Communication pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Graphite Intercalation by Mg Diamine Complexes Wei Xu, Hanyang Zhang, and Michael M. Lerner* Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331-4003, United States
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S Supporting Information *
ternary GIC was obtained.11 Kawaguchi et al. employed BC2N, rather than graphite, to successfully obtain a stage 2 Mg-BC2N product by a vapor-phase reaction.12 In electrochemical preparations, Takeuchi et al. evaluated the effect of Mg2+ on the electrochemical intercalation/deintercalation of Li+ into graphite and found no evidence for Mg2+ cointercalation.13 Recently, God et al.7 reported the electrochemical intercalation of Mg2+ into graphite in N,N-dimethylformamide with a maximum specific capacity of 35 mAh/g (as compared with 372 mAh/g in LiC6), indicating that only a high-stage GIC was obtained, and the product structure and composition were not reported. Since 2011, our group has reported on syntheses of ternary stage-1 GICs containing alkali metal cations M+ (M = Li, Na, K) complexed with diamines. The inclusion of a cointercalate such as a diamine or ether can significantly increase the graphite intercalation onset potential and thereby allow the formation of new GICs.14,15 In the following report, we show that stage-1 ternary GICs containing Mg and ethylenediamine (en) are obtained by a simple, one-pot synthesis and establish the product composition and structure. The Mg-en-GIC was prepared by adding ethylenediamine (2 mL, Tokyo Chem. Ind.) to a glass test tube containing Mg(m) powder (35 mg, 99.8%, Alfa Aesar) and graphite powder (100 mg, SP-1 grade, avg. diameter 100 μm, Union Carbide) in an Ar atmosphere. The tube was sealed, and the reaction mixture stirred at 100 °C for 60 h. After centrifugation (3500 rpm, 5 min) to remove the supernatant solution, the bottle-green solid product was rinsed briefly with en and then dried in vacuo at 25 °C for 12 h. A synthetic scheme is illustrated in Figure 1.
ABSTRACT: The first structural and compositional details of a low-stage graphite interaction compound (GIC) containing Mg are reported, with the GIC obtained by combining magnesium metal and graphite powder in ethylenediamine (en) at 100 °C under an inert atmosphere. Thermal analyses indicate the bottle-green stage 1 product has a composition of [Mg(en)1.0]C13. Xray diffraction shows a c-axis expansion of 0.55 nm, indicating the presence of intercalate monolayers with the en cointercalate oriented perpendicular to the encasing graphene layers. Redox titration indicates two electrons are transferred per Mg. A structural model is proposed with dimeric [Mg2(en)2]2+ intercalate species.
T
he graphite intercalation compound (GIC) LiC6 remains the workhorse anode of commercial lithium-ion batteries due to its excellent electrode performance. K, Rb, and Cs are all known to form GICs with a maximum metal cation content of C8M, and recently the electrochemical preparation and anode properties of C8K were explored by Jian et al.1−3 Of the alkali metals, sodium alone does not form a low-stage binary GIC due to the higher reduction potential of Na+.4 The alkaline earth elements also have low reduction potentials, and due to the possibility of a higher charge than +1, are of great interest as electrode materials. Binary GICs with the alkaline earth metals have been reported for Ca, Sr, and Ba.5 Reversible Mg anodes are of particular interest due to the intrinsic properties of Mg that include abundance, low cost, low toxicity, and potential for high specific capacity. However, metallic Mg anodes are limited by the formation of barrier passivation surfaces.6 Thus, as with lithium chemistry, reversible Mg anodes with carbon or other host substrates have been explored.7,8 Several groups have explored chemical or electrochemical routes to the intercalation of Mg into graphite. In 1988, Maeda et al.9 reported a stage 7 ternary GIC obtained via electrochemical reduction in a MgCl2/dimethyl sulfoxide electrolyte but did not obtain product structure or composition. Since stage number relates to the proportion of expanded galleries and is inversely related to intercalate content, a stage 7 GIC contains very little intercalate and will be difficult to characterize. Stumpp et al. prepared a MgNH3-GIC by the reaction of graphite with Mg dissolved in liquid ammonia at −60 °C, but the product was unstable and there was limited structural and compositional characterization.10 The preparation of ternary GICs containing Li and alkaline earth intercalates was explored by Pruvost et al., but in that case Mg did not cointercalate and only a graphite−Li−Ca © XXXX American Chemical Society
Figure 1. Schematic diagram of the stage 1 Mg-en-GIC, showing the dimeric Mg-en complex orientation and monolayer arrangement.
Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Miniflex II diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15406 nm), in the 2θ range from 3 to 60° at a scan rate of 3°/min. A Shimadzu TA-50 thermogravimetric analyzer (TGA) was used to evaluate the weight loss of GICs from ambient to 800 °C under flowing N2 or O2 (60 mL/min) at a heating rate of 10 °C/min. Received: May 7, 2018
A
DOI: 10.1021/acs.inorgchem.8b01250 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry GIC products were titrated with 0.05 M iodine solution in cyclohexane under an inert atmosphere. Typically, the sample was stirred in 2 mL of cyclohexane and the titrant added in 20 μL increments until a constant light pink end point was obtained. Although a stage 1 Na-en-GIC can be prepared by direct reaction with Na(m) or Li(m) with graphite in en at 60−90 °C within 24 h,16 these conditions result in higher stage Mg-enGICs. A higher reaction temperature and longer reaction time are required to obtain a stage 1 Mg-en-GIC. Since an electride intermediate must form in order to reduce graphite, this difference in reactivity may be associated with the higher ionization energy for Mg: 7.65 eV (Mg) vs 5.39 eV (Li) or 5.14 eV (Na). The behavior of the Mg-en-GIC product is shown in Figure 2. Under a N2 flow (b), two mass loss regions occur at 50−200
Figure 3. Graphite and the Mg-en GIC (A) ex situ powder X-ray diffraction patterns and (B) product colors showing the bottle-green GIC product. Miller indices for the graphite (G) and the GIC are included.
greater expansion in Li-en-GIC (0.51 nm) than in Na-en-GIC (0.36 nm) to a difference in en orientation, which outweighed the opposite trend for the cationic radii. Similarly, ternary GICs with a 1,2-diaminoproane (DAP) cointercalate gave expansions of K-DAP-GIC (0.36 nm), Na-DAP-GIC (0.41 nm), and Li-DAP-GIC (0.45 nm) that again are opposite the trend in cationic radii but were attributed to changes in the DAP cointercalate orientation. For the present study, the expansion observed in Mg-en-GIC (0.55 nm) is the same as that calculated for the longest dimension of en, indicating that the cointercalate is oriented with a long axis perpendicular to the graphene sheets. The reason for changes in diamine orientation has not been established but could reflect differences in cationic charge density or the relative significance of chelation vs electrostatic interactions in the GIC. Structural and composition data for the three known M-enGICs are summarized in Table 1, including a gallery packing fraction (Vi/Vh) where Vi is the total calculated volume of intercalate and Vh is the total expansion volume in forming the GIC from graphite.18 Previous studies on binary GICs containing alkaline earth metal cations have indicated that the metal cation charge is less than +2. X-ray photoemission19 and neutron-diffraction20 experiments on BaC6 suggest that Ba transfers only about one of its two valence electrons to the graphene host. There have been no Mg-containing GICs available for comparable studies, but the higher ionization energy of Mg could result in a different oxidation state in GICs than for the heavier congeners. Work by Green et al. has shown that dimeric Mg(I) complexes can form with bulky diamine ligands under strongly reducing conditions.21 They prepared [L2Mg2]2+ complexes with a covalent M−M bond by reduction of Mg(II) with K metal in toluene. Redox titration on Na-en-GIC and Mg-enGIC using a tri-iodide titrant (see SI) indicates that approximately 1.0 and 2.0 electrons are required to fully oxidize the GICs to graphite and Na+ or Mg2+ salts,
Figure 2. Thermal analysis of graphite under N2 flow (a) and under O2 flow (d). Mg-en-GIC product under N2 flow (b) and under O2 flow (c).
°C and 200−350 °C, with both losses attributed to the volatilization and/or decomposition of the en cointercalate. These two losses give a combined mass loss of 25.1%. The Mg content in the Mg-en-GIC product was separately assessed by TGA under an O2 flow. (Figure 2c). In this case, the residual after high temperature thermolysis (16.8 mass %) is a white MgO powder; all carbon has been eliminated by oxidation to CO or CO2. Taken together, these thermal analyses provide both en and Mg contents and yield a product composition of [Mg(en)1.0]C13. The PXRD pattern obtained for the Mg-en-GIC product (Figure 3) shows sharp reflections that can all be indexed according to a single-phase GIC product, with no detectable Mg(m) remaining. The coherence length along the stacking direction obtained by the Sherrer relation17 is 22.84 nm, indicating a well-ordered GIC stacked structure. The basal repeat distance (Ic) is 0.88 nm; this is alternately expressed as the expansion height along the c direction, Δd = 0.88 − 0.335 = 0.55 nm, and is consistent with the composition obtained in suggesting a monolayer of intercalate complex present in each gallery. The expansion heights of ternary GICs containing diamines depend primarily on the diamine orientation, because the cointercalate dimensions are typically larger than for the cations. For example, Maluangnont et al.14 attributed the B
DOI: 10.1021/acs.inorgchem.8b01250 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Table 1. Structural and Composition Data of M-en-GICs with Li, Na and Mg intercalate cation
GIC stage
Δd/nm
intercalate arrangement
experimental composition
packing fraction
ref
Mg Li Na
1 1 1
0.55 0.34 0.36
monolayer monolayer monolayer
[Mg(en)1.0]C13 [Li(en)0.8]C15 [Na(en)1.0]C15
0.36 0.40 0.51
this work Maluangnont et al. Maluangnont et al.
(3) Jian, Z. L.; Luo, W.; Ji, X. L. Carbon electrodes for K-ion batteries. J. Am. Chem. Soc. 2015, 137, 11566−11569. (4) Jung, S. C.; Kang, Y. J.; Han, Y. K. Origin of excellent rate and cycle performance of Na+-solvent cointercalated graphite vs. poor performance of Li+-solvent case. Nano Energy 2017, 34, 456−462. (5) Hick, S. M.; Griebel, C.; Blair, R. G. Mechanochemical synthesis of alkaline earth carbides and intercalation compounds. Inorg. Chem. 2009, 48, 2333−2338. (6) Song, J.; Sahadeo, E.; Noked, M.; Lee, S. B. Mapping the challenges of magnesium battery. J. Phys. Chem. Lett. 2016, 7, 1736− 1749. (7) God, C.; Bitschnau, B.; Kapper, K.; Lenardt, C.; Schmuck, M.; Mautner, F.; Koller, S. Intercalation behaviour of magnesium into natural graphite using organic electrolyte systems. RSC Adv. 2017, 7, 14168−14175. (8) Chen, L.; Bao, J. L.; Dong, X.; Truhlar, D. G.; Wang, Y.; Wang, C.; Xia, Y. Aqueous Mg-ion battery based on polyimide anode and Prussian blue cathode. ACS Energy Lett. 2017, 2, 1115−1121. (9) Maeda, Y.; Touzain, P.; Bonnetain, L. Electrochemical formation of graphite intercalation compound in magnesium chloridedimethylsulfoxide solution. Synth. Met. 1988, 24, 267−270. (10) Stumpp, E.; Alheid, H.; Schwarz, M.; Janssen, J.; MüllerWarmuth, W. Ternary graphite intercalation compounds of type M(NH3)xCy with M = Be, Mg, Al, Sc, Y, La. electrochemical synthesis, stability and NMR studies. J. Phys. Chem. Solids 1996, 57, 925−930. (11) Pruvost, S.; Hérold, C.; Hérold, A.; Lagrange, P. Cointercalation into graphite of lithium and sodium with an alkaline earth metal. Carbon 2004, 42, 1825−1831. (12) Kawaguchi, M.; Kurasaki, A. Intercalation of magnesium into a graphite-like layered material of composition BC2N. Chem. Commun. 2012, 48, 6897−6899. (13) Takeuchi, S.; Kokumai, R.; Nagata, S.; Fukutsuka, T.; Miyazaki, K.; Abe, T. Effect of the addition of bivalent ions on electrochemical lithium-ion intercalation at graphite electrodes. J. Electrochem. Soc. 2016, 163, A1693−A1696. (14) Maluangnont, T.; Lerner, M. M.; Gotoh, K. Synthesis of ternary and quaternary graphite intercalation compounds containing alkali metal cations and diamines. Inorg. Chem. 2011, 50, 11676−11682. (15) Maluangnont, T.; Bui, G. T.; Huntington, B. A.; Lerner, M. M. Preparation of a homologous series of graphite alkylamine intercalation compounds including an unusual parallel bilayer intercalate arrangement. Chem. Mater. 2011, 23, 1091−1095. (16) Sirisaksoontorn, W.; Adenuga, A. A.; Remcho, V. T.; Lerner, M. M. Preparation and characterization of a tetrabutylammonium graphite intercalation compound. J. Am. Chem. Soc. 2011, 133, 12436−12438. (17) Zhang, Z.; Zhou, F.; Lavernia, E. J. On the analysis of grain size in bulk nanocrystalline materials via x-ray diffraction. Metall. Mater. Trans. A 2003, 34, 1349−1355. (18) Zhang, H.; Wu, Y.; Sirisaksoontorn, W.; Remcho, V. T.; Lerner, M. M. Preparation, characterization, and structure trends for graphite intercalation compounds containing pyrrolidinium cations. Chem. Mater. 2016, 28, 969−974. (19) Dean, M. P. M.; Howard, C. A.; Saxena, S. S.; Ellerby, M. Nonadiabatic phonons within the doped graphene layers of XC6 compounds. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 045405. (20) Fischer, J. E.; Kim, H. J.; Cajipe, V. B. Neutron-diffraction studies of BaC6: c-axis compressibility, carbon−carbon bond length, and charge transfer. Phys. Rev. B: Condens. Matter Mater. Phys. 1987, 36, 4449.
respectively. As expected, the composition [Na(en)1.0]C15 thus represents Na with a +1 oxidation state and full transfer of the electron from Na to the graphene host. For [Mg(en)1.0]C13, although two electrons are required per formula unit to oxidize the GIC back to graphite and Mg(II), the extent of electron transfer is not indicated by the redox data. The electrons could reside on either the graphene host or the Mg intercalates, at one limit to form [Mg(en)1.0]2+C132− or at the other extreme, [Mg(en)1.0]0C130. Because of the similar ratios of metal to graphitic carbon in the Na-en-GIC analog, a one-electron transfer provides a similar level of graphene reduction. The prior literature indicates that Mg(I) diamine complexes can be stabilized as dimers.21 A most likely structure therefore is proposed to be [Mg2(en)2.0]C26, with dimeric +2 complex intercalates present. In any of these models, the specific capacity will be approximately twice that observed for the GIC analogs with Na or other alkali metals. In summary, the first stage [Mg(en)1.0C13] was successfully synthesized by a mild route, and solid powder X-ray diffraction evidence for the intercalation was obtained. The gallery structure, composition, and redox properties were characterized, and a structure model was proposed with Mg(I) complex dimers in monolayer galleries. The intercalation chemistry to form low-stage ternary GICs with alkaline earth cations may still prove useful for anodic charge storage.
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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.inorgchem.8b01250.
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Details on the iodometric titration data for [Mg(en)1.0]C13 and [Na(en)1.0]C15 (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel.: 1-541-737-6747. Fax: 1-541-737-2062. E-mail: Michael.
[email protected]. ORCID
Michael M. Lerner: 0000-0001-5604-4430 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the TGA and PXRD instrument support by Prof. Mas Subramanian (OSU Chemistry) and Prof. Xiulei Ji (OSU Chemistry), who provided helpful suggestions.
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REFERENCES
(1) Ouvrard, G.; Guyomard, D. Intercalation chemistry. Curr. Opin. Solid State Mater. Sci. 1996, 1, 260−267. (2) Dresselhaus, M. S.; Dresselhaus, G. Intercalation compounds of graphite. Adv. Phys. 2002, 51, 1−186. C
DOI: 10.1021/acs.inorgchem.8b01250 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (21) Green, S. P.; Jones, C.; Stasch, A. Stable magnesium(I) compounds with Mg-Mg bonds. Science 2007, 318, 1754−1757.
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DOI: 10.1021/acs.inorgchem.8b01250 Inorg. Chem. XXXX, XXX, XXX−XXX
