Ionothermal synthesis and electrochemical properties of a selenidostannate containing the mixed cations of Na+ and enH+

Author’s Accepted Manuscript Ionothermal synthesis and electrochemical properties of a selenidostannate containing the mixed cations of Na+ and enH+ Cheng-Feng Du, Jian-Rong Li, Nan-Nan Shen, Xiao-Ying Huang www.elsevier.com/locate/yjssc

PII: DOI: Reference:

S0022-4596(16)30108-6 http://dx.doi.org/10.1016/j.jssc.2016.03.036 YJSSC19332

To appear in: Journal of Solid State Chemistry Received date: 31 December 2015 Revised date: 10 March 2016 Accepted date: 20 March 2016 Cite this article as: Cheng-Feng Du, Jian-Rong Li, Nan-Nan Shen and Xiao-Ying Huang, Ionothermal synthesis and electrochemical properties of a selenidostannate containing the mixed cations of Na+ and enH+, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2016.03.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ionothermal synthesis and electrochemical properties of a selenidostannate containing the mixed cations of Na+ and enH+ Cheng-Feng Dua,b. Jian-Rong Lia. Nan-Nan Shena,b. Xiao-Ying Huanga* a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, People’s Republic of China. b University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China. *

Tel: +86 591 83793727; fax: +86 591 83793727; [email protected]

Abstract In this report, by taking advantages of the competitive and synergistic effects of mixed-cations under the ionothermal conditions, a novel selenidostannate compound containing the mixed cations, namely (enH)3Na[Sn3Se7]2·1.5Me2NH·1.5H2O (1, en = ethylenediamine, Me2NH = dimethylamine) was obtained in the presence of the ionic liquid (IL) [DAMS]I (4-N,N-Dimethylamino-4′-N′-methylstilbazolium iodide) with a bulky and more structurally rigid organic cation as the main solvent and en as the auxiliary solvent. The compound features anionic [Sn3Se7]n2n- layers that are interconnected by sodium ions to form a three-dimensional (3D) structure. The syntheses, structure, thermal, optical and electrochemical properties of 1 were investigated. 1 showed good thermal stability and a Na+ ion storage property without capacity fading over 150 cycles.

Keywords: ionothermal synthesis; selenidostannate; Na-ion storage.

1. Introduction Crystalline main-group metal chalcogenides (MMCs) have received increasing attention in the past decades due to their fascinating architectures, topologies and unique properties.[1-4] Particularly, the crystalline MMCs with porous structure are desirable for potential applications in fields of ion exchange, [5] photocatalysis,[6] fast ion conductivity[7], and energy storage.[8] In the case of IV-VI MMCs, much effort has been made in the synthesis of crystalline compounds via the so-called “one-pot hydro/solvothermal synthesis” in the presence of various structure-directing agents (SDAs), including alkali metal ions,[9] transition metal complexes[10-13] and organic amine molecules.[14-18] As a result, a series of crystalline chalcogenidostannates with varied structural dimensionalities have been obtained, including the one-dimensional (1D) chain,[11, 12] two-dimensional (2D) layer [9-12, 14, 16-28] and three-dimensional (3D) framework.[12, 22, 25, 29, 30] Recently, a new method known as ionothermal synthesis has attracted increasing attention and ionic liquid (IL) was found to be a promising reaction media for inorganic synthesis,[31, 32] which can simultaneously act as solvent, reagent, template, SDA and charge compensating agent. However, such applications for chalcogenide chemistry are still in an early stage,[33-36] and the multiple roles of ILs and the structure-directing effect of extra SDAs under ionothermal conditions have rarely been investigated.

In recent years, a series of crystalline selenidostannates have been obtained ionothermally from the imidazolium-based ionic liquid (IL), clearly demonstrating that the ILs are suitable reaction media and SDA for the formation of novel porous chalcogenidometalates that are inaccessible by using the traditional SDAs and traditional synthetic routes.[33-39] Very recently, we further reported the competitive and synergistic effect of metal–amine complex (MAC) and IL cation in directing selenidostannates.[13] Therefore, it is anticipated that by utilizing the ionothermal method and the competitive and synergistic effect of SDAs, novel

porous

selenidostannates

can

be

obtained.

Herein,

by

choosing

the

IL

[DAMS]I

(4-N,N-Dimethylamino-4′-N′-methylstilbazolium iodide) with a bulky and more structurally rigid organic cation as the solvent and ethylenediamine (en) as the auxiliary solvent, a novel selenidostannate with an anionic [Sn3Se7]n2n- layered

structure and

mixed

counterions

of

(enH]+

and

Na+, namely,

(enH)3Na[Sn3Se7]2·1.5Me2NH·1.5H2O (1, Me2NH = dimethylamine), was ionothermally synthesized. Interestingly, the [Sn3Se7]n2n- layers were interconnected by sodium ions to form a 3D {Na[Sn3Se7]2}n3nstructure. 1 showed an open tunnel along the [211] direction filled by enH+ cation and Me2NH and H2O molecules. The synthesis, structure, and thermal, optical and electrochemical properties of 1 were investigated. 1 showed good thermal stability and a Na+ ion storage property without capacity fading over 150 cycles based on an intercalation/deintercalation mechanism.

2. Experimental section 2.1. Materials and physical measurements Na2SnO3·H2O (99.5%) was purchased from Sinopharm Chemical Reagent, selenium (analytical grade) powder was purchased from Tianjin Yingda Rare Chemical Reagent, and [DAMS]I was synthesized according to the literature.[40] N-Methyl-2-Pyrrolidinone (NMP, 99%), sodium trifluomethanesulfonate (NaSO3CF3, (98%) were purchased from Acros, diethylene glycol diethyl ether (DGE, 98%) and ethylenediamine (99%) were purchased from Adamas. Carbon Black and polyvinylidene fluoride (PVDF) were purchased from Shenzhen kejingstar technology LTD. All of the chemicals were used without further purification. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Miniflex II diffractometer by using CuK radiation. Elemental analysis was performed on a German Elementary Vario EL III instrument. Thermogravimetric analysis-mass spectroscopy (TG-MS) was carried out at a heating rate of 5 °C min-1 in a N2 atmosphere from 35 to 600 °C on a STA449C-QMS403C thermal analysis-quadrupole mass spectrometer. Fourier-transform infrared (FTIR) spectroscopy was recorded on a Vertex 70 FTIR spectrometer photometer as a KBr pellet within the range of 4000–400 cm-1. Energy dispersive X-ray (EDX) spectrum was recorded on a JSM 6700 instrument. 2.2. Synthesis of (enH)3Na[Sn3Se7]2·1.5Me2NH·1.5H2O (1). A mixture of Na2SnO3·3H2O (0.267 g, 1 mmol), Se powder (0.198 g, 2.5 mmol) and 0.500 mL en in 1.000 g [DAMS]I was sealed in a 20 mL Teflon–lined stainless–steel autoclave at 160 °C for 6 days, followed by slowly cooling to room temperature under a cooling rate of ~5.4 K h–1. The product was washed with N,N-dimethylformamide (DMF) and ethanol for several times. Red crystals of 1 were obtained by manually selection (yield: 0.250 g, 60.8% based on Sn). Elemental analysis: calcd. (%) for C9H40.5N7.5NaO1.5Se14Sn6: C 5.10, H 1.92, N 4.95; found: C 4.98, H 1.93, N 5.10. 2.3. X-ray Crystal Structure Determination Single-crystal X-ray diffraction data were collected on an Xcalibur CCD diffractometer for compound 1 at 100(2) K with MoKα radiation (λ = 0.71073 Å). Analytical absorption correction was applied. The structure

was solved using direct methods and refined by full-matrix least-squares on F2 using the SHELX-2014 program.[41] Anisotropic thermal factors were assigned to all the non-hydrogen atoms. The hydrogen atoms bonded to C and N atoms were positioned with idealized geometry except that the H atoms attached to O of disordered water molecule were not added. In 1, some restraints (DFIX, SIMU, and SADI) were applied to the disordered en and Me2NH molecules to obtain chemically reasonable models and reasonable atomic displacement parameters. The elementary composition of 1 was confirmed by EDX spectrum (for detail, see ESI). The empirical formula was confirmed by TG-MS analysis and elemental analysis results. Detailed crystallographic data and structure refinement parameters of 1 are summarized in Table 1. 2.4. Electrochemical Measurements The electrochemical tests were performed via CR2025 coin-type test cells. To fabricate the working electrodes, 80 wt.% active material, 10 wt.% conductivity agent (TIMCAL SUPER C45 Carbon Black), and 10 wt.% polyvinylidene fluoride binder (PVDF) were mixed with NMP and then pasted on Cu foil. The electrodes were dried at 60 °C for 3 h and 100 °C for 15 h in a vacuum oven. Cells were assembled in an Ar-filled glove box with the concentration of moisture and oxygen below 1 ppm. Pure sodium was pressed on a stainless steel sheet with diameter of 16 mm and used as both counter and reference electrode. The electrolyte was 1 M NaSO3CF3 in DGE. A Celgard 2325 membrane was used as the separator. The galvanostatic discharge/charge cycles were carried out on a LAND 2001A system over a voltage range of 1.2 to 3.0 V (vs. Na / Na+) at 30 °C. Cyclic voltammetry (CV) tests were performed on a CHI 660E Electrochemical Workstation. The specific capacities in this article were calculated based on the overall mass of 1. Talbe 1. Crystallographic data and structural refinement details for compound 1.

Compound CSD Formula Mr (g mol-1) Crystal system Space group Dcalcd (g cm-3) Flack parameter a (Å) b (Å) c (Å) V (Å3) Z  (Å) T (K) μ (mm-1) Rint Measured refls. Independent refls. No. of parameters R1a [I > 2σ(I)] wR2b [I > 2σ(I)] Goodness of fit a

1 CCDC 1440520 C9H40.5N7.5NaO1.5Se14Sn6 2118.66 trigonal R32 3.343 0.355(18) 13.8647(6) 13.8647(6) 18.9623(14) 3156.8(4) 3 0.71073 100(2) 15.646 0.0333 5425 1709 99 0.0250 0.0527 1.006

R1 = ∑(Fo − Fc)/∑Fo. b wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2

3. Results and discussion 3.1. Description of Crystal Structure

1 crystallizes in the trigonal space group of R32 and contains a lamellar fragment of nanoporous [Sn3Se7]n2n-. The crystallographically asymmetric unit of 1 is composed of half an enH+ cation, one-sixth Na ion, a quarter of Me2NH molecule, a quarter of water molecule and an anionic [SnSe 7/3]2/3− group. As shown in Fig. 1a, each Sn4+ atom is coordinated to five Se atoms and a [Sn 3Se10] unit forms. The [Sn3Se10] units are further connected by two bridging μ2-Se atoms to form a 2D [Sn3Se7]n2n- layer with regular hexagonal windows. The Sn–Se distances are located in the range of 2.4885(10)–2.8822(10) Å and the bridging Sn– Se–Sn bond angles for μ2-Se atoms between [Sn3Se10] units are bent in the range of 83.87(3)–96.96(3)°. The [Sn3Se7]n2n- layers are stacked in an ABC sequence with an inter-lamellar spacing of 6.419 Å. As shown in Fig. 1b, the Na+ ions are located in between two [Sn3Se10] units, developing an octahedral coordination with six Se atoms from two [Sn3Se10] units with the Na–Se distances of 3.1117(7) Å. The enH+ cations are all sandwiched in the inter-lamellar spaces as counter ions (Fig. 1c). Although the Na–Se interactions are ionic in character,[42-48] a 3D {Na[Sn3Se7]2}n3n- structure can be decribed by [NaSe6] octahedra interconnecting the [Sn3Se7]n2n- layers (Fig. 1c). Confirmed by the TG-MS, FTIR and EA analysis, the disordered organic components were Me 2NH and H2O molecules, which reside in the nanopores of the [Sn3Se7]n2n- layers (Fig. 1a). The Me2NH might be derived from the in situ decomposition of [DAMS] + cation during the ionothermal process.[49] Fig. 1d shows the perspective view of the structure of 1 along the [211] direction, which clearly shows the tunnels along this direction where the enH+, Me2NH and H2O molecules are located.

Fig. 1 (a) A [Sn3Se7]n2n- layer in 1 with regular hexagonal nanopores filled with the disordered Me 2NH and H2O. (b) The Na+ ion sandwiched between two [Sn3Se10] units. (c) Perspective view of 1 viewed along the b-axis. (d) Perspective view of 1 along the [211] direction. The yellow tubes denote the tunnel along the [211] direction. NaSe 6 units are drawn in polyhedron. H atoms are omitted for clarity.

To identify the guest molecules and cations in 1, FTIR spectroscopy and TG-MS method were conducted. As shown in Fig. 2a, the FTIR spectra of 1 exhibit two peaks at 3309 and 3247 cm−1, which might mainly originate from the stretch of N–H bond from en and Me2NH molecules.[50, 51] The two peaks at 1586 and 1493 cm−1 can be assigned to the bending vibration of (N–H) and the vibration of C–H from en and Me2NH molecule.[51-54] The existences of peaks at 2942, 1384, 1332, 1136, 1022 and 768 cm−1 are also in accordance with the standard spectra of en and Me2NH.[54, 55] As shown in Fig. 2b, the TG profile of 1 show a weight loss of 0.7% between 85 °C and 155 °C, which can be assigned to the partial evaporation of Me2NH molecule confirm by the MS spectrum. A sharp weight loss of 1 began at 200 °C and finished at 295 °C, which is due to the pyrolysis of organic components fill in the framework. From the MS spectrum, the removal of neutral guest molecules (Me2NH and H2O) and the thermal decomposition of enH+ and Me2NH resulted in the evaporation of NH 3, H2O and Me2NH species, which was confirmed by the MS

signals with m/z = 17, 18 and 45. The absence of MS signal with m/z = 60 indicated the totally decomposition of enH+.[56, 57] In addition, the MS signal with m/z = 81 confirmed the elimination of a small amount of H2Se. The overall observed weight loss (27.6% from RT to 580 °C) was attributed to the pyrolysis of all the organic compounds and the removal of H2Se molecules during the phase transformation from 1 to SnSe2 and SnSe. As observed on the MS spectrum, there are two MS signals with m/z = 44 appeared at higher temperature, which might correspond to the liberation of organic molecules occluded in the compound.[58] The post-TGA residue for the compound was characterized by PXRD, verifying that the pyrolysis process of 1 resulted in a mixture of SnSe2 (JCPDS card No. 89-3197) and SnSe (JCPDS card No. 48-1224) as major phases (Fig. S1).

Fig. 2 (a) FTIR spectrum of 1. (b) TG-MS curves of 1.

Up to now, the Na-containing selenidostannates were all obtained from the high-temperature solid state reaction or solution methods in traditional solvents.[43-48, 59, 60] In the pure inorganic ternary Na-Sn-Se compounds, the anionic parts usually feature low-dimensional structures, such as the discrete units in Na4SnSe4[48] and Na6Sn2Se7,[45] and 1D chain-like structure in Na2SnSe3.[43, 46, 47] While Na2Sn2Se5[44] is the only compound features a 2D layered structure of [Sn2Se5]n2n-. In the organic-containing

Na-Sn-Se

compounds

(e.g.

[Na4(en)4][SnSe4][59]

and

[Na(MeOH)2][R1(R1H)2Sn3Se4][R12(R1H)Sn3Se4][60]), the anionic part all feature isolated structures. In fact, there is still no report on the 3D Na-containing selenidostannate compound. Herein, by employing [DAMS]I as solvent, a novel Na-containing selenidostannate with 3D {Na[Sn3Se7]2}n3n- structure was ionothermally synthesized. Interestingly, as shown in Scheme 1 and Table S1, compound 1 could only be obtained under the particular ionothermal conditions. Neither the reaction taking en (25 mmol) as the solvent and [DAMS]I (1 mmol) as the auxiliary solvent, nor the reaction merely taking [DAMS]I as the

solvent when keeping all the other reaction parameters unchanged, would result in the title crystalline products. Instead, only a red solution or red gel could be obtained from the control experiments. These control experiments clearly demonstrated that the addition of organic amine species as auxiliary solvent to the ionothermal reaction system is important for the formation of crystalline selenidostannates .[33, 35]

Scheme 1. Overview of different reaction pathways and results for the synthesis of compound 1.

In order to verify the structure and phase purity of 1, the crystals were manually selected and used for powder X-ray diffraction (PXRD) analysis. As shown in Fig. 3a, the experimental PXRD pattern of the title compound is in accordance with the simulated PXRD pattern from single-crystal X-ray structure, demonstrating the phase purity of 1. However, preferred orientation was unavoidable due to the laminar nature of the title compound as evidenced by the intense (003) reflection in the experimental PXRD pattern of 1, Fig. 3a. Solid-state absorption spectrum of compound 1 at room temperature is plotted in Fig. 3b. The spectrum indicates a sharp absorption edge at about 1.93 eV for 1, which is consistent with its red colour, and is located in the visible region. Compared to other ionothermally synthesized lamellar selenidostannates with organic cations or second metal atom in anionic backbone such as [Bmmim]2[Sn3Se7] (Eg = 2.2 eV) and [Bmmim]7[AgSn12Se28] (Eg = 2.2 eV),[35] the compound 1 exhibits a red shift in the absorption edge after the introduction of Na+ into the structure.

Fig. 3 (a) PXRD pattern of compound 1 (top) and the one simulated ones from the single-crystal X-ray data (bottom). (b) Solid-state optical absorption spectrum of 1.

The electrochemical performance of 1 was evaluated by both cyclic voltammetry (CV) and galvanostatic charge-discharge cycling. According to the literatures,[61-63] the sodiation process of tin chalcogenides (Q = S or Se) can be elucidated as follows: Na en Na n (1) n 2 Na 2 en Na2 (2) n 3. Na 3. eNa3. n (3) Reaction (1) represents the sodium intercalation into the chalcogenide layers without phase decomposition, while (2) and (3) represent the conversion and alloying reactions of sodium below 1V. In order to evaluate the Na + ion storage performance based on an intercalation/deintercalation mechanism, the CV and galvanostatic charge-discharge cycling were carried out over a voltage range of 1.2 to 3.0 V (vs. Na / Na+). The sodium ion storage behaviour of 1 was first characterized by CV at a scanning rate of 0.1 mV s −1 (Fig. 4a). During the first negative scan, two reduction peaks appeared at 1.92 and 1.62 V. The reduction peak at 1.92 V disappeared in the subsequent cycles, which may correspond to the decomposition of electrolyte to form the solid electrolyte interphase (SEI) film. During the first anodic sweep, the sample exhibited three peaks at 1.63, 1.83 and 2.05 V, which were gradually fused into one broad peak during the successive cycles. All the redox peaks of 1 in the CV curves are well matched to the charge/discharge plateaus (Fig. 4b). Thus, after the first several cycles, one pair of oxidation/reduction peaks at 1.87/1.62 V

clearly emerged and then remained steady, which should be attributed to the similar reaction according to reaction (1). Interestingly, there is an obviously increase of area in the successive cycles, which is in accordance with the cycling process. As shown in Fig. 4c, the cycling performance of 1 was evaluated between different voltage ranges at a current density of 50 mA g -1. When carried out the galvanostatic charge-discharge cycling over a voltage range of 0.05 to 2.0 V (vs. Na / Na +), 1 exhibited a first discharge capacity of 163.8 mA h g-1 but a sharply capacity fading during the successive several cycles (Fig. 4C, blue). This might be related to the structural pulverization during the conversion and alloying reaction of Sn according to formulas (2) and (3).[64] However, when carried out the cycling over the voltage range of 1.2 to 3.0 V, 1 exhibited a first discharge capacity of 16 mA h g-1 and a reversible Na-ion capacity of about 35 mA h g-1 without capacity fading over 150 cycles (Fig. 4C, red). Based on the theoretical capacity of complete deintercalation of Me 2NH and water molecules per formula (37.5 mA h g -1), the result might indicate that the Na[Sn3Se7]2 skeleton of 1 could be embedded a little amount of Na+ ions into the tunnels without phase decomposition. Interestingly, although a small quantity of crystalline water existed in 1, 1 could remain stable during cycling. The effect of crystalline water in sodium ion battery has been previously studied, which showed that the incorporation of crystalline water enhanced Na + ion diffusion both in the host and at the interface of the cathode.[65]

Fig. 4 (a) CV curves for the first three cycles of 1 at a scanning rate of 0.1 mV s-1. (b) Galvanostatic charge–discharge curves of 1 at a current density of 50 mA g-1. Point 1 and 2 represent the electrodes at different state of charge in the 20th cycle for ex situ PXRD analysis. (c) Cycling performance of 1 between different voltage ranges at a current density of 50 mA g-1. (d) A comparison of the experimental PXRD patterns for 1, and the fresh electrode and the electrodes collected at two points as indicated in the corresponding voltage profile in Fig. 4b with the simulated ones from single-crystal X-ray structure. The two strong peaks above 40° in the patterns of electrodes before and after cycling originate from the Cu current collector of electrode.

In order to verify the electrochemical stability of 1 during charge-discharge process, PXRD analyses of the as-prepared crystalline sample, the fresh electrode of 1 and ex situ PXRD analysis of the electrodes at different state of charge in the 20th cycle were performed and taken into comparison (Fig. 4d). As shown in Fig. 4d, the PXRD pattern of the as-prepared crystalline sample of 1 is in accordance with the corresponding simulated PXRD pattern from the single-crystal X-ray structure, demonstrating the phase purity of the title compound. As for the ex situ PXRD patterns of the fresh electrode and the electrodes collected at two points as indicated in the corresponding voltage profile (Fig. 4b), when exclude the two strong peaks above 40° in the patterns originate from the Cu current collector of electrode, the three patterns are in agreement with the simulated pattern. Although the intensities decreased, the three weak peaks at 14.1°, 18.8° and 26.5° were retained and comparable with those of the fresh electrode of 1, suggesting that the compound still survived after the galvanostatic charge-discharge cycling while with a decrease of crystallinity.

4. Conclusions In summary, based on the competitive and synergistic effect of mixed-cations in the ionothermal conditions, by

taking

a

bulky

and

more

structurally

rigid

organic

cation

+

4-N,N-Dimethylamino-4′-N′-methylstilbazolium ([DAMS] ) as ionic liquid cation, a novel Na-containing selenidostannate with 3D framework was obtained, which cannot be accessed from the traditional synthetic methods. Through an electrochemical process, the neutral guest molecules could be up taken from the tunnels of 1. The Na-ion storage property of the nanoporous selenidostannate 1 was studied, exhibiting Na+ ion storage without capacity fading over 150 cycles. The intercalation/deintercalation mechanism of Na + ion in the framework of 1 was discussed in detail, revealing that the reversible Na + ion storage property is derived from the nanoporous nature of 1. This work affords an example for the design and synthesis of novel porous crystalline chalcogenide materials.

Acknowledgements This work was supported by the 973 program (no. 2014CB845603) and the NNSF of China (no. 21371001 and 21521061).

Appendix A. Supplementary data CCDC 1440520 contains the supplementary crystallographic data for compound 1. The data can be obtained free of charge via , or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: [email protected]. Additional details of control experiments, PXRD result and EDX spectrum are available as electronic supplementary information in the online version, at doi: xx.xxxx/j.inoche, 2014.xxx.xx

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Graphical abstract

A selenidostannate (enH)3Na[Sn3Se7]2·1.5Me2NH·1.5H2O prepared in the ionic liquid of 4-N,N-Dimethylamino-4′-N′-methylstilbazolium iodide exhibited the reversible Na-ion storage property.

Highlights 1. A 2D selenidostannate interconnected by sodium ions to form a 3D structure. 2. The selenidostannate containing the mixed cations of Na+ and enH+ 3. The compound shows a Na+ ion storage property without capacity fading over 150 cycles.