Asymmetric-Layered Tin Thiophosphate: An Emerging 2D Ternary

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Asymmetric-Layered Tin Thiophosphate: An Emerging 2D Ternary Anode for HighPerformance Sodium Ion Full Cell Qinghua Liang,† Yun Zheng,‡ Chengfeng Du,† Yubo Luo,† Jin Zhao,† Hao Ren,† Jianwei Xu,‡ and Qingyu Yan*,† Downloaded via UNIV OF GOTHENBURG on December 7, 2018 at 02:23:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡ Institute of Materials Research and Engineering, A*STAR (Agency for Science; Technology and Research), 2 Fusionopolis Way Innovis #08-03, Singapore 138634, Singapore S Supporting Information *

ABSTRACT: The emerging sodium ion batteries (SIBs) are believed to be prospective substitutes for lithium ion batteries (LIBs) because of the wide distribution of sodium resources. However, to compensate for the sluggish reaction kinetics and higher intrinsic potential of Na+ compared to Li+, costeffective, reliable, and sustainable electrode materials must be explored for practical applications. Herein, 2D ternary tin thiophosphate (SnP2S6) nanosheets (∼10 nm thickness) grown on graphene (denoted as SPS/G hybrid) are demonstrated as intriguing anodes for SIBs. The asymmetric-layered structure and ternary composition enable the SPS/G hybrid with a high reversible capacity (1230 mAh g−1 at 50 mA g−1), superior rate capability (200 mAh g−1 at 15 A g−1), and an exceptional capacity retention of 76% after 1000 cycles at 2.0 A g−1. More importantly, a prototype sodium-ion full cell constructed by pairing with the Na3V2O2(PO4)3F cathode affords a high capacity of 470 mAh g−1 at 30 mA g−1 (on the basis of anode weight) and good cyclic capacity of 360 mAh g−1 at 150 mA g−1. Such 2D ternary chalcogenides with low-cost elements are promising materials for superior SIBs. KEYWORDS: tin thiophosphate, sodium ion battery, 2D nanomaterials, asymmetric-layered structure, ternary chalcogenides

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still challenging and of fundamental importance to enable the practical applications of SIBs in the future.25−29 It is known that the energy storage capability of a Na ion full cell is closely related to the electrochemical performance of the as-used anode. An ideal anode should be inexpensive and show a high capacity, good rate capability, as well as low reaction potential. The nanostructured conversion-type materials are of great interest due to their extremely high theoretical specific capacities.24,30−32 Notably, the electrochemical performance of such conversion-type anodes depends highly on their intrinsic chemical compositions.33 In particular, the phosphorus-based compounds like black phosphorus have the advantages of exceptional theoretical capacity (2596 mAh g−1), lower sodiation voltage, and elemental abundance.17,34 However, the large volume change and poor chemical stability seriously limit the rate and cycling performance, and the extremely high cost impedes the practical application of black phospho-

ince being commercialized in 1991, lithium ion batteries (LIBs) have dominated the global markets as energy storage systems (ESSs) for powering portable electronics, smart electrical grids, and electric vehicles because of their satisfactory energy density.1−9 However, the scarce abundance of Li resources, concerns about operational safety, and high costs have encouraged researchers around the world to develop alternative economic ESSs for renewable applications in the long run.1,10−15 Since sodium has a redox chemistry similar to that of lithium and is much more earth abundant and low cost, sodium ion batteries (SIBs) are being extensively studied as promising substitutes for LIBs.12,16−20 Nevertheless, the higher intrinsic redox potential, larger ionic radius, and heavier weight of Na+ compared to Li+ result in lower energy density, faster capacity decay, and poorer cycling life of the SIBs when adapting the active electrode materials, especially the anodes, commonly used for LIBs.21−24 Although many achievements have been made in the past few years, developing suitable electrode materials with inexpensive elements, high specific capacity, and good cyclic stability is © XXXX American Chemical Society

Received: October 27, 2018 Accepted: December 3, 2018 Published: December 3, 2018 A

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Figure 1. (a) Crystal model showing the asymmetric-layered structure of SnP2S6. (b) Schematic illustration of the in situ growth of ultrathin SnP2S6 nanosheets on graphene. (c) XRD patterns of the SnO2 nanosheets/graphene and SnP2S6 nanosheets/graphene hybrids. The standard XRD patterns of SnO2 (JCPDS No. 70-4177) and SnP2S6 (JCPDS No. 83-0721) are also listed for comparison.

A g−1 for 1000 cycles). More significantly, the as-constructed full battery delivers a specific capacity of 470 mAh g−1 at 30 mA g−1 as well as a capacity retention of 90.6% after 50 cycles at 150 mA g−1 in a potential range of 1.0−3.6 V (based on the weight of anode). Such impressive sodium-storage properties coupled with the simple preparation method enable such SPS/ G hybrid to be promising anode for practical applications.

rus.35−40 Exploring other potential anodes with high theoretical capacity and low cost is urgently desirable for commercial applications. In consideration of chemical composition, ternary layered tin thiophosphate (SnP2S6) is an intriguing candidate because of the following expected merits: (1) the intrinsically asymmetric-layered structure of SnP2S6 with internal electric field can locally accelerate the ion or electron migrations and improve electrochemical reaction dynamic;41 (2) the spacious configuration of SnP2S6 with a much larger interlayer spacing (∼0.65 nm) than Na+ radius (∼0.10 nm) ensures easy insertion and extraction of Na+; (3) an extremely high theoretical specific capacity of ∼1560 mAh g−1 is deemed to be expected due to the capacity contribution from forming Na3P, Na15Sn4 alloy, and Na2S intermediates; (4) the in situ formed discharge product of Na15Sn4 alloys with high electrical conductivity could improve charge-transfer kinetics and rate capability during the sodiation process. In spite of the above-mentioned advantages, such materials have not yet been studied for battery electrodes. Motivated by these findings, we here report the construction of a high-performance Na ion full cell based on the our newly developed asymmetric-layered SnP2S6 nanosheets as anode and the Na3V2(PO4)2O2F as cathode. To further enhance the electronic conductivity and decrease volume changes caused by the sodiation process, ultrathin SnP2S6 nanosheets were designed to in situ grow on the graphene matrix by forming a well-defined 2D heterostructural nanojunction. As demonstrated for anodes for the half-cell of SIBs, the 2D SnP2S6 nanosheets wrapped with a graphene (SPS/G) hybrid show an outstanding reversible capacity (1230 mAh g−1 at 50 mA g−1) and exceptional rate performance (330 mAh g−1 at 5.0 A g−1) as well as good cyclic stability (450 mAh g−1 after cycling at 2.0

RESULTS AND DISCUSSION As shown in Figure 1a, the ternary rhombohedral SnP2S6 has the same layered structure as Ni2P2S6 with an interlayer spacing of ∼0.65 nm.42,43 However, half of the cation positions are vacant, characterized by the occupation of octahedral sites (P2S6) by the P−P bonds between sulfur layers (Figure 1a).44,45 The intrinsic cation vacancies enable the SnP2S6 to be an asymmetric layered structure with internal polarity. Thus, SnP2S6 is a promising material for use in nonlinear optics, while there is as yet no report for applications in battery.44,45 Normally, the bulk SnP2S6 compounds were prepared by gastransport methods.44−46 The SnP2S6 nanostructures have yet to be reported up to now. We here achieve the in situ growth of 2D ultrathin SnP2S6 nanosheets on graphene through a twostep process as shown in Figure 1b. The first step refers to the growth of SnO2 nanodots on graphene by a hydrothermal treatment of Sn4+ in the aqueous dispersion of graphene oxide (GO). The abundant negatively charged oxygen-containing groups of GO are able to capture positively charged Sn4+ by strong electrostatic interactions. The subsequent heating results in the formation of SnO2 nanodots owing to the confinement effect of GO. The second step involves the in situ growth of 2D ultrathin SnP2S6 nanosheets on graphene by transforming the SnO2 nanodots/graphene with simultaneous B

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Figure 2. (a, b) SEM, (c) TEM, and (d) HERTEM images of the SnO2 nanodots/graphene composite. The inset in (c) is the corresponding SAED pattern. The inset in (d) is the zoomed inverse fast Fourier transform image.

Figure 3. (a, b) Low- and high-magnified SEM, (c) TEM, and (d and e) HRTEM images and (f) HAADF-STEM images showing C, P, S, and Sn elemental mapping of the SPS/G hybrid. The inset in (c) is the FFT pattern of (e).

adjacent SnO2 nanodots are easily merged together when reacting with S and P vapor under vacuum conditions, forming 2D ultrathin SnP2S6 nanosheets anchored on graphene (SPS/ G hybrid). It is noted that the graphene served as a growth template for the in situ formation of ultrathin SnP 2S6

sulfuration and phosphatization procedures through a solidstate method.47,48 Notably, this step could be accomplished in 10 min by heating the mixture with S and P powder at a relative low temperature of 360 °C because of the highly active and exposed surface of SnO2 nanodots. In addition, the C

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Figure 4. (a) XPS survey spectrum, (b) high-resolution P 2p, (c) S 2p, (d) Sn 3d, (e) C 2s XPS spectra, and (f) Raman spectrum of SPS/ graphene hybrid.

that the XRD pattern of SnP2S6 shows a much higher intensity ratio (∼3.0) of (003)/(11-3) than the standard value (∼0.72). This indicates that the resultant SnP2S6 shows a preferred orientation growth along the c direction, probably leading to the formation of lamellar structure. No characteristic diffraction peaks from graphene could be seen from the XRD pattern, demonstrating that the as formed SnP2S6 effectively prevents graphene from stacking during the thermal treatment process. More importantly, the SPS/G is very stable in atmosphere, as confirmed by the nearly unchanged XRD pattern after being placed in air for more than 6 months (see Figure S1).

nanosheets. The easy operation steps, inexpensive reactant, and mild reaction conditions make this method suitable for large-scale preparation. The powder X-ray diffraction (XRD) was performed to trace the phase evolution during the preparation process. As shown in Figure 1c, the XRD profile of the SnO2/graphene composite is consistent with the standard tetragonal SnO2 pattern (JCPDS No. 70-4177). All diffraction peaks show broad shape and weak intensity, indicating the small size of the asproduced SnO2. After thermal reaction with P and S, all prominent diffraction peaks can be perfectly indexed to the characteristic lines of rhombohedral SnP2S6 (JCPDS No. 830721) with the R3 space group. A careful observation reveals D

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Figure 5. Electrochemical performance of SnP2S6 nanosheets/graphene hybrid for Na ion half-cell. (a) CV curves for the first three cycles at 0.1 mV s−1. (b) Charge−discharge curves for the first three cycles at 0.05 A g−1. (c) Rate capability and corresponding Coulombic efficiency at different current densities ranging from 0.05 to 20 A g−1. (d) CV collected at 2.4 mV s−1 and the corresponding capacitive contribution (shaded area). Inset shows the plots for determining b values for anodic and cathodic peaks at 2.06 and 0.45 V, respectively. (e) Long-term cycling performance at 0.1, 0.5, and 2.0 A g−1.

Scanning electron microscopy (SEM) together with transmission electron microscopy (TEM) were conducted to verify the morphological evolution from SnO2/G to the SPS/G hybrid. As shown in Figure 2a, the panoramic SEM image of SnO2/G shows the hierarchical structure composed of abundant wrinkled nanosheets with an average lateral size and thickness of 1.0 μm and 8 nm, respectively. The magnified SEM image in Figure 2b reveals the rough and corrugated surface of the nanosheets. A more careful observation shows the presence of numerous nanodots with a mean size of ∼5 nm embedded evenly on the surface of graphene (inset in Figure

2b). It can be concluded that the hierarchical SnO2/G composite is composed of SnO2 nanodots anchored on the surface of corrugated graphene, as further verified by TEM observation. As displayed in Figure 2c, a homogeneous sheetlike structure covered with many black nanodots can be clearly observed. A series of concentric polycrystalline rings in the selected area electron diffraction (SAED) spectrum could be well indexed to all crystal planes of SnO2 and graphene (inset in Figure 2c). The high-resolution TEM (HRTEM) image also clearly reveals that numerous ultrasmall SnO2 nanodots connect tightly with each other and are uniformly E

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while the latter two bands are attributed to the associated Eg vibration modes of SnP2S6, giving further evidence of the formation of SnP2S6.44 Besides, two broad shifts located at 1350 and 1600 cm−1 represent the representative D and G peaks of graphene, respectively.50 Of note, the typical D and G bands move to higher wavenumbers compared with those of pure graphene (1340 and 1570 cm−1 for the D and G bands, respectively),50 indicating the strong electronic interaction between SnP2S6 and graphene in the SPS/hybrid. This is also supported by the obvious shift of P−S vibration after coupling with graphene in the Fourier transform infrared spectroscopy of the SPS/G hybrid (see Figure S3). The thermogravimetric analysis reveals that the weight percentage of SnP2S6 in the SPS/G hybrid is determined to be about ∼86 wt % (see Figure S4). The electrochemical Na-storage behavior of the resultant SPS/G hybrid was first studied in detail by assembling the halfcell with fresh sodium foil as anode. The cyclic voltammetry (CV) profiles for evaluating the electrochemical reactions with sodium metal of the initial three cycles are presented in Figure 5a. As can be seen from the initial cathodic sweep, a small peak located at 2.10 V is ascribed to the intercalation of layered SnP2S6 by sodium ions. Another redox peak at ∼1.54 V corresponds to the gradual sodiation of SnP2S6 as a result of forming NaxS intermediates.49 One broad peak located at 1.0 V could be caused by forming solid−electrolyte interphase (SEI) layers step by step.33 Another shoulder peak (∼0.50 V) could be induced by the alloying processes of Na15Sn4 and Na3P.51 In the inverse anodic sweep, two obvious peaks at 1.56 and 2.06 V are possibly indicative of the NaxS desodiation reactions. A bump centered at ∼1.20 V is possibly indicative of the dealloying process of Na15Sn4 and Na3P.52 The outlines of CV profiles almost overlap in the following two cycles, suggesting that the followed electrochemical reaction is reversible. On the basis of above results, the sodium storage mechanism of SnP2S6 can be summarized as following equation by delivering a theoretical capacity of ∼1560 mAh g−1.

distributed in the graphene matrix (Figure 2d). The lattice fringe of 0.34 nm can be assigned to the d-spacing of (110) lattice plane of tetragonal SnO2 (inset in Figure 2d). Both the SEM and TEM observations confirm the formation of the SnO2 nanodots/graphene composite after hydrothermal treatment. The morphology and microstructure of the resultant SPS/G hybrid were also examined by SEM and TEM in detail. As shown in Figure 3a, the SPS/G hybrid retains the 3D hierarchical structure of SnO2 nanodots/graphene. The EDS pattern shows the main presence of C, O, P, S, and Sn elements (see Figure S2). The SnP2S6 nanosheets with an average lateral size of ∼100 nm distribute uniformly on the flexible graphene substrate. A highly magnified SEM image reveals that the SnP2S6 nanosheets have an average thickness about 10 nm (Figure 3b). The TEM observation also shows a similar result in that the SnP2S6 nanosheets are well anchored on graphene (Figure 3c). Notably, there are many voids between adjacent SnP2S6 nanosheets. In addition, the HRTEM images clearly reveal two sets of interplanar distances of ∼0.66 and 0.50 nm (Figure 3d ,e), which correspond to the (003) and (101) planes of rhombohedral SnP2S6, respectively. The corresponding fast Fourier transformation (FFT) pattern shows the characteristic spots with a hexagonal symmetry (inset in Figure 3c), suggesting the single crystalline nature of the resultant SnP2S6 nanosheets. Additionally, the elemental mapping images taken under the mode of high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) demonstrate that C, P, S, and Sn elements are homogeneously distributed in the sample (Figure 3f). The SEM and TEM observations further verify the complete transformation of SnO2/G to SnP2S6/G after vacuum annealing under P and S atmosphere. To elucidate the chemical compositions and valence state of the SPS/G hybrid, we carried out X-ray photoelectron spectroscopy (XPS) measurements. The full XPS spectrum in Figure 4a reveals the predominant signals of Sn, P, S, C, and O elements without any other impurity. The atomic ratio of Sn, P, and S is in close proximity to the theoretical value (1:2:6). The oxygen signal mainly comes from the water or oxygen absorbed on the surface of the SPS/G hybrid. The narrowly scanned XPS spectrum of P 2p can be fitted to double peaks at 134.8 and 132.6 eV, which are in line with the P 2p1/2 and P 2p3/2 states, respectively. The doublet spin−orbit of S 2p1/2 and S 2p3/2 at 163.8 and 162.6 eV of the S 2p spectrum confirm the existence of the S2− oxidation state of the SPS/G hybrid (Figure 4c), similar as that of NiPS3.48,49 More indicatively, the high-resolution Sn 3d profile can be mainly fitted with two components at 495.9 eV (3d3/2) and 487.5 eV (3d5/2), matching well with the binding energy of Sn(IV) (Figure 4d).46 Furthermore, two characteristic peaks at 286.6 and 285.0 eV, which are, respectively, ascribed from the binding energies of C−O and C−C bonds,50 were obtained after a deconvolution of the broad peak of C 1s (Figure 4e). A semiquantitative analysis by calculating the peak area ratio of 286.6 and 285.0 eV shows a much higher content of C−C component (∼90%), demonstrating the nearly complete removal of the oxygen-containing groups in GO in the SPS/ G hybrid. The coexistence of SnP2S6 and graphene in the SPS/ G hybrid is further confirmed by Raman test. As shown in Figure 4f, a set of intense peaks at 141, 266, 187, and 310 cm−1 can be observed in the low wavenumber region. The former two bands are assigned to the Ag vibration modes of SnP2S6,

SnP2S6 + 21.75Na + + 21.75e− → 0.25Na15Sn4 + 2Na3P + 6Na 2S

The ex situ TEM analysis and XRD measurements of the electrode at different potentials also confirm the formation of Sn, NaxP, NaxS, and Na15Sn4 intermediates (see Figures S5− S7). The nearly reversible desodiations of NaxSn, NaxP, and NaxS enwrapped by graphene contribute to the high specific capacity during the subsequent charging process. Accordingly, the galvanostatic charge−discharge (GCD) technology was conducted for quantitatively evaluate the Nastorage performance of the SPS/G hybrid. As shown in Figure 5b, the first discharge curve mainly contains four plateau regions at 2.25−1.8 V, 1.65−1.45, 1.3−1.0, and 0.8−1.3 V, while the followed charge profile shows three sloping regions at 1.5−1.8, 1.0−1.3, and 0.4−0.8 V. The initial GCD curves reveal a similar feature of electrochemical reaction as that of the above CV results. In the first cycle, the SPS/G electrode delivers the charge/discharge capacities of 1253 and 2062 mAh g−1 at 50 mA g−1 (based on the total weight of SPS/G hybrid), respectively, showing the first cycle colombic efficiency of 60.8%. The capacity loss could be owing to the SEI layer formation. The subsequent two GCD cycles show a reversible capacity of ∼1230 mAh g−1. This value approaches the theoretical sodium storage of SnP2S6 after normalizing the weight of SPS/G (∼1450 mAh g−1), corresponding well with above-mentioned reaction mechanism. Notably, the discharge F

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Figure 6. (a) Schematic illustration of the structure of Na ion full cell. (b) The typical charge/discharge curves of SnP2S6 nanosheets/ graphene and Na3V2O2(PO4)2F at 0.05 A g−1. (c) Charge/discharge curves of the full cell at different current densities. The specific capacity of the full cell is calculated based on the weight of anode. (d) Cycling performance of the Na ion full cell at the current density of 0.15 A g−1. The inset is the charge/discharge curves of the first and fiftieth cycles.

extracting the slope (b) from the plot of (log i) vs (log v), in which i refers to the tested current and v is the sweep speed.53 Both the as-determined values of b for both oxidation and reduction peaks are larger than 0.5 (inset in Figure 5d and see Figure S8), implying that the electrochemical kinetics of the SPS/G hybrid are controlled by both diffusion insertion and surface capacitive processes.54,55 The insertion reaction contributes more capacity when the charging/discharging at a low current density, while the capacitive contribution from surface reaction is the dominative process as subjected to a high current density. For example, according to the typical calculation method proposed by Dunn et al.,53 about 58% of the total capacity comes from the capacitive contribution when the sweep rate is 2.4 mV s−1 (Figure 5d). This is related to the improved electronic conductivity for fast electron transfer during fast charging and discharging processes. More importantly, the SPS/G hybrid shows a stable sodiumstorage capability during the long-term repeated sodiation/ desodiation processes. As shown in Figure 5e, capacity retentions of 91% (after 200 cycles), 88% (after 400 cycles), and 76% (after 1000 cycles) were obtained at 0.1, 0.5, and 2.0 A g−1, respectively. Such cycling stability of the SPS/G hybrid is better than that of many newly developed Sn-based anodes for SIBs (Table S1). Such a good cycling performance of the

capacity is more than 2-fold higher than that of the SnO2 nanodots/graphene precursor at 50 mA g−1 (∼480 mAh g−1). Such sodium-storage capability of the SPS/G hybrid surpasses that of most recently developed tin- or sulfur-containing anodes (Table S1). This is mainly attributed to the asymmetric-layered structure and intrinsic ternary composition of the 2D ultrathin SnP2S6 nanosheets. In addition, the coupling with good geometric confinement by graphene is anticipated to improve the Na-storage capacity, reversibility, and stability. Furthermore, the SPS/G anode also shows a good rate capability. As the current density is boosted from 0.05 to 2.0 A g−1 (Figure 5c), the average discharging capacity of SPS/G hybrid (600 mAh g−1) is about 8 times than that of SnO2 nanodots/graphene (75 mAh g−1). More predominantly, the SPS/G hybrid still delivers average capacities of 458, 338, 266, 200, and 118 mAh g−1 even at 5.0, 8.0, 10, 15, and 20 A g−1, respectively. This indicates the reversible electrochemical reaction of SPS/G hybrid with Na+ even at fast charging/ discharging processes. To gain more insight into the underlying reasons for such a high rate performance of the SPS/G hybrid, we further study the electrochemical reaction kinetics by collecting the CV profiles at different sweep rates. An insightful analysis of the CV curves was performed by G

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could be also retained after 1000 cycles (2.0 A g−1). More significantly, the Na ion full cell delivers stable discharge capacity of 260 mAh g−1 after 50 cycle (0.15 A g−1). The favorable composition with low cost elements, simple preparation process, the intriguing Na-storage performance enables the 2D asymmetric-layered SnP2S6 nanosheets to be promising anodes for sodium-ion batteries.

SPS/G hybrid can be attributed to the following aspects. On one hand, the space voids between adjacent SnP2S6 nanosheets enwrapped by flexible graphene ensures good accommodation of volume change caused by the repeated sodiation/ disodiation processes. Besides, the ultrathin 2D heterostructural nanojunction of SnP2S6 nanosheets and graphene is favorable for fast mass transport and rapid electron transfer due to the intimate contact with electrolyte and good electrical conductivity. This is also confirmed by the similar electrochemical impedance spectrum after cycling process (see Figure S9). The SEM observation also verifies the good structural integrity of SPS/G hybrid electrode without obvious generation of cracks after cycling (see Figure S10). Such a high specific capacity coupled with a good rate capability enable the SPS/G hybrid to be a promising anode for practical applications. As a proof of demonstration for that, a prototype Na-ion full cell was constructed by pairing with Na 3 V 2 O 2 (PO 4 ) 2 F cathode (Figure 6a). The as-used Na3V2O2(PO4)2F cathode shows similar phase structure and morphology to a previous report (see Figure S11).56 The representative charging/discharging profiles of the resultant Na3V2O2(PO4)2F electrode are shown in Figure 6b for comparison with the SPS/hybrid. When testing in half-cell, the Na3V2O2(PO4)2F cathode achieves a discharging specific capacity of ∼120 mAh g−1 with two evident plateaus at 4.0 and 3.5 V. As shown in Figure 6c, the typical discharge profile of the sodium ion full battery shows an exceptional specific capacity of 470 mAh g−1 at 30 mA g−1 in the potential range of 1.0−3.6 V (based on the weight of anode). Apparently, there are two discharge plateaus located at 3.2−2.4 and 1.8−1.2 V, possibly resulting from two discharge plateaus of the Na3V2O2(PO4)2F cathode. Moreover, as the current density is elevated to 1.0 A g−1, a specific capacity of 260 mAh g−1 could be obtained as well, indicating the high rate capability of the sodium ion battery. Notably, the different charge/discharge profiles collected at slow and fast charging/discharging processes may be caused by the mismatch of the rate capability between the cathode and anode. Furthermore, the Na ion full cell also shows good cycling stability, maintaining 86% capacity of the fist cycle after 50 cycles at 150 mA g−1 (Figure 6d). The charge/discharge curves before and after cycling are also nearly overlapped. In addition, the Na ion full cell keeps 78% of the capacity after testing at 1.0 A g−1 for 100 cycles (see Figure S12), suggesting the good stability of both cathode and anode. Such a superior performance of the Na ion full cell indicates the potential of the SPS/G hybrid as an anode for practical applications.

EXPERIMENTAL METHODS Preparation of SnO2 Nanodots/Graphene Composite. The freeze-dried graphene oxide (GO) was well dispersed in water for 4.0 h by tip sonication with a concentration of 4.0 mg mL−1. Then, SnCl4 (100 μL) was diluted with cold water (20 mL) for immediate use to avoid the hydrolysis of Sn4+. Afterward, the as-obtained aqueous Sn4+ solution was dropwise added to GO aqueous dispersion (20 mL) under continuous stirring. Subsequently, the resultant mixed solution was transferred to a 50 mL Teflon reactor for hydrothermal treatment at 80 °C for 6 h. After an alternative wash with water and alcohol three times, the final SnO2 nanodots/graphene was obtained after freeze-drying. During this step, GO could be partially reduced and prevented from restacking owing to the separation by SnO2 nanodots. Preparation of SnP2S6 Nanosheets/Graphene Hybrid. The SnP2S6 nanosheets/graphene (SPS/G) hybrid was obtained by an easy and scalable solid-state route by the use of the resultant SnO2 nanodots/graphene as precursor. Typically, 100 mg of SnO2 nanodots/graphene was uniformly mixed with stoichiometric red phosphorus and sulfur. Afterward, the as-obtained mixture was filled into a quartz tube (length = 200 mm, inner diameter = 10 mm). Then the quartz tube was sealed by oxy-hydrogen flame under vacuum (Partulab MRVS-1002).48 Subsequently, the sealed tube was subject to a thermal treatment at 360 °C for 10 min. Finally, the SPS/G hybrid was obtained after removing the unreacted species by washing with HCCl3 and CS2. During this step, the residual oxygen groups of graphene could be further subjected to reduction after thermal treatment. Safety notes: The quartz glass tube should be opened carefully because of the possibly high internal pressure. Electrochemical Measurements. The CR2032 coin-type cell constructed inside an argon-filled glovebox was carried out to evaluate the electrochemical properties of the electrodes. The mixture of active electrode material (80 wt %), single-wall carbon nanotube (10 wt %), and polyvinylidene fluoride (10 wt %) uniformly coated on copper or aluminum film was used as working electrode after total vacuum drying. The half-cells were assembled with a fresh sodium slice as the anode. NaClO4 dissolved in a propylene carbonate (1.0 mol L−1) with the presence of 5 vol % fluoroethylene carbonate (FEC) was explored as the electrolyte. The full-cell was assembled using the Na3V2(PO4)2O2F coated on Al film as cathode (Supporting Information). The weight ratio of cathode and anode is about 9:1 to improve the utilization of cathode. A piece of glass fiber was used as the separator. The anode was first activated at 50 mA g−1 in half-cell for one cycle before assembling full cell. The cyclic voltammetry (CV) measurements were collected on a Solartron electrochemical analyzer (England). The galvanostatic discharge−charge (GCD) test was carried out on a Neware BTS station. The potential window of 0.01− 3.0 V (vs Na+/Na) is selected for testing the half-cell. All batteries were placed for 24 h before testing. Details of the other instrumental characterization and measurements testing setup of materials can be found in the Supporting Information.

CONCLUSIONS In summary, we report the design of a high-performance sodium ion full cell based on the use of asymmetric-layered SnP2S6 nanosheets as anode and Na3V2O2(PO4)2F as cathode. A facile and scalable strategy was achieved for in situ formation of 2D ternary SnP2S6 nanosheets on graphene (SPS/G hybrid). The resultant SPS/G hybrid possesses the structural advantage of a 2D heterostructural nanojunction. Benefiting from the combined structural and compositional advantages, the SPS/G hybrid shows good electrical conductivity and mechanical resilience to realize rapid mass and electron transfer as well as accommodate volume change during electrochemical process. When explored as the cathode in SIB half-cell, the SPS/G hybrid exhibits a specific capacity of 1230 mAh g−1 at 50 mA g−1. A high capacity of 450 mAh g−1

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b08229. Experimental details, Figures S1−S12, and Table S1. Instrumental characterization and measurements; preparation of Na3V2O2(PO4)2F; XRD pattern of SPS/G hydrid, EDS pattern, FTIR, and TGA curves; TEM and H

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HRTEM images, SAED and XRD patterns of the SPS/G hybrid after discharging at different potentials; CV profiles of the SPS/G hybrid at various sweep rates; EIS curves of the Na ion half-cell; EDS pattern of the SPS/ hybrid after cycling; XRD and EDS patterns of the Na3V2O2(PO4)2F cathode; cycling performance of the Na ion full cell (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jianwei Xu: 0000-0003-3945-5443 Qingyu Yan: 0000-0003-0317-3225 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the Singapore EMA project EIRP 12/ NRF2015EWT-EIRP002-008, Singapore MOE AcRF Tier 1 under Grant Nos. RG113/15 and 2016-T1-002-065, and Singapore MOE AcRF Tier 2 under Grant Nos. 2017-T2-2069 and 2018-T2-01-010 are greatly appreciated. The authors also greatly acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS) of Nanyang Technological University, Singapore, for using their TEM, SEM, and XRD equipment. REFERENCES (1) Nayak, P. K.; Yang, L.; Brehm, W.; Adelhelm, P. From LithiumIon to Sodium-Ion Batteries: A Materials Perspective. Angew. Chem., Int. Ed. 2018, 57, 102−120. (2) Wang, P.-F.; Yao, H.-R.; Liu, X.-Y.; Yin, Y.-X.; Zhang, J.-N.; Wen, Y.; Yu, X.; Gu, L.; Guo, Y.-G. Na+/Vacancy Disordering Promises High-Rate Na-Ion Batteries. Sci. Adv. 2018, 4, eaar6018. (3) Zhang, J.; Wang, D.; Lv, W.; Zhang, S.; Liang, Q.; Zheng, D.; Kang, F.; Yang, Q. H. Achieving Superb Sodium Storage Performance on Carbon Anodes through Ether-Derived Solid Electrolyte Interphase. Energy Environ. Sci. 2017, 10, 370−376. (4) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117, 10403−10473. (5) Zhao, Y.; Li, X.; Yan, B.; Xiong, D.; Li, D.; Lawes, S.; Sun, X. Recent Developments and Understanding of Novel Mixed TransitionMetal Oxides As Anodes in Lithium Ion Batteries. Adv. Energy Mater. 2016, 6, 1502175. (6) Niu, S.; Wang, Z.; Yu, M.; Yu, M.; Xiu, L.; Wang, S.; Wu, X.; Qiu, J. MXene-Based Electrode with Enhanced Pseudocapacitance and Volumetric Capacity for Power-Type and Ultra-Long Life Lithium Storage. ACS Nano 2018, 12, 3928−3937. (7) Kim, C.; Song, G.; Luo, L.; Cheong, J. Y.; Cho, S.-H.; Kwon, D.; Choi, S.; Jung, J.-W.; Wang, C.-M.; Kim, I.-D.; et al. Stress-Tolerant Nanoporous Germanium Nanofibers for Long Cycle Life Lithium Storage with High Structural Stability. ACS Nano 2018, 12, 8169− 8176. (8) Yang, X.; Wang, J.; Wang, S.; Wang, H.; Tomanec, O.; Zhi, C.; Zboril, R.; Yu, D. Y. W.; Rogach, A. Vapor-Infiltration Approach Toward Selenium/Reduced Graphene Oxide Composites Enabling Stable and High-Capacity Sodium Storage. ACS Nano 2018, 12, 7397−7405. (9) Sun, J.; Lv, C.; Lv, F.; Chen, S.; Li, D.; Guo, Z.; Han, W.; Yang, D.; Guo, S. Tuning the Shell Number of Multishelled Metal Oxide Hollow Fibers for Optimized Lithium-Ion Storage. ACS Nano 2017, 11, 6186−6193. I

DOI: 10.1021/acsnano.8b08229 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.8b08229 ACS Nano XXXX, XXX, XXX−XXX