Structure and Dynamic Behavior of Sodium–Diglyme Complex in the

Nov 21, 2016 - Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Ishikawa 920-1192, Japan. ∥. Graduate School of...
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Structure and Dynamic Behavior of Sodium−Diglyme Complex in the Graphite Anode of Sodium Ion Battery by 2H Nuclear Magnetic Resonance Kazuma Gotoh,*,†,‡ Hisashi Maruyama,† Tatsuya Miyatou,§ Motohiro Mizuno,§ Koki Urita,∥ and Hiroyuki Ishida† †

Graduate Elements § Graduate ∥ Graduate ‡

School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Strategy Initiative for Catalysts and Batteries, Kyoto University, Nishikyo-ku, Kyoto 615-8245, Japan School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Ishikawa 920-1192, Japan School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan

S Supporting Information *

ABSTRACT: Cointercalation systems consisting of graphite, sodium, and some linear glycol ethers (glymes) are anticipated for use as negative electrodes in sodium ion batteries because of their superior properties such as outstanding cycle performance. We synthesized a ternary intercalation compound consisting of sodium, deuterated diglyme (diglyme-d14), and graphite by a solution reaction. Then we investigated the dynamics and coordination structure of diglyme-d14 molecules by 2H solid-state NMR. Two diglyme molecules coordinate to each sodium ion rigidly, except for rotation of the methyl groups at low temperatures below 233 K. At room temperature, diglyme weakly coordinates to a Na ion through one oxygen atom of the ligand and rotates around the O−Na axis. The active motion of sodium−diglyme complexes is favorable for Na diffusion between graphene layers in the graphite intercalation compound. g−1 capacity and long-term cycle stability.13 Kim et al. also reported greater than 150 mA·h·g−1 desodiation capacity in diglyme15 and investigated intercalation−deintercalation behavior using operando X-ray diffraction.16 Zhu et al.17 reported stable ultrafast electrode sodiation/desodiation in tetraglyme for over 6000 cycles. The electrochemical properties of the electrodes with different glymes have been compared and the structures of sodium−diglyme complexes in graphite layers described.14 However, the dynamic behavior and temperature dependence of the sodium−diglyme complex intercalate have not been clearly determined, although they are critical for a fuller understanding of the promising performance of these novel NIB anodes. Solid-state deuteron (2H) nuclear magnetic resonance (NMR) can provide significant information on the dynamics of deuterated organic molecules. For the current research, we prepared sodium−diglyme-d14−GIC by chemical reduction using a partially deuterated diglyme solvent, which is analogous to a procedure reported previously for the synthesis of several sodium ternary GICs (Na−en-d4−GIC, Na−thf-d8−GIC, etc.).18,19 The obtained sample was characterized by high-

1. INTRODUCTION Secondary batteries such as lithium ion batteries (LIBs) are commonly used in our modern world as energy storage systems for electronic devices and vehicles. Nevertheless, because of increasing demand for LIBs, issues related to lithium resources and cost will grow in importance in the future. Sodium ion batteries (NIBs) are promising candidates for use as alternatives to LIBs because of their comparable performance and the natural abundance of sodium.1−4 Carbon materials such as nongraphitizable carbon (hard carbon),5,6 porous carbon,7−9 and modified carbon10 have been reported as efficacious negative NIB electrodes.11,12 Graphite anodes have a significant advantage for practical use because of their low cost and availability, and they are the most commonly used anode in most commercial LIBs. However, graphite has not been used in NIBs because low-stage secondary graphite intercalation compounds (GICs) of sodium (NaCx) do not form electrochemically or chemically. Recently, Jache et al.13,14 demonstrated the potential for graphite electrodes in NIBs by using glymes (symmetric linear glycol ethers) such as monoglyme (1,2-dimethoxyethane), diglyme (diethylene glycol dimethyl ether), and triglyme (triethylene glycol dimethyl ether) as electrolyte solvents. Na cations and diglyme cointercalate into graphite to form ternary GICs during charging (sodiation) of the NIB negative electrode. The process allows over 100 mA·h· © XXXX American Chemical Society

Received: October 31, 2016 Revised: November 18, 2016 Published: November 21, 2016 A

DOI: 10.1021/acs.jpcc.6b10962 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C resolution scanning transmission electron microscopy (HRSTEM) and powder X-ray diffraction (XRD). The behavior of the diglyme-d14 molecules in the GIC was investigated by 2H static NMR over a wide temperature range of 123−293 K. 23Na NMR spectra were also collected to observe the state of the Na ion.

2. EXPERIMENTAL SECTION Sodium−diglyme-d14−GIC was prepared by a solution-phase reaction where 50 mg of graphite powder (Timrex SLP50; Timcal) and 30 mg of Na metal (Wako Pure Chemical) were added to 0.5 mL of 1:1 diglyme/diglyme-d14 (Cambridge Isotope Laboratories) liquid under an N2 atmosphere. The mixture was stirred for a week at room temperature, yielding a dark blue product. The obtained solid product was separated by centrifugation and dried at reduced pressure for 12 h. For comparison, sodium−diglyme−GIC (nondeuterated) was also prepared by the same procedure. The products were characterized by XRD and HR-STEM by use of a MiniFlex II diffractometer (Rigaku) and an ARM200CF microscope (JEOL) with energy-dispersive X-ray spectrometry (EDS), respectively. The product compositions were examined by CHN elemental analysis and inductively coupled plasma (ICP) analysis for Na. 2H NMR spectra (resonance frequency at 45.3 MHz) were obtained from 123 K to ambient temperature on an ECA-300 spectrometer (JEOL) and a quadrupole echo pulse sequence with 2.8 μs pulse length. 23Na magic-angle spinning (MAS) NMR spectra (resonance frequency at 132.2 MHz) were recorded on a DD2 spectrometer (Agilent Technologies) with a single pulse sequence of 2.0 μs pulse length. D2O and 1 M NaCl aqueous solution were used as references for the 2H and 23Na chemical shifts, respectively.

Figure 2. 2H static NMR spectra of sodium−diglyme-d14−GIC between 293 and 123 K. A simulation spectrum from two powder patterns of I = 1 nuclei, which have qcc = 50 kHz (η = 0.04) and 150 kHz (η = 0.04), is shown as a red line.

3. RESULTS AND DISCUSSION 3.1. High-Resolution Scanning Transmission Electron Microscopic and Energy-Dispersive X-ray Spectrometric

Figure 3. Observed and fitted 2H NMR spectra of (a) sodium− diglyme-d14−GIC at 293 K. (b) Proposed model of diglyme-d14’s molecular motion. Figure 1. Dark-field images and their EDS mapping images of C and Na for (a) pristine graphite and (b) sodium−diglyme-d14−GIC.

exfoliation. EDS mapping of the sample clearly illustrates a uniform dispersion of sodium ions over the entire GIC particle area (Figure 1b). XRD patterns of the GIC products (Figure S2) show diffraction peaks assigned to stage 1 and stage 2 phases with a basal repeat distance of 1.16 nm; this gallery dimension is similar to that reported for the electrochemically prepared sodium−diglyme−GIC.14 The composition of the

Analysis. Dark-field (DF) STEM images and EDS analysis of pristine graphite and product GIC are presented in Figure 1. Bright-field (BF) TEM images are also shown in Figure S1. The product shows a thin flake morphology resembling that of the graphite precursor: the reaction does not cause significant B

DOI: 10.1021/acs.jpcc.6b10962 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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consistent with previous reports of rigid deuterated methylene groups (-CD2-, 167 kHz) and rotating methyl groups (-CD3, 50 kHz) around the C3 axis of the -CD3.25 Therefore, the diglyme molecules are coordinating rigidly to sodium ions at low temperature. We recently reported the dynamic behavior of some deuterated organic molecular cointercalates with sodium or potassium ions into graphite, for example, ethylenediamined4 (en-d4) or tetrahydrofuran-d8 (thf-d8) with Na and piperazine-d10 (ppz-d10) or 1,4-diazabicyclo[2.2.2]octane-d12 (dabco-d12) with K.19 Interactions between the organic ligands and coordinated cations was seen to depend on both the alkali metal ion and the ligand. Most en-d4 and thf-d8 molecules that cointercalate with Na are rigid at low temperatures, whereas the ppz-d10 and dabco-d12 that cointercalate with K show axial rotations. The diglyme-d14 in sodium−diglyme-d14−GIC at low temperatures is rigid except for CD3 rotation, similar to en-d4 and thf-d8, although its behavior at higher temperatures is more complicated than for en-d4 and thf-d8 in GICs. At room temperature, at least three powder pattern components having different qcc values were observed, indicating that diglyme rotational motions are activated at room temperature. However, weak interaction between diglyme molecules and Na ion remains because diglyme-d14 is not under isotropic rotation. The 23Na MAS NMR spectra for sodium−diglyme−GIC measured at 293 and 223 K (Figure S3) exhibit similarly complicated signals. The main peak at 9 ppm and the edge structure at −24 or −27 ppm are apparently ascribable to a predominant second-order quadrupolar component (quadrupolar coupling constant; qcc =1.8 and 2 MHz, respectively) of the sodium ion, which was also observed for Na−en−GIC (qcc = 3 MHz) in an earlier study.19 The smaller qcc values than those of Na−en−GIC mean that Na in Na−diglyme−GIC is in a more isotropic state than Na−en−GIC. The slight decrease of the qcc values by increasing temperature suggests reduction of anisotropy around Na ions by diglyme molecular motion. 3.3. Arrangement and Dynamics of Sodium−Diglyme in Graphene Layers. The 2H spectrum of sodium−diglymed14−GIC at 293 K was fitted with a combination of two methylene (-CD2-) groups and one rotating methyl group (-CD3) with some motional narrowing (Figure 3a). The simulated spectrum shown as a red line in Figure 3a was obtained by assuming linear diglyme molecules with rotation perpendicular to the molecular long axis (Figure 3b). The fitted peaks in Figure 3a are adjusted by tilting two methylene (-CD2) groups (4.7° and 3.9°) and one methyl group (10.0°) slightly from the provisional molecular plane including the rotation axis. The “rotation angle” in Figure 3a stands for the angle between the C−D axis in CD2 and the molecular rotation axis, or for the angle between the rotation axis of (rotating) CD3 group and the molecular rotation axis. They correspond to tilting angles of 4.7°, 3.9°, or 10.0° between the C−O axes and the molecular rotation axis (Figure S4). An asymmetric peak at +2.2 kHz, marked by #, is ascribed to a minor impurity of free diglyme-d14 not intercalated into graphite. Molecular motion of the linear diglyme-d14 does not correspond to the rigid diglyme−sodium complex model proposed at lower temperature. At ambient temperatures, where NIBs would be generally used, two diglyme molecules might be expected to coordinate weakly to each Na ion via the central oxygen atom of the diglyme and to rotate about an axis along the oxygen atoms and Na ion (Figure 4). However, both narrowed and rigid components were observed in 2H NMR spectra taken at 273 and 233 K, which suggests that some

Figure 4. Models of diglyme and sodium in graphite, (a) below 123 K and (b) at ambient temperature, derived from 2H NMR spectra.

nondeuterated GIC was estimated to be C22−26(diglyme)1.8−2.2Na1.0 from results of elemental analyses (shown in Table S1), indicating that, on average, two diglyme molecules can coordinate to each Na cation in the GIC. This composition differs from that reported by Kim et al.16 for the electrochemically prepared GIC; they estimated the ratio of Na/diglyme as 1:1 and proposed a corresponding structure of the complex. However, the 2:1 molecule/Na ion ratio was also inferred from cyclic voltammograms examined by Jache et al.,14 and several examples of stable [Na(diglyme)2] complexes are reported in crystal structures.20−24 Indeed, the optimized structure of a [Na(diglyme)2] complex, when calculated with a 6-31G basis set and Hartree−Fock method, has a diameter of 0.7−0.8 nm and therefore fits well into the expanded galleries of the GIC (0.84 nm). We therefore conclude that sodium ions in the GIC sample prepared in this research are coordinated by two diglyme molecules. 3.2. Solid-State NMR. Figure 2 presents 2H NMR spectra obtained for sodium−diglyme-d14−GIC between 293 and 123 K. At 123 K, the spectrum includes two components of the typical powder pattern signal of the I = 1 nucleus. By fitting the spectra, the quadrupole coupling constant (qcc) and the asymmetry parameter (η) of the broader component were estimated respectively as 165 kHz and 0.05. The qcc and η values for the narrower component were found to be 50 kHz and 0.00 (red spectrum in Figure 2). These qcc values are C

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diglyme molecules remain partially uncoordinated and under rotation. The active motion of diglyme at ambient temperature is favorable for rapid Na diffusion along the galleries and therefore good rate performance in NIB electrodes. The flexible structure of the sodium−diglyme complex may facilitate intercalation into graphite layers without exfoliation.

4. CONCLUSION In sodium−diglyme−GIC synthesized by chemical reduction, sodium ions are dispersed uniformly over the entire GIC. The estimated composition of the nondeuterated GIC, C22−26(diglyme)1.8−2.2Na1.0, indicates that two diglyme molecules can coordinate to each Na cation in the GIC. NMR results reveal that the diglyme molecules coordinate to the sodium ion rigidly, except for rotation of the methyl groups at low temperatures below 233 K. At room temperature, diglyme weakly coordinates to the sodium ion through one oxygen atom of the ligand and rotates around the O−Na axis. The active motion of diglyme at ambient temperature is favorable for rapid Na diffusion and might contribute to good rate performance of NIB. The flexible structure of the sodium−diglyme complex may facilitate intercalation into graphite layers without exfoliation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10962. Four figures showing TEM images of pristine graphite and sodium−diglyme−GIC, powder XRD patterns of sodium−diglyme−GIC and sodium−diglyme-d14−GIC, 23 Na MAS NMR spectra of sodium−diglyme−GIC, and schematic for the tilt of C−D bonds in CD2 groups; one table listing results of elemental and ICP analyses (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Kazuma Gotoh: 0000-0002-8197-5701 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS Kakenhi (Grant 26870385) and New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b10962 J. Phys. Chem. C XXXX, XXX, XXX−XXX