capacities of the electrode.20,21 Consequently, the development of an amorphous tin oxidebased anode with an appropriate nanostructure for effectively

accommodating the volume changes during the charge− discharge processes via a surfactant-free fabrication process is strongly desired for the technological advancement of SIBs. 6514

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(Figure 2c) shows only a sharp peak of Cu(200) (JCPDS No. 04-0836) from the Cu substrate, indicating that the asdeposited SnOx NHs are amorphous. In contrast, the c-SnO2 NP and c-SnO NP electrodes show a number of XRD peaks (JCPDS No. 41-1445 and No. 06-0395, respectively) from the crystalline phase of SnO2 and SnO, respectively. The Raman spectra in Figure 2d show Raman peaks at 476.5 cm−1 (Eg), 634 cm−1 (A1g), and 775 cm−1 (B2g) for c-SnO2 and at 211 cm−1 (A1g) for c-SnO, whereas a very broad peak ranging from 300 to 800 cm−1 appears for the a-SnOx sample.24 The TEM, XRD, and Raman spectroscopic analyses confirm that the SnOx NHs grown on Cu foil by OAD are amorphous. Note that, with bottom-up approaches including solution coating of NPs, obtaining an amorphous phase metal oxide thin film is difficult because a high-temperature annealing process is necessary for removing unwanted materials, such as solvents, surfactants, or structure-directing agents, incorporated during the synthetic process. In contrast, with OAD, a physical vapor deposition process using an electron beam evaporator, an array of amorphous SnOx NHs can be easily fabricated directly on virtually any substrate at room temperature in a simple and easy way that is compatible with current microelectronics fabrication processes. Further, atomic-selective X-ray absorption spectroscopy of the tin oxide samples was performed using the 10C beamline of the Pohang Accelerator Laboratory. In the extended X-ray absorption fine structure (EXAFS) profiles shown in Figure 3a, the differences in the Fourier transform (FT) magnitudes in the second-shell interaction, which is dominated by the Sn−Sn interaction (ranging from a radial distance of 3 to 4 Å), between c-SnO2, c-SnO, and a-SnOx clearly show the existence of anisotropy in the a-SnOx owing to a lower local structural

In this study, we present an array of amorphous tin oxide (aSnOx) nanohelixes (NHs) on copper (Cu) foil fabricated by an oblique angle deposition (OAD) method as an efficient anode for SIBs. Compared to electrodes based on crystalline tin dioxide (c-SnO2) and tin monoxide (c-SnO) nanoparticles (NPs), an a-SnOx NH with high porosity and high aspect ratio shows superior electrochemical performance, including reversible specific capacity and stability even without the use of the carbon additive and polymeric binder.

RESULTS AND DISCUSSION Synthesis of Amorphous SnOx Nanohelixes on Cu Foil. An array of SnOx NHs was fabricated on Cu foil by OAD using an electron beam evaporator. During OAD, the vapor flux of SnOx has an incident angle (θ) with respect to the substrate surface-normal direction by tilting the substrate, as schematically demonstrated in Figure 1a. At the initial stage of deposition, nanoscale nuclei form on the substrate, creating a self-shadowed region where subsequent vapor flux can no longer deposit (Figure 1b). With the progression of the deposition process, the SnOx vapor deposits preferentially on top of the nuclei, forming a porous film composed of an array of slanted nanorods when the tilted substrate is held static or vertically aligned NHs when it is rotated. By adjusting the rotation speed and the deposition rate, one can control the geometrical shape of the NHs from vertically aligned nanorods to spring-like NHs with different diameters. Figure 1c shows a schematic illustration of the changes occurring in the structure of SnOx NHs after charge and discharge processes. Note that individual NHs in the vertically aligned array are laterally equidistant from each other and have empty spaces, which can enable great tolerance for the volume changes during the charge/discharge processes. The a-SnOx NH materials achieved better electrochemical performance compared with crystallized materials with the same nanostructure (see further discussion). Therefore, in order to investigate the reaction mechanism, commercially available tin oxides, crystalline SnO2 (c-SnO2) NPs, and crystalline SnO (c-SnO) NPs were also fabricated to serve as control electrodes. Figure 2a shows a scanning electron micrograph (SEM) of an array of SnOx NHs. A vertically aligned array consisting of 6-turn NHs with a high aspect ratio and large surface area is well-formed on the Cu substrate, with direct contact between the NHs and the current collector. This contact can facilitate a more facile electron transport as compared with the electrodes obtained by conventional coating of a slurry of NPs on the substrate. Vertically aligned individual NHs that are laterally separated from each other with equidistant empty space and not irregularly tangled up can enable facilitated diffusion of Na ions, promoting the reaction with large surface area of NHs, as well as substantial tolerance for isotropic volume changes during the charge/discharge processes. In addition, spring-like NH shape would be beneficial for mechanical stability during repeated operations.22 The empty spaces between the laterally equidistant NHs can be very beneficial not only for the diffusion of Na ions but also for accommodating the volume changes during charging−discharging. The high-resolution transmission electron micrograph (HR-TEM) shown in Figure 2b and the selected area electron diffraction pattern acquired from the region enclosed in a yellow box reveal the amorphous nature of the SnOx NHs formed by OAD via electron-beam evaporation at room temperature.23 The X-ray diffraction (XRD) pattern of the as-deposited SnOx NHs on the Cu foil

Figure 3. (a) EXAFS and (b) XANES profiles of the a-SnOx NH, cSnO2 NP, and c-SnO NP electrodes and Sn foil used as a reference. 6515

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Figure 4. (a, b, c) CV and (d, e, f) galvanostatic discharge−charge curves of SIBs with the a-SnOx NH electrode, c-SnO2 NP electrode, and cSnO NP electrode, respectively (CV scan rate: 0.1 mV s−1, current density: 20 mA g−1).

combination of the microstructural properties, i.e., the amorphous phase with a low oxidation number and the geometrical structure with a large surface area, high porosity for facilitating Na-ion dynamics, and accommodating the volume changes, enabled by the array of a-SnOx NHs is expected to result in highly enhanced electrochemical performance of the SIBs. Electrochemical Performance of a-SnOx NH Electrodes. Cyclic voltammetry (CV) measurements of the a-SnOx NH, c-SnO2 NP, and c-SnO NP electrodes were performed to investigate their electrochemical performances and the relevant mechanism of each electrode. The CV profiles of the different electrodes show quite different discharge−charge characteristics. The a-SnOx NH electrode exhibits an intense cathodic peak at 0.65 V at the first discharge (sodiation) process (Figure 4a). This peak is related to the initiation of the conversion reaction and the formation of a solid−electrolyte interphase (SEI) along the electrode surface, which will be discussed with the EXAFS analysis. A cathodic peak related to formation of a Sn−Na alloy is observed at the end of the cathodic scan. During the charging (desodiation) process, there are two

symmetry, further confirming the amorphous nature of the aSnOx NHs.12,25 The oxidation state of the transition metal in a transition metal oxide affects the electrochemical reaction for Na ion storage and, therefore, can play an important role in the electrical conductivity, reversibility, and stability of SIBs. The average oxidation number of Sn atoms in the a-SnOx NH electrode is estimated to be 2.2 (Figure S1), which is between those of c-SnO2 and c-SnO, based on the X-ray absorption near edge structure (XANES) profiles shown in Figure 3b.26,27 The oxidation state of Sn in a-SnOx NHs was determined by correlating the oxidation states of Sn metal, c-SnO, and c-SnO2 references and their edge energies, which were decided based on the points at which the second derivatives are zero in the XANES profiles.26,27 Metal oxides formed by physical vapor deposition under a high-vacuum condition (an oxygendeficient condition) typically have a lower oxidation number. It has been reported that such reduced form of transition metal oxides shows enhanced electrical conductivity and high electrochemical activity, resulting in a low electrode resistance, high reversibility, and high capacity.28−30 Therefore, the 6516

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Figure 5. Discharge−charge curves for the first cycle: (a) a-SnOx NH, (d) c-SnO2 NP, and (g) c-SnO NP electrodes. (b, c) Sn K-edge EXAFS profiles of the a-SnOx NHs at key reaction stages during the discharge and charge processes. (e, f) Sn K-edge EXAFS profiles of the c-SnO2 NPs and (h, i) Sn K-edge EXAFS profiles of the c-SnO NP electrode for discharge and charge processes.

distinct peaks, at 0.27 and 0.95 V, respectively, related to the dealloying and reoxidation of Sn to the SnOx phase. As the discharge−charge process continues, the cathodic and the anodic peaks get closer and reach a much higher reversible electrochemical reaction. However, the c-SnO2 NP electrode shows broad peaks during both discharging and charging processes (Figure 4b). Although reversible conversion and alloying reactions occur in the c-SnO2 NP electrode in the initial cycle, the cathodic and anodic (reoxidation at 1.0 V) peaks suffer from continuous decline in the intensity, indicating that the sodiation/desodiation processes are not occurring effectively in the c-SnO2 NP electrode. Owing to the lower oxidation state, the sodiation process in the c-SnO electrode (see CV profile in Figure 4c) begins at a lower potential compared with that of the c-SnO2 electrode. The cSnO NP electrode exhibits a similar profile to the a-SnOx NH electrode during the desodiation processes. However, peaks at ∼1.0 V for the c-SnO NP electrode during the charging processes are not stable with cycling. The reason for the enhanced reversibility of all the redox peaks in the CV profile of the a-SnOx NH electrode, compared with those of the crystalline electrodes, is probably due to the larger amount of defects including vacancies in the amorphous phase.31

Galvanostatic discharge−charge curves of the a-SnOx NH, cSnO2 NP, and c-SnO NP electrodes, shown in Figure 4d, e, and f, respectively, were obtained at 20 mA g−1 current density in the potential range of 2.5−0.002 V (vs Na/Na+). During the discharge process, the a-SnOx NH electrode shows a plateau at 1.0 V, followed by a sloped curve. During the charging process, it exhibits two capacity contributions in the 0.1−0.3 and 0.7− 1.3 V regions. The capacity of the 0.1−0.3 V region gradually increases with cycling, as shown in Figure 4a. Discharge and charge capacities in the first cycle for the a-SnOx NH electrode are 1414 and 751 mA h g−1, respectively (initial Coulombic efficiency (ICE) value is 53.1%). Compared with the a-SnOx NH, the c-SnO2 NP electrode shows a quite different discharge−charge profile including a steep decrease in the potential, a sloped curve up to 0.1 V, and a plateau, as shown in Figure 4b. On one hand, the discharge and charge capacities in the first cycle for the c-SnO2 NP electrode are 537 and 124 mA h g−1, respectively (ICE: 23.1%). On the other hand, the Na ion insertion plateau at 0.4−0.25 V contributes to most of the discharge capacity of the c-SnO NP electrode. The c-SnO NP electrode shows similar discharge−charge profiles to those of a-SnOx after the initial cycle (531 and 327 mA h g−1, ICE: 61.6%), and its reversible capacities increase with the number of cycles, consistent with the CV results. The relatively low 6517

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annealed a-SnOx NH electrode are shown in Figure S4. The annealed a-SnOx NH electrode shows lower reversible capacities than the bare a-SnOx NH electrode, indicating that the amorphous phase is more advantageous than the crystalline phase for electrochemical reaction of tin oxide with Na ions. The a-SnOx NH electrode annealed under H2/Ar ambient exhibits improved capacity compared with the same material annealed under Ar ambient, which may imply that crystalline Sn-based oxide with low oxidation state has a better electrochemical reaction with Na ions. It is consistent with better electrochemical performance of the c-SnO NP electrode than that of the c-SnO2 NP electrode, as shown in Figure 6a. On the other hand, in order to investigate the effect of geometrical shape and NH structure on the electrochemical performance, an a-SnOx thin-film electrode, i.e., non-nanostructured electrode, was prepared by electron-beam evaporation. The thin-film a-SnOx showed a similar initial sodiation

ICE of the a-SnOx NH electrode is attributed to the formation of an SEI layer on the NH structures with a large surface area. In order to trace the chemical reaction of the three electrodes during the discharge−charge processes, Sn K-edge EXAFS profiles of the a-SnOx NH, c-SnO2 NP, and c-SnO NP electrodes were analyzed ex situ at each key stage of the reaction (Figure 5a, d, and g). The a-SnOx NH electrode shows a Sn−O peak at the open-circuit voltage (OCV). During the discharge process, the Sn−O peak declines and the Sn−Sn peak grows, which is evidence for the conversion reaction (Figure 5b). This Sn−Sn peak becomes bigger at the fully sodiated state, indicating a further conversion reaction process occurs at lower potential. There is no direct evidence of the formation of a Na−Sn alloy during the sodiation process in the EXAFS profiles. This is likely caused by competitive processes between the formation of Sn metal by the conversion reaction and the formation of a Na−Sn alloy phase, decreasing the Sn− Sn peak (see TEM data in Figure S2). Yoon and co-workers reported that the reaction of SnO2 with Li+ makes Sn−Sn peaks grow during the conversion process, while the intensity of the peak decreases during further lithiation (alloying).8 During the charging process, there is a small difference in the intensity of the Sn−O and Sn−Sn peaks at 0.002 V (point 2) and 0.5 V (point 3) (Figure 5c) despite the contribution of a high capacity, ∼290 mA h g−1 (Figure 5a), which is correlated to the dealloying process. The Sn−O peak recovers whereas the Sn−Sn peak decreases during the charging process, with the EXAFS profile becoming similar to that of the OCV state, suggesting a highly reversible discharge−charge process. In the EXAFS profiles of the c-SnO2 NP electrode (Figure 5e and f), the first and second Sn−O peaks decrease during the discharging process, but the Sn−Sn peak in the profiles is negligible. Although the Sn−O bond is partially recovered, this reaction had low reversibility, as confirmed in the discharge− charge curves of the c-SnO2 NP electrode (Figure 5d). The disappearance of the second Sn−O peak after a full discharge− charge cycle indicates that the highly crystalline grain structure could not be recovered very well. In contrast, the EXAFS profiles of the c-SnO NP electrode (Figure 5h and i) exhibit a similar trend to those of the a-SnOx NH electrode. The results imply that the low oxidation state of Sn atoms in tin oxide, which can make less Na2O phase, is required for both conversion and alloying reactions to achieve a highly efficient Na-ion storage. The ex situ EXAFS results showing the conversion and alloying reactions of a-SnOx with Na ions are further supported by TEM and XRD analyses (Figures S2 and S3). Selective area electron diffraction (SAED) patterns of the a-SnOx NH electrode in Figure S2b show the existence of Na2O, Sn, and Na15Sn4 when the a-SnOx NH electrode is discharged to 0.35 V, which indicates the conversion and alloying reactions of the a-SnOx with Na ions. The SAED patterns of the a-SnOx NH electrode charged to 2.5 V in Figure S2d mainly show Sn and SnO phases. In the XRD study (Figure S3b), we found that amorphous SnOx was converted to Sn metallic phase after full discharge and converted to Sn and SnO phases after the charge process, which is coincident with the SAED pattern of the aSnOx NH electrode in Figure S2d. In order to investigate the effect of the amorphous phase on electrochemical performance, the a-SnOx NH was annealed under Ar (inert) ambient and H2/Ar (reductive) ambient at 350 °C for 2 h, respectively. Discharge−charge curves and cycle life of the SIBs with the a-SnOx NH electrode and the

Figure 6. (a) Cycle life (25 mA g−1 for the first 5 cycles and 100 mA g−1 for further cycles) and (b) rate capability profiles of the SIBs with a-SnOx NH, c-SnO2 NP, and c-SnO NP electrodes. (c) Comparison of the electrochemical performance of the electrode examined in this study with previously reported tin oxide-based anodes for SIBs.9,11,20,34−38 6518

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exhibiting an excellent electrochemical performance compared to SIBs with tin oxide-based anodes.

profile but a poor reversibility compared to the a-SnOx NH electrode, as shown in Figure S5. The capacity contribution from the reoxidation process (>0.5 V) disappeared from the second cycle, and the dealloying capacity (