Communication pubs.acs.org/IC
Preparation of Graphite Intercalation Compounds Containing Crown Ethers Hanyang Zhang and Michael M. Lerner* Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, United States S Supporting Information *
to form new GICs with novel intercalates or cointercalates, such as alkylammonium cations27−29 or polyethers.15 Ethylenediamine (en) has proven useful for this exchange as it can form electride solutions with all the alkali metals. Alternately, electrocatalysts can be used to promote graphite reduction and the direct synthesis of the GIC containing cointercalate.5−9,15,30 These two approaches, which we will call the exchange and the direct methods, help broaden the scope of graphite intercalation chemistry. As detailed below, both can be used to incorporate crown ethers into graphite. To prepare M-crown-GICs, the alkali metal M, graphite, and the crown ether were placed into glass tubes and either en or naphthalene was added (see SI section 1 for experimental details). Caution! Crown ethers and naphthalene are toxic. Alkali metals are f lammable when exposed to air and/or water and should be caref ully handled. The tubes were heated under an inert atmosphere to 90 °C and then centrifuged briefly. The solid products were rinsed and dried in vacuo at 60 °C. Reaction products are labeled as M-CECat-T with M = alkali metal, CE = crown ether, Cat = en or naphthalene (naph), and T = reaction time (h). PXRD data for products of reactions using en and 12c4 (Figure S1) do not indicate the formation of any low-stage GIC products after reaction times from 2 to 48 h. The products observed are mixtures of graphite with some reflections consistent with those expected from high-stage GICs. Stage n indicates that n graphene layers are present between intercalate galleries. High-stage GICs contain little intercalate, whereas stage = 1 indicates the maximum extent of intercalation, with all graphene sheets separated by intercalate galleries. Under these same conditions, en and alkali metals react with graphite to rapidly generate stage-1 GICs such as [Na(en)1.0]C15.10 The addition of 12c4 therefore suppresses this reaction. Since the crown ether concentration is in stoichiometric excess relative to solubilized metal cations, essentially all cations in solution are in crown ether complexes rather than in [M(en)x]+. While the latter will rapidly intercalate into graphite under these reaction conditions, the former do not. Thus, only graphite or only high-stage GIC products are obtained. All direct reactions (using 12c4 with the naphthalene electrocatalyst) similarly result in only graphite or high-stage GIC phases. In contrast, when 15c5 is used as a reagent, low-stage GICs are obtained for some reactions. With Li(m), no low-stage GICs form after 2−24 h (Figure 1a, b). Reactions with Na(m) generate
ABSTRACT: Crown ethers are well established as cointercalates in many layered hosts, but there are no reports of crown ethers incorporated into graphite. Here, we describe the preparation of the first graphite intercalation compounds (GICs) containing crown ethers. These GICs are obtained either by reductive intercalation of an alkali metal-amine complex followed by cointercalate exchange or by the direct reaction of graphite with a crown ether, alkali metal, and an electrocatalyst. Structural and compositional characterization of these new GICs using powder X-ray diffraction, thermal analysis, and GC/MS indicates the formation of well-ordered, stage-1 bilayer galleries.
I
n lithium-ion battery chemistry, the graphite anode is reduced to LiC6 during charging. This workhorse of rechargeable batteries is applied in devices ranging from mobile phones to electric vehicles.1,2 A comparable Na-ion chemistry has proven elusive due to the significant difference in the intercalation chemistries of Li and Na cations. Unlike Li+, unsolvated Na+ cannot form a low-stage GIC.3 There are many reports4−15 on the preparation of stable GICs that contain solvated Na+; the ionsolvate complexes intercalate into graphite at significantly higher potential than desolvated metal cations.16 Most cointercalates have been amines or ethers, due to their ability to solvate alkali metals and their reductive stabilities. The etheric cointercalates include THF derivatives,5−9 oligo- or polyethers,15 and glymes.4,5,7−9,12−14 This chemistry shows promising electrochemical charge/discharge behavior for Na-ion storage.12−14,17 Recently, Jache et al. thoroughly investigated a series of short linear chain glymes and proposed that tri- and tetra-glymes bind to Na+ to form “crown ether-like” configurations in GICs. However, attempts to directly cointercalate crown ethers by electrochemical reduction under similar conditions did not result in GIC formation.14 Similarly, crown ethers employed as electrolyte additives were not reported to cointercalate into graphite anodes.18,19 Crown ethers have been introduced as cointercalates into numerous two-dimensional hosts, including phyllosilicates,20 V2O5,21 layered zirconium phosphonates,22 LDHs,23 MS2,24 and MPS3.25 These crown ethers include the simple macrocyclics, 12crown-4 ether (12c4), 15-crown-5 ether (15c5), 18-crown-6 ether (18c6),21 and their derivatives (e.g., dibenzo- and dicyclohexano-18c6),26 as well as azacrown ethers.23 Our group previously has reported that amine−cation complex intercalates can be rapidly and quantitatively displaced © XXXX American Chemical Society
Received: July 18, 2016
A
DOI: 10.1021/acs.inorgchem.6b01689 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
The intercalation of the M-en complex is much more rapid than direct intercalation of M-crown ether complexes without using en. For example, the stage-1 Na-15c5-en-GIC forms within 2−4 h, while naph-assisted reactions to generate GICs with crown ethers require 6−12 h to produce significant stage-1 content. The reaction mechanisms are still under investigation, but based on the above observations and the high formation constants of these cations with both crown ethers and en, it is probably that the M-en cation complex initially intercalates and is subsequently displaced by the M-crown ether complex. For both exchange and direct reactions using Li and 18c6, only graphite and high-stage GICs are produced. Na-18c6-en-12 is predominantly a stage-1 bilayer phase with gallery expansion Δd = 0.70 nm (Figure S2c), but the product shows small unidentified PXRD reflections indicating a minor impurity phase. A similar bilayer result but different expansion in size was seen using the direct method (Figure S3c). Longer reaction times for this combination do not improve phase purity but result in decomposition of the stage-1 product. K and 18c6 reacting by the exchange method show mainly unreacted graphite with the formation of high-stage GICs at longer reaction times (Figure S2e,f). When the direct method is used, a mixed product with stage-1 GIC and graphite is obtained. Table 1 summarizes the reaction products obtained with the combinations of different alkali metals and 12c4, 15c5, or 18c6 Table 1. Reaction Products Obtained for Different Alkali Metals and Crown Ethersa
Figure 1. Ex situ PXRD patterns for reaction products of graphite with alkali metals, en and 15c5. Reaction times (in h) are indicated at the end of each label. Miller indices (00l) are provided for low-stage GIC products.
crown ether
Li
Na
K
12c4 15c5 18c6
G G G
M GIC1 GIC1
G GIC1 M
a
GIC1 = stage-1 GIC; M = mixed-phase, including low-stage GICs; G = graphite and/or high-stage GICs.
a new stage-1 GIC in 2h (Figure 1c,d). The basal repeat for this new phase, di = 1.19 nm, corresponds to a gallery expansion of Δd = di − 0.335 nm = 0.86 nm, consistent with a bilayer arrangement for the 15c5 cointercalate. For comparison, [Na(en)1.0]C15 is a stage-1 monolayer phase with Δd = 0.36 nm. K(m) under these conditions also produces a stage-1 GIC, with Δd = 0.86 nm. K-15c5-en-2 shows only the (003) reflection for the new phase, but a well-ordered single-phase GIC is obtained after 24 h of reaction (Figure 1e,f). Direct reaction with naphthalene results in a similar GIC bilayer phase (Figure S3a,b). A schematic illustration of the bilayer intercalate arrangement consistent with the known intercalate dimensions and observed gallery spacing is provided in Figure 2.
examined in this study. Products are classified as either new stage-1 GICs (GIC1), mixed-phases that include a low-stage GIC (M), or those containing graphite and/or high-stage GICs (G). GC-MS was used to identify and quantify the cointercalate in one of the new GIC phases. Following digestion in dichloromethane, Na-15c5-en-24 releases 15c5 but no en is observed. Quantitative analyses based on MS peak areas indicate a 15c5 mass percent of 41.5(3.4) % (N = 4) for Na-15c5-en-24 (Figure S4). This agrees well with TGA compositional analyses (15c5 = 43.2%; see below). The en content was found to be
