Two-Dimensional SnO Anodes with a Tunable Number of Atomic

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Two Dimensional SnO Anodes with Tunable Number of Atomic Layers for Sodium Ion Batteries Fan Zhang, Jiajie Zhu, Daliang Zhang, Udo Schwingenschlögl, and Husam N. Alshareef Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05280 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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Two Dimensional SnO Anodes with Tunable Number of Atomic Layers for Sodium Ion Batteries Fan Zhang, † Jiajie Zhu, † Daliang Zhang, ‡ Udo Schwingenschlögl, † Husam N. Alshareef*,† †Materials Science and Engineering, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ Imaging and Characterization Core Laboratories, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

* E-mail: [email protected]

ABSTRACT We have systematically changed the number of atomic layers stacked in 2D SnO nanosheet anodes and studied their Sodium Ion Battery (SIB) performance. The results indicate that as the number of atomic SnO layers in a sheet decreases, both the capacity and cycling stability of the Na ion battery improve. The thinnest SnO nanosheet anodes (2-6 SnO monolayers) exhibited the best performance. Specifically, an initial discharge and charge capacity of 1072 and 848 mAh g-1 were observed, respectively, at 0.1 A g-1. In addition, an impressive reversible capacity of 665 mAh g-1 after 100 cycles at 0.1 A g-1 and 452 mAh g-1 after 1000 cycles at a high current density of 1.0 A g-1 was observed, with excellent rate performance. As the average number of atomic layers in the anode sheets increased, the battery performance degraded significantly. For example, for the anode sheets with 10-20 atomic layers, only a reversible capacity of 389 mAh g-1 could be obtained after 100 cycles at 0.1 A g-1. Density functional theory calculations coupled with experimental results were used to elucidate the sodiation mechanism of the SnO nanosheets. This systematic study of monolayer-dependent physical and electrochemical properties of 2D anodes

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shows a promising pathway to engineer and mitigate volume changes in 2D anode materials for sodium ion batteries. It also demonstrates that ultra-thin SnO nanosheets are promising SIB anode materials with high specific capacity, stable cyclability, and excellent rate performance. KEYWORDS: Two dimensional SnO, SnO nanosheets, Sodium ion battery, High-rate batteries.

INTRODUCTION Two-dimensional (2D) materials, with atomic or molecular thickness and large lateral lengths, have emerged as important new materials due to their unique structure and properties1-3. Within each 2D monolayer, atoms are tightly connected by strong chemical bonds, while the layers are bound together by the weaker van der Waals bonds. Inspired by the intriguing properties of graphene, many efforts have been devoted during the last few years to preparing 2D inorganic materials beyond graphene4-8. Among the various groups of 2D materials, transition metal dichalcogenides (TMDs) such as MoS2, SnS2 and NbSe2 have attracted substantial attention, since many of them have an appropriate band gap and exhibit enhanced ionic diffusion properties. Thus, they are considered promising materials for developing next-generation semiconductor, electronic, and energy storage devices9-11. However, many TMDs are not sufficiently stable in air and will degrade or oxidize gradually. In contrast, 2D metal oxides are relatively more stable, and have lower atomic mass compared with the same cation based sulfides and selenides. Tin monoxide (SnO) is one such layered metal oxide, which has been demonstrated to have excellent physical and chemical properties12, 13. We were motivated to study this oxide in part because of its high theoretical capacity and because of its unique layered structure, as discussed in detail later in this introduction. Up to now, different kinds of 2D nanosheets have been used in the energy storage field14-


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. It is a commonplace to use graphene as an inactive matrix intermixed with the active material

to form composite structures for battery anodes. The special 2D nanostructure, superior electronic conductivity, and large specific surface area of graphene enhances the electrochemical performance of oxides used in such composites18, 19. Most of TMD nanosheets also show great potential as electrodes for lithium and sodium ion batteries (LIBs and SIBs)20, 21. It has been demonstrated that the large interlayer spacing of 2D nanosheets can effectively buffer the big volume expansion of these electrode materials, and prevent their collapse during the battery charge/discharge process. This nanoarchitecture greatly improves cycling stability and leads to high reversible capacity. Beyond that, ultra-thin nanosheets with the large specific surface area can significantly shorten the diffusion distance and provide faster diffusion channels and more reaction sites for ions and electrons, leading to a fast charge-discharge capability for lithium and sodium storage22,

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. Thus, it can be seen that the 2D nanosheet structures offer significant

advantages as electrodes for the batteries. 2D nanosheets, with thickness of 1-20 nm, and lateral size from tens to hundreds of nanometers, have been investigated for many applications such as catalysis24, batteries25, light harvesting26, and solid lubricants27. The layer-dependent parameters of nanosheets, layer thickness and lateral size, are known to strongly influence their physical properties, such as band-gap structure, optical absorption spectrum and so on28-30. However, in the electrochemical field, there are only a few reports on how the dimensions of nanosheets can affect electrochemical device performance. Gholamvand et al. used liquid cascade centrifugation method to prepare a series of dispersions of size-selected WS2 nanosheets, and utilized them as a model system to determine the influence of nanosheets dimensions in electrochemical double layer capacitor and hydrogen evolution electrocatalytic electrodes. The results show that the


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volumetric double layer capacitance scales inversely with nanosheets thickness and the catalytic density scales inversely with nanosheets lateral size31. So we have reason to believe that controlling the 2D nanosheets thickness will also have a significant influence on the electrochemical performance when the materials are used as SIB anodes. Sn-based materials are known high capacity anode materials for SIBs, since tin has a theoretical capacity of 847 mAh g-1 when alloying with sodium to form Na3.75Sn. However, the large volume expansion of tin (~420%) during the alloying process seriously degrades the cycling stability of Sn-based electrodes32. SnO, as mentioned earlier, can form a layered structure in the [001] crystallographic direction with Sn-O-Sn sequence. The tin atoms of SnO lose their two 5p orbital electrons to the more electronegative oxygen atoms, and the electrons of 5s orbital constitute a lone pair because they do not participate in the bonding process. In SnO, these lone pairs point toward the interlayer spacing and the resulting dipole–dipole interaction leads to a van der Waals gap with a large c parameter (4.84 Å) between the monolayers. The special 2D structure and large interlayer spacing can buffer the deleterious volume changes that occur during the charge/discharge process12. In addition, SnO can deliver a high theoretical capacity of 1150 mAh g-1, considering that 1 molar SnO can store 5.75 molar sodium through the conversion (2 molar) and alloying (3.75 molar) reactions. In this work, we used chemical synthesis to carefully prepare SnO nanosheets with controlled number of atomic layers and lateral size, and studied how these changes affect SnO properties and electrochemical performance as SIB anodes. Several chemical synthesis parameters were optimized to obtain the desired SnO nanosheet anodes with controlled number of atomic layers, including precursors, solvents, reducing agents, pH values, and reaction times.


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RESULTS AND DISCUSSION Material synthesis and characterization. Both bare SnO ultrathin nanosheets and SnO@Carbon Cloth (CC) samples were prepared by the hydrothermal method. Several process parameters were studied extensively to develop and optimize the SnO phase and its nanostructure (including precursors, solvents, reducing agents, pH values, and reaction time). Please see detailed discussions and Table S1 in the Supporting Information: Part 1. The samples selected for battery evaluation were SnO nanosheets which are 1-3 nm thick (2-6 SnO monolayers), 3-5nm thick (6-10 monolayers), and 5-10 nm thick (10-20 monolayers). These samples are henceforth referred as SnO-2L (2-6 SnO monolayers), SnO-6L (6-10 SnO monolayers), and SnO-10L (10-20 SnO monolayers), respectively. Figure 1a shows the X-ray diffraction (XRD) patterns of as-prepared samples. For the bare SnO nanosheets, all the characteristic peaks can be indexed as the tetragonal SnO phase with JCPDS No. 03-0695 (space group of P4/nnm), and lattice constants are a=b=3.80 Å and c=4.84 Å. The tetragonal layered crystal structure of SnO is shown in Figure 1b, where a large interlayer spacing can accommodate Na+ insertion and the layered structure provides twodimensional pathways along the ab plane for Na+ diffusion33. The SnO-2L samples show all the diffraction peaks of SnO; beyond that, one broad peak at about 25° and one small peak at about 43° can be observed, which can be assigned to the carbon cloth. Raman spectra of bare SnO and SnO-2L samples are shown in Figure 1c and d. The peaks at 120 cm−1 and 203 cm−1correspond to the A1g and Eg(1) vibration modes of SnO, respectively (the position of these peaks actually depends on the thickness of SnO nanosheets, which will be explained in more detail later). Figure S6 shows the vibration modes of SnO that are considered in this work. The A1g mode is the symmetry preserving mode, representing vibration along the c axis, which means Sn atoms


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vibrate towards or away from the O plane. While the Eg(1) mode involves movements in the ab plane, which means the vibration within each monolayer or the vibration between adjacent monolayers within the ab plane34-36. Carbon cloth can be characterized by the disorder-induced D-band and graphitic G-band at 1348 cm−1 and 1580 cm−1, respectively. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface composition of SnO-2L and chemical states of C, Sn and O elements, as shown in Figure S7. The XPS survey in Figure S7a shows that elemental C, Sn and O are present in the SnO-2L sample, as expected. In the high-resolution C 1s spectrum (Figure S7b), the detected peaks can be fitted into four peaks of carbon atoms in different functional groups: non-oxygenated carbon (sp2 C at 284.8 eV and sp3 C at 285.1 eV), carbon in C-O bonds (at 286.2 eV) and carboxyl carbon (O-C=O at 289.2 eV)37. Those non-oxygenated carbon derived from the carbon cloth itself, and the existence of oxygen-containing groups indicates that the carbon cloth has been successfully activated by concentrated hydrochloric acid before the hydrothermal process. The peaks at 486.2 and 495.1 eV in Figure S7c correspond to the Sn 3d5/2 and Sn 3d3/2 photoelectron emissions of Sn2+, respectively38, which means that Sn exists principally in the Sn2+ oxidation state. In Figure S7d, the binding energy of O 1s located at 531.0 eV is assigned to the bond of Sn-O, and two other peaks located at 531.8 and 533.0 eV are ascribed to the C-O and C=O bond39. Furthermore, the elemental ratio of Sn to O in SnO-2L is measured to be 1:1.08 by compositional analysis, which confirms the synthesis of pure SnO phase. Since the carbon cloth contains a few oxygen groups, it is expected that O content is slightly higher than Sn. Characterizations of different layered SnO ultra-thin nanosheets As we know, many properties of 2D materials depend strongly on the nanosheet dimensions. We have prepared three samples, designated as SnO-2L, SnO-6L and SnO-10L and varied their dimensions by increasing


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the reaction time, resulting in 2D SnO nanosheets with different number of atomic layers and lateral sizes. The effect of the size of these nanosheets on their physical and electrochemical properties was systematically investigated, as discussed in the following sections. Figure 2a, e, and i show SEM images of the SnO-2L, SnO-6L, and SnO-10L nanosheets at low magnification, respectively. Firstly, we can clearly see that densely packed and highly ordered SnO nanosheets were formed on the carbon cloth, such that the entire surface of carbon cloth is uniformly covered. Secondly, from 2L to 6 L to 10 L, the size of SnO nanosheets increases and the number of nanosheets per unit area becomes smaller. Higher magnification SEM and TEM images shown in Figure 2 exhibit more detail about the nature of SnO nanosheets. Figure 2b, f, and j demonstrate that the nanosheets are intimately interconnected, and form a network structure. However, their lateral size and thickness show significant differences. The three samples have their lateral size in the range of 30-50 nm (SnO-2L), 50-100 nm (SnO-6L), and 100-150 nm (SnO-10L), which can be confirmed by their TEM images of Figure 2c, g, and k. Since all the samples are ultra-thin nanosheets, they have a clear boundary and look transparent in their TEM images. In addition to this, combined with Figure S9, we can determine how the evolution of SnO nanosheets proceeds with time. Early in the process(2-h reaction time, as shown in Figure S9a and S9b), tiny seed crystals grow uniformly on the surface of the carbon cloth. However, there are some big particles attached on the surface, which are thought to be unreacted Sn2O2(OH)2 (see Supporting Information: Part 1). Increasing reaction time to 5 h (Figure S9c and S9d), the SnO ultra-thin nanosheets start to grow gradually while unreacted Sn2O2(OH)2 is dissolved and its amount are reduced. After 10 h reaction time, a homogeneous SnO nanosheet network will form, and with longer time, these nanosheets continue to grow bigger.


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High-resolution transmission electron microscopy (HRTEM) was used to better understand the structure of the SnO nanosheets. Figure 2d, h and l correspond to SnO-2L, SnO6L, and SnO-10L, respectively. We have intentionally selected nanosheets that are oriented such that we can examine the change in the thickness and number of atomic layers in each nanosheet. It can be clearly seen that the nanosheets of SnO-2L have the smallest thickness of about 1-3 nm, corresponding to 2-6 SnO monolayers (c=4.84 Å). The SnO-6L nanosheets have a thickness of 3-5 nm, corresponding to 6-10 monolayers. The SnO-10L nanosheets have a thickness of 5-10 nm, corresponding to 10-20 monolayers. It is worth mentioning that from the HRTEM images (Figure S10a, c and e), we can observe some black and white stripes, which correspond to the SnO monolayers and space between them, which is direct evidence of the layered nature of the SnO nanosheets22. From the top view images of the SnO nanosheets (Figure S10b, d, and f), we can see that their lateral dimensions also change with increasing reaction time. However, all nanosheets, independent of their size, exhibit the same crystal plane and selected area electron diffraction (SAED) pattern, indicating that they have the same phase and crystal structure. Energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were used to study the composition of the SnO nanosheets. As shown in Figure S11b, EDS data verify that all nanosheets consist of elemental Sn and O, further indicating the formation of pure SnO. Typical EELS spectra are shown in Figure S11c. It can be seen that the M4 and M5 edges of Sn were observed blow 530 eV, while the O-K edge makes the main contribution above 530 eV. SnO shows two characteristic peaks of almost equal intensity at around 540 eV with a narrow split, and exhibits a slightly less pronounced energy-loss near-edge structure (ELNES) around 570–575 eV, which can be used as a fingerprint for the SnO phase compared with other tin oxides40, 41. Beyond that, the elemental mapping of Sn and O from the STEM image (Figure S12)


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show a homogeneous distribution through the entire scan area, further indicating the uniform composition of SnO nanosheets. To acquire more accurate data about the thickness of SnO nanosheets, we performed atomic force microscopy (AFM) analysis. As shown in Figure 3a, b and c, the thinnest thickness of SnO-2L, SnO-6L and SnO-10L nanosheets are 1 nm, 3 nm and 5 nm, which correspond to two, six, and ten atomic layers of SnO, respectively. Overall, the thickness of these three samples are in the range of 1-3 nm, 3-5 nm and 5-10 nm, and the lateral sizes are in the range of 30-50 nm, 50-100 nm and 100-150 nm, respectively. According to the dimensional distributions shown in Figure 3d, the thickness and lateral size grow gradually with increasing reaction time. More information about the distribution of the thickness and lateral size are shown in the histograms of Figure S13. Thus, we have been able to chemically prepare 2D SnO nanosheets with controlled number of atomic layers and sizes, and we can use this system to study layer-dependent physical and electrochemical properties of two dimensional oxide anodes. Similar to graphene and TMD, the atoms within each SnO monolayer are joined together by covalent bonds, while different layers are joined together by van der Waals forces, which makes the physical and chemical properties of 2D SnO nanosheets dependent on the number of 2D layers. It has been shown that the Raman and UV-absorbance spectrum of 2D materials sensitive to the thickness and size of 2D nanosheets28-30; we show here, for the first time, that this trend is also observed in the case of SnO nanosheets, as shown in Figure 3e and f. From Figure 3e, we can observe the evolution of the peak parameters of Raman modes from bulk to ultrathin SnO nanosheets (bulk SnO was synthesized by a similar hydrothermal method which is described in Sample Preparation, and the XRD pattern and SEM images are shown in Figure S14). As the SnO nanosheet thickness increases from SnO-2L to bulk, the A1g and Eg(1) peaks


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show opposite trends: the A1g mode shows a red-shift of 9 cm-1 and the Eg(1) mode shows a blueshift of 7 cm-1, respectively. We have already explained that the A1g mode represents vibration along the c axis, while the Eg(1) mode involves movements in the ab plane. So, these shifts can be explained as follows. When the number of SnO atomic layers increases, the interlayer van der Waals force will be strengthened in the c direction of A1g mode, and the structure and long-range coulombic interlayer interactions for the Eg(1) mode will change28, 29. As shown in Figure 3f, the UV-absorbance spectroscopies of SnO nanosheets with different dimensions also show obvious differences. The confinement effects shift the wavelength of peaks, associated with the A-exciton, to higher values as the thickness increases from SnO-2L to bulk. Due to the edge effect, the spectral shape also has a significant change31, 42. Then N2 adsorption/desorption isotherm curves were used to obtain the specific surface area of the different size SnO nanosheets, as shown in Figure S15. The Brunauer–Emmett– Teller (BET) specific surface area of bare SnO, SnO-2L, SnO-6L, SnO-10L samples and bare carbon cloth are calculated to be 34.5, 163.6, 103.4, 49.2 and 3.5 m2 g-1. There are three points to note: (1) The SnO@CC sample has a significant bigger specific surface area compared with the bare SnO sample, even though it was prepared using a similar hydrothermal process. This is because the activated carbon cloth can provide more heterogeneous nucleation sites for SnO crystal, which results in uniform SnO nanosheet growth on the carbon cloth substrate without any aggregation. In contrast, the bare SnO sample is comprised of stacked nanosheets that have somewhat agglomerated, resulting in a decrease in the surface area; (2) With increasing reaction time, the specific surface area of SnO@CC samples show dramatic changes. In fact, the SnO-2L sample has the largest specific surface area, while the SnO-10L sample has the smallest one. We can understand this result in an intuitive way. It is clear that the outer layers make the main


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contribution to the specific surface area; thus, when the nanosheets grow thicker and bigger, more and more layers will become inner layers, which means that the surface area per unit mass will decrease; (3) The specific surface area of carbon cloth is very small so that we can neglect the contributions of carbon cloth to the whole SnO@CC samples. Specific surface area is a crucial material parameter considering their electrochemical properties because more specific surface area means shorter diffusion distance and more active reaction sites19. Electrochemical properties. Since many electrochemical properties of 2D materials depend on the thickness and size of nanosheets, we employed the SnO nanosheets as prepared in our study as a model system to systematically illustrate how the number of atomic layers in a 2D material can influence its performance as SIB anode. As-prepared bare SnO, SnO-2L, SnO-6L and SnO-10L samples were tested as anodes for SIB. Figure 4a and Figure S16a show the cycle voltammetry (CV) curves of the first three cycles of SnO-2L and bare SnO electrodes at a scan rate of 0.2 mV s-1 between 0.005 and 2.50 V. For the SnO-2L electrode, in the first cathodic process, the large reduction peak from 0.5 to 0.005 V can be attributed to the conversion reaction of SnO with Na to form Sn and Na2O matrix (Equation 1), and the alloying reaction between Na and Sn to form Na3.75Sn in the Na2O matrix43, 44

(Equation 2), as well as some contribution from irreversible decomposition of electrolyte,

which will form solid electrolyte interphase (SEI)18. In the reversed anodic process, two distinct peaks at 0.25 and 1.0 V are detected, which can be assigned to the reversible de-alloying reaction of Na3.75Sn and the restitution of SnO, respectively33. The voltammograms are superimposable perfectly after the first cycle, indicating excellent reversibility of SnO-2L for the sodiation and desodiation reactions. In comparison, the CV curves of bare SnO electrodes (Figure S16a) have similar peaks, indicating the same electrochemical reactions are taking place. However, the CV


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curves for the following two cycles do not overlap very well with each other, indicating poor cyclic performance for bare SnO electrodes. 2Na + +SnO+2e - → Na 2 O+Sn

(1)

Sn+3.75Na + +3.75e - → Na 3.75Sn

(2)

To gain a deeper insight into the electrochemical reactions and reversibility of SnO electrodes, ex-situ XRD was used to study the SnO anodes at different states of discharge and charge process in the first cycle as shown in Figure 4c. When the cell was fully discharged to 0.005 V, the XRD pattern shows that the SnO phase has almost disappeared, suggesting that SnO has been fully reduced by the Na+. In addition, the peaks of Na2O and Na3.75Sn can be detected, which can be attributed to the complete conversion of SnO to Sn and Na2O, followed by an alloying reaction between Sn and Na+. For the first charge process at 0.5 V, pure Sn and Na2O phases appear due to the de-alloying of Na3.75Sn. After fully charging to 2.5 V, the broad peaks of nanosized SnO reappear, but the peaks of Sn and Na2O still exist, with lower intensities, which means that the conversion reaction is not fully reversible. According to some previous reports, the Na2O matrix can form a backbone to retain the uniform morphology of nanoparticles (Sn and SnO); although this partially reversible reaction will somewhat reduce capacity, the matrix will hold together the nanoparticles, leading to an excellent stability and cyclability in the following cycles44. Based on all the experimental evidence, the schematic illustration of Figure 4d demonstrates the alloying and conversion reaction mechanism taking place in our SnO nanosheet electrodes during the charging and discharging process. It is worth mentioning that for many 2D Sn compounds such as SnS2 and SnSe2, there will be an obvious intercalation reaction during the first discharge process, but for our SnO electrode we cannot find any peaks corresponding to intercalation reaction in either the CV test or ex-situ XRD pattern. The DFT 12 ACS Paragon Plus Environment

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simulations confirm that intercalation reactions do not happen during the charge/discharge process of SnO electrode as shown Figure S21. Figure 4b and Figure S16b show the galvanostatic charge-discharge profiles for the 1st, 2nd, 10th, 50th, and 100th cycles of SnO-2L and bare SnO electrodes at a current density of 0.1 A g-1, respectively. For the SnO-2L electrode, the initial discharge and charge capacities are 1072 and 848 mAh g-1, corresponding to a coulombic efficiency of 79.1%, and the irreversible capacity loss can be attributed to the formation of SEI. (All the capacity values in this work are based on the mass of SnO, and we have confirmed that the carbon cloth contributes negligible capacity of 21 mAh g-1 which has been subtracted from the capacity of SnO anodes, see Figure S17). The discharge plateau located between 0.5 and 0.005 V corresponds to the conversion and alloying reactions, and the charge plateaus located at 0.25 and 1.0 V are assigned to the dealloying and reversed conversion reactions. All of these results are consistent with the CV results. From the second cycle, the electrode shows similar charge-discharge profiles, suggesting the sodiation and desodiation reactions are quite reversible, and after 100 cycles, the SnO-2L electrode exhibits a high reversible capacity of 665 mAh g-1. In contrast, not only the initial capacity of bare SnO electrode is lower than the SnO-2L electrode, but there is also severe capacity fading in the following cycles resulting in a reversible capacity of only 236 mAh g-1 after 100 cycles (see Figure S16b). Figure 4e shows the cycling performances of as-synthesized SnO-2L and bare SnO electrodes at a current density of 0.1 A g-1. The SnO-2L electrode demonstrates excellent reversibility and cycling stability and a reversible capacity as high as 665 mAh g-1 was sustained after 100 cycles (8.3% decay from the 10th to the 100th cycle). However, in the case of bare SnO, the first discharge and charge capacities were 972 and 745 mAh g-1, respectively, but the reversible capacities seriously declined to 236 mAh g-1 (45% decay from the


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10th to the 100th cycle). Figure 4f presents the rate performances of the SnO-2L and bare SnO electrodes at different current densities from 0.1 to 2 A g-1. As can be seen, the cell with SnO-2L electrode has a better rate capability, and the discharge capacities are 658, 602, 543, 472 and 410 mAh g-1 at current densities of 0.2, 0.3, 0.5, 1.0 and 2.0 A g-1, respectively. Moreover, the electrode has a capacity retention of 668 mAh g-1 when the rate abruptly switched back to 0.1 A g-1, further indicating the excellent cycling durability of the SnO-2L electrode. However, the bare SnO electrode only maintains a discharge capacity of 125 mAh g-1 at the current density of 2.0 A g-1, and a reversible capacity of 258 mAh g-1 is recovered when the current density is returned to 0.1 A g-1. The obvious electrochemical differences between bare SnO and SnO-2L electrodes can be explained by the following three points: (1) Carbon cloth can enhance heterogeneous nucleation of SnO nanosheets and provide more nucleation sites, which results in uniform SnO nanosheet growth on the carbon cloth substrate without any aggregation. Compared with bare SnO, the SnO-2L sample has a more uniform morphology and higher specific surface area (SEM & BET data from Figure S8 and S15), which will provide more electrochemical reaction sites and allow more space to buffer the large volume change that occurs during cycling processes. Beyond that, porous carbon cloth with a high capacity for electrolyte absorption can provide more diffusion channels for Na+; (2) The SnO-2L electrode has a higher electronic conductivity compared with the bare SnO electrode, which is demonstrated by Figure S18a and S18b, in which SnO-2L have a lower resistance both before cycling and after 100 cycles; (3) The excellent rate performances of SnO-2L indicates the high structural stability and low polarization


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of the SnO-2L hybrid structure. The high structural stability means better cycling performance, while low polarization suggests fast charge transfer and mass diffusion kinetics. Now we focus on the effect of the number of atomic layers in each SnO nanosheet. Figure 5a shows the schematic illustration of the SnO nanosheets with different number of layers and the half-cell structure which is used to study layered SnO as anode for Sodium ion battery. Figure 5b shows the cycle performances of the as-prepared SnO-2L, SnO-6L and SnO10L electrodes at 0.1 A g-1 between 0.005 and 2.5 V versus Na+/Na. Among the three samples, the SnO-2L electrode exhibits the highest reversible capacity and best cycling performance. In the first cycle, the discharge capacity of SnO-2L is 1072 mAh g-1, which is very close to its theoretical capacity (1150 mAh g-1, based on the complete alloying and conversion reactions). The initial charge capacity for SnO-2L is 848 mAh g-1 and the corresponding CE is 79.1%, which are reasonable values, considering the SEI formation and partially reversible conversion reaction we have mentioned (The coulombic efficiency data of SnO-2L, SnO-6L and SnO-10L electrodes at 0.1 A g-1 are shown in Figure S19a). In contrast, the initial discharge and charge capacities of SnO-6L are 985 and 830 mAh g-1, and for SnO-10L, they are 908 and 792 mAh g-1. Both of these values are lower than the SnO-2L anode. After the first cycle, CE of all the three samples quickly increases to about 99% in the following 10 cycles and remains at nearly 100% after that. The SnO-2L and SnO-6L anodes have similar cycling performance, where the capacity decay is very slow after 20 cycles, and a reversible capacity of 665 and 544 mAh g-1, respectively, are maintained after 100 cycles (this represents 5.4% and 9.2% decay from the 20th to the 100th cycle, respectively). However, for the SnO-10L electrode, the capacity continuously decreases with cycles, and a reversible capacity of 389 mAh g-1 is reached after 100 cycles (23.7% decay from the 20th to the 100th cycle).


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Besides stable cycling performance, the high-rate capacity of the electrodes is also critical for practical applications. For the evaluation of the high-rate and long-term capacity and cycle stability of the SnO-2L, SnO-6L and SnO-10L electrodes, they were galvanostatically discharged and charged at 0.1 A g-1 during the first 10 cycles for activation, and then at 1 A g-1 for the following 1000 cycles. From Figure 5c, we can see that after 10 cycles, the SnO-2L electrode delivers an initial discharge capacity of 492 mAh g-1 at 1 A g-1. The capacity retentions at 500th cycles and 1000th cycles are 94.3% (464 mAh g-1) and 91.8% (452 mAh g-1), respectively, indicating a superior high-rate cycling capacity and stability of SnO-2L electrode. In contrast, the SnO-6L electrode also has a good cycling performance, but the reversible capacity is just about 286 mAh g-1 after 1000 cycles (35% decay from the 10th to the 1000th cycle), which is much lower than the SnO-2L electrode. Moreover, the reversible capacity of SnO-10L electrode drops very fast from a maximum 445 mAh g-1 to 0 after about 200 cycles. The coulombic efficiency data of SnO-2L, SnO-6L and SnO-10L electrodes at 1.0 A g-1 are shown in Figure S19b. The significantly enhanced high-rate performances are closely related to the layered structure of SnO. Specifically, the large interlayer spacing of SnO and the number of monolayers in the nanosheets have strong effects on the transport of Na+ at the high rate without causing irreversible changes to the structure. The SnO-2L sample, which has the thinnest layers, the smallest lateral size, and the highest specific surface area exhibits the best performance. Electrochemical impedance spectroscopy (EIS) analysis was performed to understand the different electrochemical behavior between SnO-2L, SnO-6L and SnO-10L nanosheet anodes. The data before and after 100 cycles are shown in Figure S18c and S18d. The inset shows the equivalent circuit model used to fit the experimental data. It can be seen that all the plots exhibit a semicircle in the high-frequency region and a sloped line in the low-frequency region. From


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Figure S18c, we can see that the fresh electrodes have a small semicircle before cycling, and the charge-transfer resistance Rct of SnO-2L, SnO-6L and SnO-10L electrodes are 44, 60 and 103 Ω, respectively. While after 100 cycles, the Rct increased significantly to 85, 208 and 320 Ω, which may be due to the formation of the SEI during the cycling process. Compared with the other two samples, SnO-2L electrode demonstrates a much lower Rct and a smaller change after 100 cycles. This impedance change matches very well with the electrochemical performance of these three electrodes discussed earlier. In addition, the lowest resistance of SnO-2L electrode can be attributed to the following reasons: Thinner SnO nanosheets result in shorter electron transport distance and good contact between SnO and carbon cloth, which reduces the diffusion length of ions and improves the electrical conductivity of the electrodes. Ex-situ SEM images of SnO-2L, SnO-6L and SnO-10L electrodes before cycling and after 100 cycles at a current density of 100 mA g-1 are shown in Figure S20. It can be seen that the SnO nanosheets network of SnO-6L and SnO-10L electrodes significantly degraded after 100 cycles, and the structure of SnO nanosheets has been totally destroyed and pulverized. Even worse, parts of the structure of SnO have fallen off from the surface of carbon cloth, which resulted in significantly degraded electrochemical performances. In contrast, the SnO-2L electrode shows obviously better stability by retaining its nanosheet structure, which explains the high reversibility of the charge/discharge reactions. The superior electrochemical performances of the SnO-2L electrode and the dependence of electrochemical performance on the number of SnO atomic layers in the nanosheet has been clearly demonstrated. Several reasons can be invoked to explain these results. (1) From SEM, TEM and AFM data, we can see that SnO-2L nanosheets have fewer number of layers, smaller size, and a uniform morphology. The SnO-2L nanosheets form a robust network, just like a shell


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conformally covering the carbon cloth core. The conductive carbon fiber core and uniform boundaries between the nanosheets can provide faster diffusion channels for ions and electrons leading fast charge-discharge capability. (2) From BET data, it has been shown that SnO-2L nanosheets have the largest specific surface area compared to thicker nanosheets. Thus, thinner nanosheets expose more surface to the electrolyte and provide more activation sites for electrochemical reactions. In addition, larger surface area can more effectively buffer the large volume change that occurs during sodiation and desodiation process. (3) From the EIS results, we can see that the SnO-2L electrode has the lower charge-transfer resistance both before cycling and after 100 cycles. This high conductivity can allow the transport of sodium ions at high current density without irreversible change to the structure of the ultra-thin nanosheets, indicating an excellent high-rate performance. (4) The SnO-2L nanosheet also has the best structural stability, as demonstrated by the ex-situ SEM data. After 100 cycles, no obvious agglomeration or detachments were found in the SnO-2L. In contrast, the nanosheets with more atomic layers had partial or complete destruction of the network structure. Sn-based materials, including metallic Sn and Sn compounds, such as tin oxide (SnO, SnO2), tin sulfide (SnS/SnS2), tin selenide (SnSe/SnSe2), tin phosphide (SnP3/Sn4P3) and so on, have very high theoretical capacities. The reason is that tin is one of the most promising SIB anode materials, which alloys with up to 3.75 Na, leading to a capacity of 847 mAh g-1. Furthermore, in case of the fourth group Sn compounds, the conversion reactions that occur before Na-Sn alloying also have a large contribution to the whole capacity. What’s more, most of them have a 2D crystal structure (e.g., SnO, SnS2, Sn4P3). It has been proven that the large interlayer spacing between 2D monolayers is beneficial for the transport of Na+ at the high rate without causing irreversible changes to the structure23. Up to now, there have been many reports


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about the sodium storage abilities of Sn-based materials, as shown in Supporting Part 5: Table S2. They are promising candidates as the anode materials for SIBs with very high reversible capacities, excellent rate performance, and good stability. Among all these Sn-based materials, our ultra-thin SnO nanosheets electrode shows the best performance, reaching a high reversible specific capacity of 665 mAh g-1 at 0.1 A g-1 after 100 cycles and excellent performance at highrate.

CONCLUSIONS In this work, SnO nanosheets with controlled number of atomic layers were chemically prepared and systematically studied. The scaling of the number of atomic layers in SnO nanosheets was found to significantly influence their physical and electrochemical properties. The optimized sheets exhibit a high initial discharge and charge capacity of 1072 and 848 mAh g-1, a reversible capacity of 665 mAh g-1 after 100 cycles at 0.1 A g-1 , and a reversible capacity of 452 mAh g-1 after 1000 cycles at a high current density of 1.0 A g-1, with an excellent rate performance. These results, to our knowledge, are among the best performances for Sn-based SIB anodes. Our results indicate that the large volume changes typically observed in SnO anodes can signficantly be reduced by controlling the numebr of atomic layers in each SnO nanosheet. This work clearly demonstrates the importance of nanoscience for realizing aplications.

ASSOCIATED CONTENT Supporting Information


promising materials for SIB

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The Supporting Information is available free of charge on the ACS Publications website via the Internet at http://pubs.acs.org. Experimental section; Material characterization data including XPS, SEM, TEM, EDS, EELS, BET and elemental mapping of SnO nanosheets; XRD and SEM of bulk SnO; Histograms showing the distribution of the thickness and lateral size; Electrochemistry characterization data of bare SnO and carbon cloth; EIS and SEM data before and after 100 cycles; DFT calculation; Comparison between different Sn-based anodes in SIBs.

AUTHOR INFORMATION Corresponding Author *E-mail (Husam N. Alshareef): [email protected] Notes The authors declare no competing financial interest. Author contributions F.Z. and H.N.A. conceived and designed this work; F.Z. prepared all the samples and performed most of the material and electrochemical characterizations; J.J.Z. and U.S. performed DFT calculations; D.L.Z. carried out the HRTEM imaging and STEM mapping observation; F.Z. and H.N.A. wrote the paper; all the authors participated in analysis and discussions of the results and in preparing the manuscript.

ACKNOWLEDGEMENTS Research reported in this manuscript was supported by funding from King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia. Fan Zhang


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acknowledges supports from the KAUST Graduate Fellowship. Fan Zhang also thanks Dr. Hanfeng Liang, Mr. Guan Sheng, Dr. Bilal Ahmed, Mr. Qiu Jiang and Dr. Dhinesh Velusamy for their help. Figure 1b and 5a were produced by Mr. Heno Hwang, scientific illustrator at King Abdullah University of Science and Technology (KAUST).

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Figure 1. (a) XRD patterns of bare SnO and SnO-2L samples; (b) Schematic illustration of the crystal structure of 2D SnO; (c) and (d) Raman spectra of as-prepared bare SnO and SnO-2L in different scan ranges.


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Figure 2. (a-d) SEM and TEM images of SnO-2L nanosheets; (e-h) SEM and TEM images of SnO-6L nanosheets; (i-l) SEM and TEM images of SnO-10L nanosheets.


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Figure 3. (a-c) AFM micrographs with corresponding thickness and lateral size measurements of SnO-2L, SnO-6L and SnO-10L, respectively; (d) Thickness and lateral size distributions of three samples; (e) Raman spectrum of SnO-2L, SnO-6L, SnO-10L and bulk SnO, ranging from two layers to bulk material. This is the first report showing shift in Ramn peaks as a function on number of SnO 2D layers; (f) UV-absorbance spectrum

of

samples

containing

different


nanosheet

size.

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Figure 4. (a) CV curves of SnO-2L nanosheet electrodes; (b) Galvanostatic charge/discharge profiles of SnO2L electrode in the 1st, 2nd, 10th, 50th and 100th cycles at 0.1 A g-1; (c) Ex-situ XRD profiles of bare SnO electrode over the first charge/discharge cycle; (d) Schematic illustration of the reaction mechanism during the sodiation process; (e) Cycling performances of bare SnO and SnO-2L electrdes at 0.1 A g-1 for 100 cycles; (f) Rate capabilities of bare SnO and SnO-2L electrdes.


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Figure 5. (a) Schematic illustration of the SnO nanosheets with different number of layers and the half-cell structure; (b) Cycling performances of SnO-2L, SnO-6L and SnO-10L nanosheet electrodes at 0.1 A g-1 for 100 cycles; (c) Long-term cycle stabilities of SnO-2L, SnO-6L and SnO-10L electrodes at 1 A g-1 for 1000 cycles (0.1 A g-1 for the first 10 cycles and1 A g-1 for the next 1000 cycles).


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Two-Dimensional SnO Anodes with a Tunable Number of Atomic

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