NMR Detected Insights into the Dynamics of Sodium Co-Intercalation

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C: Energy Conversion and Storage; Energy and Charge Transport

NMR Detected Insights into the Dynamics of Sodium Co-Intercalation with Diglyme Electrolyte into Graphite Anodes Linked to Prolonged Cycling Nicole Leifer, Miryam Fayena Greenstein, Albert Mor, Doron Aurbach, and Gil Goobes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06089 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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The Journal of Physical Chemistry

NMR Detected Insights into the Dynamics of Sodium Co-Intercalation with Diglyme Electrolyte into Graphite Anodes Linked to Prolonged Cycling Nicole Leifer,a Miryam Fayena-Greenstein, a Albert Mor, a Doron Aurbacha and Gil Goobes a,* Department of Chemistry, Faculty of Exact Sciences, Bar-Ilan University, Ramat-Gan, 5290002 Israel

ABSTRACT

Following the success of Li-ion batteries, Na-ion batteries are becoming an important economical alternative, particularly where weight and density considerations are not of primary importance. Graphite, the anode of choice for nearly all commercial Li-ion battery applications, has only recently been successfully used as such in Na systems. This unprecedented success was due to the proper choice of solvent, e.g. diglyme. Interestingly, lithium performs poorly under such conditions, which is the converse of their respective behavior in standard carbonate solvents. These phenomena have been attributed to co-intercalation of the alkali ions upon their complexation with the smaller solvent molecules. In the case of Li, the use of such solvents leads to deterioration, while in the case of Na, it improves its electrochemical performance so substantially as to make the previously irrelevant Na/graphite system viable. Several studies have

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since followed, mainly focusing on the Na-diglyme intercalation, however, a thorough understanding of the mechanisms of ternary intercalation into graphite for both Na and Li is still lacking. In particular, the characteristic differences in location and dynamics of the guest complexes in the host material upon electrochemical cycling are not yet fully understood. In this study the co-intercalation mechanisms of Na and Li in diglyme into graphite were explored via solid-state NMR spectroscopy. Direct evidence for the atomic proximity of both Na-(diglyme)2 and Li-(diglyme)2 complexes to the graphite planes in discharged electrodes was observed. Reduced mobility and stronger coupling of the Li-(diglyme)2 complex to the graphene electrons is seen, whereas higher mobility and weaker coupling to the host is detected for the Na(diglyme)2 complexes in the galleries, providing molecular cues for the difference in cycling performance of the two systems.

1. INTRODUCTION Sodium batteries are foreseen as the natural successors of lithium batteries, particularly for load-leveling applications, owing to several reasons: the higher abundance of accessible Na in the earth's crust1 ; its significantly lower processing and manufacturing costs; and its lower electropositivity compared to Li permits the use of less costly binder solvents (e.g. water, instead of N-methyl-2-pyrrolidon).2 Moreover, Na batteries can utilize Al metal as an anodic current collector instead of the more expensive copper used in lithium batteries due to sodium's disinclination to alloy with aluminum. Graphite is the most widely used anode material in current commercial lithium ion batteries, with a capacity of 372 mAh/g for LiC6. However, the same graphite intercalation compound (GIC) has been reported to have a very low capacity (< 35 mAh/g) when used as an anode in a sodium ion battery (SIB).3,4 Calculations have shown that the intercalation of Na ions into


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graphite is unstable,5 and thermodynamically disfavored. This instability was rationalized by the lower binding energy of Na ions to the (planar) graphene sheets of graphite (compared to Li and K in graphite) and the endothermic formation enthalpy of the NaCn bond, in which n carbon atoms taking part in the bond formed with a single sodium ion.6 Yet, recently it was shown that by using diglyme as the electrolyte solvent (Figure 1), reversible sodium insertion and extraction from graphite is possible, achieving up to 80 mAh/g (corresponding to approximately 20 carbon atoms per Na ion) for up to 1000 cycles with > 99 % reversible capacity.7 This appreciable improvement was attributed to the strong complexation of the Na ions by diglyme molecules and consequently the concomitant intercalation of the alkali ions with the coordinated diglyme molecules (“co-intercalation”), i.e. the existence of ternary (t)GIC complexes rather than binary (b)-GIC complexes inside the material. The migration of solvent-coordinated ions in and out of the graphite galleries had minor effect on its structure.8 Li cycled using the another ether-based solvent, tetrahydrofurane (THF), on the other hand, achieved decent initial capacities but exhibited severe capacity fading after a few cycles. This

Figure 1: 3-D Ball and stick model of the diglyme molecule, C6H14O3. ACS Paragon Plus Environment

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was attributed to exfoliation of the graphite,9 occurring during repeated co-intercalation of the Li ions with THF. The reduction of graphite in the ternary complex case is described in Equation 1:

( ) + + + ←

(1)

with Cn the n carbon atoms in the graphite lattice sharing the electron, M+ the alkali metal ion, and solv is the co-intercalating solvent.10 Computations have recently suggested that the unfavorable Na-graphene interactions, which cause instability of the binary Na-GIC formation, are effectively reduced by the screening of the diglyme molecules bound to the Na ions during co-intercalation.11 It was concluded that reversible Na intercalation into graphite is possible only for specific conditions in which Na solvation energy is high (i.e. strongly solvated Na ions). The Na-solvent complexes should remain stable inside the graphite galleries for preservation of graphite structure and consequent reversible [M(Li, Na)-solvent]+ co-intercalation. Therefore, the lowest unoccupied molecular orbital (LUMO) levels of the [M-solvent]+ molecules have to be higher than the Fermi level of the graphite. The LUMO levels of [M-ether]+ are higher than the Fermi level of graphite, suggesting that it would be difficult for electrons to flow from the graphite into these complexes (i.e. for these complexes to be reduced). Conversely, the use of propylene carbonate (PC), forming a [metal-PC]+ complex with a LUMO level that is substantially lower than the Fermi level of graphite, facilitates chemical reduction upon co-intercalation. Indeed, it is well known that intercalated [Li–PC]

+

undergoes decomposition and further reactions, leading to the

exfoliation of graphite.12,13 The operation of the first SIB combining a graphite anode with a P2Na0.7CoO2 cathode and a tetra-glyme based electrolyte was recently described.14


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Other studies investigating the performance of ternary Na–GICs followed,15 most of them focused on understanding the nature of sodium co-intercalation into graphite, using techniques such as XRD,16 EQCM,17 Raman spectroscopy, and FTIR analysis.18 While the results from these studies are compelling, these techniques can only provide indirect evidence of the cointercalation mechanism by monitoring changes in the lattice parameters of the graphite (XRD), changes in the overall graphitic order (Raman), or by detection of electrolyte in the cycled sample without the ability to determine its location (FTIR). Thus, despite the significant amount of research recently conducted on the Na-graphite system, the details of electrolyte cointercalation are still under debate. Various types of GICs were studied by solid-state 13C NMR more than four decades ago.19 The graphite

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C isotropic chemical shift was determined to exhibit a roughly linear dependence on

the average charge per C atom, upon successive intercalation of alkali species.20 Follow-up NMR research refined these models and the understanding of GICs, particularly with respect to the consideration of the Fermi level partial density of states, N(Ef), and changes in electronic distribution.21,22,23,24,25 For instance, studies on chemically prepared GICs have shed light on the differences in intercalant-to-graphite charge transfer between (t)-GICs and their (b)-GIC analogs (e.g. NaH vs. H). Also, unique

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C shifts were reported to reflect the different atomic and

electronic distributions within the graphene layers upon changes in the concentration of the intercalated species, and their distributions in the graphite lamellae.26,27,28 Previous NMR studies analyzing alkali ions electrochemically intercalated into graphite have only probed the Li ions as the charge carriers.29,30,31–34

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Na NMR studies of carbonaceous

anodes have so far only involved characterization of hard carbon and other non-crystalline carbon anodes, in which the sodium is only adsorbed and/or inserted into pores and large


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voids.35–37 One study investigating Na ions chemically prepared with diglyme in graphite focused on the coordination structure and dynamics of co-intercalant.15 Two diglyme molecules were shown to be weakly coordinated to each sodium ion and involved in rotational motions around the Na-O bonds using 2H NMR. In this study, the intercalation of Na-diglyme and Li-diglyme complexes inside graphite by electrochemical cycling are analyzed using multi-nuclear MAS NMR spectroscopy. Direct experimental confirmation of solvent co-intercalation for both nuclei is presented. New information about the local solvent-graphite-ion interactions are provided. The data shed light on intricate differences in the intermolecular distances, the dynamic state of the metal-diglyme complexes within the graphene sheets, electron density distributions between the components of the system, and subtle but significant differences in the complexation of the diglyme to the respective alkali ions. The data are discussed mainly in the context of computational studies of these systems11,38,39 and the recent NMR work.15 The achievement of Na intercalation into graphite has propelled a renewed interest in maximally exploiting this simple affordable material as the anode of choice. A greater understanding of the chemical and physical details of the intercalations mechanisms in these systems, such as provided in this report, could aid in the attainment of higher capacities and thus the realization of commercial sodium batteries.

2. METHODS 2.1 Electrochemistry Graphite (Hitachi: SMG-N-ET1-20, BET surface area ~1 m2/g) was used for the active material of the negative electrode in the prepared half-cells. Pouch-type cells were assembled


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using graphite electrodes with poly(vinylidene fluoride) (PVDF) as a binder and carbon black as electrical additive (carbon : PVDF : carbon black : 90 : 5 : 5, in wt%). Either sodium or lithium metal (Sigma-Aldrich) were employed as counter electrodes. The electrolytes were comprised of either 1M NaClO4 or LiTFSI in diglyme solution (Sigma-Aldrich). Due to technical reasons, salts containing different anions were used for the lithium and sodium solutions. However, since the emphasis in this work is on the fate of the cation and solvent in their intercalated state, the comparison was still considered reasonable and the counter ion effect assumed to be negligible. Electrochemical alkali metal insertion was performed galvanostatically at a rate of 10 mA/g on a Maccor system. All graphite half-cells underwent three full charge/discharge cycles, cycled vs. Li(s) or Na(s), and then charged/discharged to the specified potentials, and held at their final constant voltages for several hours before disassembly and analysis. The first two Na cells were cycled in the voltage range 2.00 - 0.01 V vs. Na. One cell was discharged (sodiated) to 0.01 V (Sample NaC-1), and the second cell was charged (de-sodiated) to 2.00 V (Sample NaC-2). The third Na cell was cycled in the voltage range 2.00 – 0.30 V. This cell was finally discharged to 0.30 V (Sample NaC-3). Analogous to the first two Na cells, the first two Li cells were cycled in the range 2.00 – 0.01 V using 1M LiTFSI in diglyme, one cell was discharged (lithiated) to 0.01 V (Sample LiC-1) and the second cell charged (de-lithiated) to 2.00 V (sample LiC-2).

2.2 Sample Preparation All of the carbon anodes samples were removed from their pouch cells under argon atmosphere in a glovebox (H2O < 0.9 ppm, O2 < 1.7 ppm). Half of the anode was scraped off of the copper foil current collector and immediately packed into the airtight MAS NMR rotors. The second half of each anode was left to dry under ambient conditions in the glovebox (i.e. no


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heating or vacuum was applied) and packed after several days or weeks. Throughout the manuscript these additional sample preparations are referred to as the ‘dry’ sample and indicated with ‘D’. In some cases, after the data was collected on the ‘wet’ samples, the rotors were opened and placed under vacuum for some minutes in order to exacerbate the drying; these additional samples are referred to as very dry and indicated with a ‘VD’.

2.3 NMR experiments 1

H, 7Li,

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C and

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Na NMR were measured on a 200MHz Bruker AvanceIII spectrometer at

spinning rates of 8-18 kHz, with no external temperature control. The chemical shifts reported were referenced to adamantane (1H,

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C), LiCl(aq) and NaCl(aq), respectively. Experiments

implemented on the samples included single-pulse, Hahn echo, 1H -13C INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) with low-power 1H decoupling, and 1H -13C and 1H 7

Li cross-polarization (CP) with 1H decoupling during acquisition. NMR experimental data per

experiment are detailed in Table S1. Line deconvolutions and sideband pattern calculations were conducted using both TopSpin 3.5 pl. 7, MestReNova 7.1.2 and Dmfit40 software packages. Sideband pattern and CP buildup simulations of the 1H and 7Li data were conducted using the SIMPSON simulation program41 and fittings of the CP build-up curves using Eq. (2) and (3) were conducted using a parameter minimization code employing the FMINSEARCH function in MATLAB©. The spin dynamics simulations of the 1H sideband pattern included a pair of protons with identical isotropic chemical shift evolving under a single dipolar interaction, neglecting CSA contribution and using the ZCW232 powder file to calculate the signal. The 7Li sideband pattern involved a single spin simulation of the narrow Li species separately from the broad Li species neglecting all but the quadrupolar coupling and using the REP678 powder file.


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The quadrupolar coupling constant Cq (= /ℎ) and the asymmetry parameter η were varied to achieve best fit of simulated sideband patterns to experimental ones. Structure minimizations of the M+-(diglyme)2 complex were performed under the Discovery Studio environment using a fast Dreiding-like force field. 3. RESULTS & DISCUSSION 3.1 Evidence for Co-Intercalation of Electrolyte Solvent Graphite electrodes were cycled galvanostatically vs. Na and Li metal anodes in 1M LiTFSI diglyme solutions in two types of cells. For the analytical measurements coin type cells were used, while for the preparation of graphite electrodes for the NMR measurements pouch cells were used. The electrochemical behavior of the graphite electrodes in Na and Li cells is presented in Figures 2a,b and 3a,b, respectively. In parallel to the standard specific capacity axis in the voltage profiles, the number of Na or Li ions intercalating/de-intercalating into/from the graphite electrodes per 20 carbon atoms in the graphite host is shown in Figures 2b and 3b. The results presented show the expected behavior of graphite electrodes in these solutions7. The voltage profiles in Figures 2a and 3a reflect insertion of either Na or Li ions with diglyme molecules into graphite, at a maximal specific capacity below 120 mAh/g. This is low in comparison to a maximal specific capacity of fully intercalated graphite with Li, 372 mAh/g, corresponding to a stoichiometry of LiC6.


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Figure 2: Electrochemical performance of graphite anodes cycled vs. Na(s), in 1M NaClO4 in

diglyme solution, recorded over four full cycles: (a) dQ/dV vs. V (vs. Na), second cycle. (b) voltage profile for all four cycles.

Figure 3: Electrochemical performance of graphite anodes cycled vs. Li(s), in 1M LiTFSI in diglyme solution, recorded over three full cycles: (a) dQ/dV vs. V (vs. Li), second cycle. (b) voltage profile for all three cycles.


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The co-intercalation of diglyme molecules in these solutions is possible because the graphite electrodes are not passivated by surface films at the intercalation potentials used. This is because of the relatively high cathodic stability of ethereal solutions. Hence, the electrode cannot be reduced by passivating species such as Li-alkoxides at potentials in which Li or Na ions intercalation can occur. Interestingly, the co-intercalation phenomenon into graphite in the diglyme solutions limits the specific capacity of Li ions insertion, but facilitates Na ions insertion, which fails in alkyl carbonate solutions. The voltage profiles and the derivatized dQ/dV vs. V plots of the intercalation/de-intercalation processes of both Li and Na ions indicate very well that these processes occur via reversible phase transition steps (reflected by the stages in the voltage profiles and the sets of peaks in the derivatized plots). Evidence and details of the concomitant insertion (co-intercalation) of the solvent molecules with the alkali metal ions into graphite to form ternary complexes are provided henceforth by the

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C and 1H NMR

spectroscopic measurements. For NMR measurements, graphite electrode samples were disassembled following three (Li) and four (Na) full cycles and in the following electrochemical state: A fully sodiated (intercalated) state (sample NaC-1), a fully de-sodiated (de-intercalated) state (sample NaC-2), a partially sodiated state (discharged to 0.3 V) (sample NaC-3), a fully lithiated (intercalated) state (sample LiC-1) and a fully de-lithiated (de-intercalated) state (sample LiC-2).

3.2 The Effect of the Interstitial Na- vs Li-Diglyme on the Magnetic Properties of Graphite Figure 4a-e shows the

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C single-pulse MAS NMR spectra of the graphite electrodes NaC-1

(blue), NaC-2 (green), NaC-3 (purple), LiC-1 (red) and LiC-2 (orange). The fully discharged


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samples NaC-1 and LiC-1 both exhibit a graphite peak at 123 ppm. The partially discharged NaC-3 exhibits a graphite peak at 121 ppm and charged samples do not show that peak. To further explore this spectral change, the

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C single-pulse MAS NMR spectrum of uncycled

graphite (grey) is also shown Figure 4g. The spectrum exhibits no apparent signal, however on an expanded scale (plotted in Figure S1) it shows a single resonance at 89 ppm (FWHM ~ 40 kHz) from the graphite carbons. The intensive line broadening and shift to high field (from the typical ~120 ppm value for aromatic carbons) is the result of the large anisotropic diamagnetic susceptibility20,42,43,44. It was previously shown45 that the larger the diamagnetic susceptibility χ is, the larger is the resonance shift and broadening. A general theorem describing the susceptibility effects in systems like graphite has been described45. Based on this theorem, the standard chemical shift tensor, which in small molecules results from localized currents, was supplemented by a macroscopic magnetization term to account for the significant long-range orbital contributions in these materials. In static NMR measurements, the carbon linewidth is governed by a large anisotropic magnetic susceptibility

19,20,46

Under MAS, the anisotropic

magnetic susceptibility is not averaged out and therefore the full span of chemical shifts experienced by the graphite carbons are observed giving rise to the broad line. The isotropic bulk susceptibility, on the other hand, is spun out, resulting in formation of a sideband manifold for moderate spinning. This spectral feature of the uncycled graphite sample is shown in Figure S1.


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Figure 4: 13C single-pulse MAS NMR of graphite electrodes after three full cycles disassembled in the a) fully sodiated state (blue), b) fully de-sodiated state (green), c) partially sodiated state (charged to 0.3 V) (purple), d) fully lithiated state (red), and e) fully de-lithiated state (orange). f) a

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C single-pulse spectrum of the electrolyte NaClO4 in diglyme (cyan), provided for

comparison and g) the spectrum of the uncycled graphite (grey). Spectra a-f were obtained using a 10 s recycle delay, a pulse width of 4.2 µs, 2-4 k transients, at 10 kHz spinning rate; spectrum g was collected using a 60 s recycle delay, a pulse width of 2.8 µs, 9 k transients, at 12 kHz

spinning rate. Spectra were normalized to sample mass and number of scans. Vertical lines indicate the isotropic shifts of the methylene and methyl lines in liquid diglyme. Interestingly, the graphitic carbon lines are significantly narrower (FWHM ~ 140 Hz) 28 in the discharged electrodes (NaC-1 and LiC-1) and are devoid of the large up-field shift, appearing at 123 ppm. Insertion of ions into graphite galleries is known to be accompanied by a c-axis lattice expansion of graphite and a significant drop in χ

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. The graphitic carbon resonance in Li


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intercalated to a similar concentration into graphite (C18Li state) was reportedly downfield shifted by 27 ppm relative to graphite20 in accordance with values measured here. This phenomenon is physically similar to metal doping, whereby introducing excess hole/electron density causes a shift of the Fermi energy level (EF) upwards. In graphite, it is reflected in the sharing of electrons between the conjugated π systems and the alkali ions upon intercalation. The shift of the Fermi level puts it outside of the valence band causing the largest contribution to the susceptibility by the valence electrons (χorb) to drop significantly and change sign, resulting in quenched total susceptibility χ.46 For example in the potassium intercalation compound KC24, the average total χ (in unit of 10-6 e.m.u/g) is 0.53, while for pure graphite it is -7.32

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, indicating

that for intermediate intercalation levels similar to ones used here, χ is already significantly decreased. The graphitic carbon linewidth will therefore be governed by Na+-C or Li+-C dipolar couplings in the case of intercalated graphite19,20 Under MAS, these couplings will be averaged out leading to the highly resolved lines observed. It was also shown that the change of the chemical shift in graphite with the concentration of intercalant is approximately proportional to the number of electrons in the π orbitals of graphite (partial density of states) and thus that an average charge transfer per intercalated ion can be estimated20. The 2 ppm upfield shift and broadening of the graphitic carbon in the partiallysodiated NaC-3 vs. NaC-1 is consistent with a smaller average charge transfer and a slightly larger average susceptibility. Note that the consistency of chemical shift values observed here and reported before for binary intercalation20 (without solvent co-intercalation) implies that sharing of electrons between alkali ions and conjugated π systems persists even in the presence the diglyme molecules, assuming co-intercalation does take place. The observation of an identical chemical shift and linewidth for graphite in NaC-1 and LiC-1 implies that the total χ is


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similar for both alkali ion intercalations. Recent computational studies predicted that the electron density distribution among the ion, the diglyme and the graphite should be different between Na and Li case,48,39 implying that there are differences in charge transfer between Na vs. Li and the graphite conjugated π orbitals. Indeed, the Pauli spin term, χp, in the total susceptibility, which depends on the density of states, N(EF), may be different for the two GICs. However, this term has still a smaller contribution to χ than χorb and when factoring in the lower concentration of Na vs. Li ions in the two fully intercalated samples, the net effect on the chemical shielding of the graphite may be negligible.

3.3 Co-Intercalation of Electrolyte Solvent with Charge Carriers The

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C single-pulse spectrum of the electrolyte solution (NaClO4 in diglyme) (cyan) is

provided for reference in Figure 4f. It comprises a quartet of lines at 59.0 ppm and a sextet of lines at 71.3 ppm (each line FWHM ~20 Hz wide) from the -CH3 and -CH2 groups in diglyme respectively, with their corresponding J-splitting patterns. As evident from the de-intercalated graphite materials 13C spectra (Figures 4b and 4e), similar peak multiplets are prominent around the 59 and 70 ppm chemical shifts, indicating the presence of the electrolyte solvent, in liquid form, in the charged electrodes. In the intercalated materials (Figures 4a and 4d), these multiplets appear significantly weaker, broader (FWHM ~ 45-90 Hz) and downfield shifted by 0.75 ppm in NaC-1 and by 0.5 ppm in LiC-1. They indicate that most of the diglyme molecules are no longer found as free liquid. The residual intensity (~ 8-16%) in the multiplets represents a small fraction of diglyme molecules with reduced mobility and broadened lines, experiencing a weak deshielding field from the graphite surface. These molecules are weakly adsorbed on the graphite


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outer surface retaining sufficient dynamics in the discharged materials to exhibit J-splittings in the spectra. The weak multiplets overlap two broad peaks centered around the same resonance values of the diglyme carbons. The broad peaks indicate diglyme molecules in physically restricted environment, such that is found in the graphite galleries. Signal integration shows that 84-92% of the solvent in the discharged NaC-1 and LiC-1 samples is inside the graphite layers. Concurrently, peak deconvolution of the de-intercalated NaC-2, LiC-2 materials spectra (Figures S2c and S2d) shows non-negligible intensity of the broad peaks from intercalated diglyme in these samples as well. This indicates some solvent molecule remain trapped in the graphite at full charge. The relative amount of trapped solvent electrolyte is ~25% compared to the signal in fully intercalated samples. The approximate relative line intensity of aromatic carbon vs. aliphatic intercalated carbons (lines at 72 and 58 ppm) (Figures 4a and 4d) is ~ 1.1 in the NaC-1 sample versus ~ 1.0 in the LiC-1 sample, which would correlate to the respective stoichiometries (diglyme)3.0C20 and (diglyme)3.3C20. However, from electrochemical measurements (Figures 2b and 3b) the ion-tographite stoichiometries at full intercalation are Na0.85C20 and Li1.2C20, thus, the expected stoichiometries would be Na0.85(diglyme)1.7C20 and Li1.2(diglyme)2.4C20 instead. This difference can be accounted for, by considering that 43% and 27 % of the solvent molecules are intercalated without Na and Li respectively, or by assuming that some portion of the graphite material is not visible in the spectra or by both effects. The presence of stripped down diglyme molecules inside the graphite in Na-based (t)-GIC was reported before15. Furthermore, their presence helps explain how most of the charge carriers are shuttling in and out of the electrode during cycling


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and capacity is retained between cycles while ~25% of the solvent molecules remains trapped in the fully charge state. Further evidence for co-intercalation of the solvent with the charge carrier inside the (t)-GIC is given by 1H-13C cross polarization (CP) measurements (Figure 5c). In the CP experiment, magnetization is transferred from 1H nuclei to adjacent

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C nuclei via through-space dipole-

dipole interactions in the absence of fast molecular tumbling motions. The appearance of the diglyme methylene (at 71 ppm) and methyl (at 60 ppm) peaks in the spectra of NaC-1 and LiC-1

Figure 5: a) 1H-13C CP build-up curve from samples a) LiC-1, and b) NaC-1, indicating the intensity of the three main peaks in each sample as a function of contact time, from 0.5 to 9 ms; c) a CP spectrum comparing NaC-1 (blue, bottom) and LiC-1 (red, top) at maximum contact time, 9 ms; d) table summarizing the kinetic parameter TCP and, where relevant, T1ρ, for each peak in the NaC-1 and LiC-1 samples, obtained from fittings of the curves in a) and b). The spectra were recorded using a 2s recycle delay, a pulse width of 4.2 µs, 1-2 k

transients, at 10 kHz spinning rate. Additional experimental details are provided in Table (d).


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reaffirms that the solvent molecules in the discharged electrode are inside the graphite galleries in a motionally-restricted state. This is further corroborated by the 13C magnetization build-up as a function of the CP contact time ( ) in the experiment (Figures 5a and 5b). It shows a maximal polarization of diglyme carbons by neighboring protons in less than 6 ms, typical of partially immobilized species. Further evidence for the insertion of the ether molecules to the graphitic galleries is manifested by the appearance of a relatively strong peak at 123 ppm, associated with bulk graphite carbons. As the diglyme molecules are practically the only source of protons in the sample, this is an indication of appreciable magnetic dipole-dipole interactions between the electrolyte solvent protons and the bulk graphite carbons. The diglyme carbons maximal intensity is almost equal in both samples. The graphite maximal intensity, on the other hand, is about four-fold higher in LiC-1 than in NaC-1. Since this intensity is proportional to the number of protons involved in polarization transfer, it implies fewer protons are polarizing the graphite carbons in NaC-1. As the interplanar spacings measured via XRD are similar for the two GICs, 10.44 Å for Li(digl)2Cn vs. 10.62 Å for Na(digl)2Cn

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, the

marked differences in total polarization are attributed to greater mobility of the sodium-solvent complexes inside of the graphite.


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Figure 6: 1H single-pulse spectra of graphite anodes after three full cycles disassembled in the a) fully sodiated state (blue, bottom), b) fully de-sodiated state (green), c) fully lithiated state (red) and d) fully de-lithiated state (orange). e) 1H spectrum of electrolyte solution (1 M LiTFSI in diglyme) (cyan). Inset shows spectra zoomed out on the

frequency axis and expanded on the vertical axis to show the differences in relative spinning sideband intensities of the broad peak in the intercalated samples NaC-1 and LiC1. The spectra were recorded using a 5 s recycle delay, a pulse width of 1.75 µs, 8

transients, at 10 kHz spinning rate. Spectra were normalized to sample mass and number of scans. * Indicates spinning sidebands. The 1H single-pulse MAS NMR measurements of the (t)-GIC materials offer further evidence of co-intercalation. Spectra of NaC-1 (blue), NaC-2 (green), LiC-1 (red) and LiC-2 (orange) are shown in Figure 6a-d, along with the spectrum of 1M LiTFSI in diglyme (cyan) in Figure 6e. The 1H NMR shifts of liquid diglyme (Figure 6e) typically resonate between 3.3 and 3.5 ppm50.


19


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In the spectra (Figures 6b and 6d) of the de-intercalated materials, narrow signals of liquid diglyme appear, indicating that the electrolyte solvent is out of the graphite and in free form. In the intercalated NaC-1 and LiC-1 samples, narrow signals are observed downfield-shifted by ~0.15 ppm with 2-5% of the total spectral intensity (see line deconvolution in Figures S3a and S3b). They are associated with diglyme weakly adsorbed on outer graphite surfaces. Two broad peaks are observed at 3.4-3.5 and at 5.4 ppm with relative intensity of 25:70 in NaC-1 and 26:72 in LiC-1. They represent diglyme molecules inside the (t)-GIC galleries. The former peak is attributed to diglyme protons intercalated alone inside the galleries that are not experiencing the effect of the graphite electrons while the latter peak is attributed to diglyme protons intercalated as a complex with the ions and that interact strongly with the graphite electrons, through orbital overlap11. As noted above, the susceptibility decreases by an order of magnitude but also changes sign thus acquiring a paramagnetic character rather than diamagnetic one.47 Therefore the ~2 ppm downfield shift and additional broadening observed for the co-intercalated protons is evidence for the substantial de-shielding effect via χorb by the ring currents of delocalized π electons in the material.51 This effect is expected to be smaller on the carbons and was therefore not resolved in the 13C spectra. The ratio of diglyme intercalated alone and with the alkali ion is consistent with 13C single-pulse results for the Li case (27%) and is low for the Na case (43%), however, these values are only rough estimates.

3.4 Details of Adsorbed Electrolyte The 1H-13C INEPT experiment is used to excite

13

C nuclei by transfer of polarization from

protons via a different mechanism than CP, i.e. via through-bond J-couplings. These measurements therefore are expected to show signals only in the diglyme region. The spectra are


20

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collected using 1H decoupling during acquisition so the J-splitting multiplets are removed and carbon peaks appear on their isotropic resonance. Figure 7a-d shows the INEPT spectra of NaC1 (blue), NaC-2 (green), LiC-1 (red) and LiC-2 (orange). The de-intercalated samples, NaC-2 and LiC-2, exhibit a single peak for the methyl carbon at 58.4 ppm and two peaks, at ~70.2 and ~71.6 ppm for the methylene carbons from liquid form of diglyme. The intercalated samples, NaC-1 and LiC-1, indicate two sets of three peaks (see line deconvolution in Figures S4a, S4b


21


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and Table S2). One set of peaks (FWHM ~ 45-80 Hz) is attributed to diglyme weakly bound to the outer surface of the graphite. A second set of broad intense peaks (FWHM ~130-200 Hz) is attributed to diglyme located in the graphite interplanar spaces. A summary of the shifts, line widths and intensities of the peaks is given in Table S2. Note that in the INEPT the intensities of intercalated peaks are attenuated allowing for a clearer identification of adsorbed solvent molecules.

Figure 7: 1H-13C INEPT spectra from samples NaC-1 (blue), NaC-2 (green), LiC-1 (red) and LiC-2 (orange). The accompanying table summarizes the peak positions and linewidths

obtained from line deconvolutions of each spectra (shown in Figure S4). The position values given are in ppm, the linewidths, shown in parentheses, are given in Hz. The spectra were recorded using a 2 s recycle delay, a pulse width of 4.2 µs, 4 k transients, with cnst11-

6, at a 10 kHz spinning rate. Spectra were normalized to approximate sample mass and number of scans. Further experimental details are provided in Table S1.


22

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3.5 Dynamics of the Diglyme Solvent in the Graphite Layers 13

C CP build-up curves from samples NaC-1 and LiC-1, in which line intensities are measured

as a function of contact time, are shown in Figures 5a and 5b. In order to determine the kinetic parameters (the CP time constant, and, where relevant, the rotating-frame relaxation # parameter, T!" ), of magnetization transfer from protons to carbons, the curves were fitted with

either of the following equations52: $( ) = $% [1 − $( ) = $% (1 −

*+, . +,

) -

]

(2) *+,

0+, ! 564 7 [ 23 4 ) 123

−

* 8 +, 9 6+,

],

(3)

where $% is the maximum intensity of the signal and τCP is the variable contact time in the experiment. Most of the peak build-ups were fitted with Equation 2 and for those curves only a value for TCP was derived (see table in Figure 5d). The only peak build-up for which Equation 3 # (i.e. the inclusion of T!" ) was necessary for a proper fitting, was the –CH2 diglyme peak at 71

ppm in LiC-1. The measured for the diglyme carbons are in general much larger than

anticipated for completely solid diglyme molecules in a crystal. Typical build up times for CH2 groups are ~100 µs. This indicates that the intercalated molecules in both (t)-GIC electrodes are experiencing significant motions. Marked differences are seen in the values associated with the diglyme carbons in the two charged anode materials (1.1 and 1.6 ms for LiC-1 vs. 2.0 and 2.6 ms for the NaC-1) whereas values found for polarization of the graphite carbons in LiC-1 and NaC-1 are similar. Assuming that the carbon magnetization build-up is governed by an effective dipolar coupling representing the cumulative heteronuclear couplings to all proximate diglyme protons, the CP time constant is crudely proportional to the inverse of this dipolar coupling


23


Page 24 of 48

strength (:# ) squared and thus to the effective internuclear distance ;# to the sixth power:53,54,55 = ~:# ~;#

(4)

This allows for the relative CP time constants of carbons in the two discharged (t)-GIC materials to be described in terms of the relative effective distances to polarizing protons. The >? @A / ratios for the methylene line (~ 70 ppm) and the methyl line (~ 59 ppm) are 1.81 and >B @A 1.63 corresponding to distance ratios ;# /;# of 1.105 and 1.084, respectively. These ratios

indicate longer effective distances between the diglyme carbons and their polarizing protons in the NaC-1 vs. the LiC-1 samples. As the molecules in both cases are already involved in motions, the large effective distance is linked to increased internal motions of the co-intercalated # solvent molecules in the Na case. The non-negligible T!" value for the methylene peaks in the

LiC-1 sample suggests reduced motions and larger internal 1H-1H interactions compared to the NaC-1 case. This is in line with the stronger 1H-1H interactions measured in the 1H spectra in the LiC-1 sample, which, as explained in the next section, correspond to the same set of protons (the –CH2 protons on C3 and C3’). >B @A The / for the graphite line (123 ppm) is 1.21 corresponding to a similar effective

dipolar coupling and an effective distance that is larger only by 3% in the NaC-1 sample. The internal motions inferred from the buildup rates cannot explain the lower graphite signal enhancement in NaC-1, neither can the smaller fraction of diglyme co-intercalated with Na (57%) vs. diglyme co-intercalated with Li (73%). It can, however, be explained via differences in translational mobility of the M+-diglyme complexes56. Theoretical dynamical simulations indeed show faster diffusion of Na+-diglyme vs. the Li+-diglyme in the graphite lamellae48 which could be used to explain these results. The diffusion coefficient, D, calculated in that study for the Na-


24

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diglyme complex in a Na(diglyme)C16 model was on the order of 10-8 cm2/s.48 Using the expression for the diffusion length, < D > ≅ 2(HI)!/

(5)

and taking the typical CP time as 1 ms, one can estimate < D > to be ~ 630 Å for the Na complex. Such an average distance travelled in the timescale of the experiment would not allow for any transfer of polarization and would result in no graphite signal. On the other hand, the diffusion coefficient for the Li-diglyme complex in an analogous model48 was on the order of 1013

cm2/s, corresponding to an average distance travelled during the CP experiment of only 2 Å,

only slightly larger than C-C bond distance. In this case, CP enhanced graphite signal would be possible to detect. The NMR results are therefore consistent with the predicted Li-diglyme diffusion rate and with higher rates for the Na-diglyme complexes. They are, however, not supportive of a 5 orders of magnitude higher diffusion rate for the Na(digl)2 48. While the isotropic peaks in the 1H spectra (Figures 6a and 6c) of the intercalated samples indicate approximately the same intensity, the sidebands in the LiC-1 sample, particularly the sidebands of the broad, 5.5 ppm peak, are significantly more intense than in the NaC-1 sample. The sideband patterns in the 1H spectra are dominated by 1H-1H homonuclear dipolar interactions. A simulation of these sideband patterns indicated that the cumulative dipolar strengths are ~ 2.3 times as large in LiC-1 as in NaC-1 (-6450 Hz vs. -2800 Hz, respectively), corresponding to effective 1H-1H distances of 2.65 ± 0.01 and 3.53 ± 0.21 Å in samples LiC-1 and NaC-1, respectively. For the case where the complex is held by six M+-O bonds, it is easy to rationalize, by elimination, an association of significant changes in 1H-1H distances with changes in the H3-H3' distances. The reason is that the methyl protons H1,H1' are typically involved in jumps or continuous rotations and therefore do not contribute to the dipolar couplings. The H2-


25


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H3 (H2'-H3') distances are mostly invariant as they are located on single C-C bonds restrained from rotating freely by the M+-O bonds. Even upon tighter complexation manifested by shorter M+-O distances, these distances are expected to remain unchanged. Therefore, it is only the H3H3’ protons that can vary in their distances due to possible variations in the central C3-O-C3' angle (indicated as α in Figure 1), which is dependent on the tightness of the coordination of the alkali ion. A simple structure minimization of the M-(diglyme)2 complex carried out, resulted in a near tetrahedral C3-O-C3' angle (109 o) giving rise to an average H3-H3' distance of 2.66 Å, in accordance with the measurement of the Li-(t)-GIC sample. Previous calculations of the Na(diglyme)2 complex predicted shorter Na+-O distances than analogous Li+-O distances in the Li(diglyme)2 complex37. However, a realistic variation in the C3-O-C3' angle upon tighter coordination is not expected to increase the H3-H3' distance sufficiently to account for the 0.92 Å extension measured. Therefore, it is evident that the reduced effective interaction and the longer effective distance are due to motions, in this case of the entire complex. Such motion would vary the angle of the H3-H3' distance vector with respect to the magnetic field, thereby causing the effective dipolar coupling to decrease, depending on the type of motion occurring.

3.6 Locations and Relative Quantities of the Charge Carriers Inside and Outside of the Graphite

Figures 8a-d shows the 23Na spectra of sample NaC-1 (blue), NaC-2 (green), NaC-3 (purple), and the liquid NaClO4/diglyme electrolyte (cyan). A summary of the peak chemical shifts, their widths, and their distributions in the four samples is given in Table 1. The neat electrolyte (NaClO4 in diglyme) gives rise to a narrow peak at -8.4 ppm. Sample NaC-2 (de-sodiated)


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indicates a peak of similar shift and line width as the free electrolyte (relative intensity 52%), in addition to a broad peak at -10.8 ppm. The broad peak (relative intensity 48%) is associated with a fraction of the Na ions strongly adsorbed on surfaces of graphite and a fraction of the ions irreversibly trapped inside the graphite.35–37 The severe broadening of this line (FWHM ~ 2 kHz) may stem from a significant 2nd order quadrupolar attributed to non-symmetric environments around the Na ions inside and on disordered surface sites.


27


Figure 8:

23

Page 28 of 48

Na single-pulse MAS NMR of a) NaC-1 (blue), b) NaC-2 (green), c) NaC-3

(purple), and d) 1M NaClO4 in diglyme solution (cyan). The spectra were recorded using a 1 s recycle delay, a pulse width of 2.5 µs, 2 k transients, at 12 kHz spinning rate, for the solid

samples. Spectra were normalized to approximate sample mass and number of scans. The sample inset shows the three cycled samples’ spectra, overlaid and zoomed in to highlight signal overlap. A peak of similar shift and line width is also seen in both the fully and partially sodiated samples. As expected, the relative intensity of this signal in each case (~ 26 % in NaC-3 and ~ 28 % in NaC-1; see Figures S4a and S4b) corresponds in absolute terms to the amount of irreversible capacity indicated for those samples. This irreversible capacity is associated with the ions irreversibly trapped inside the graphite, discussed above. Upon close examination of the dQ/dV data from Na cycled against disordered graphite35 it can be surmised that upon discharge of the cell from 2.0 V, only a very small amount of capacity is seen before the cell


28

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reaches 0.3 V; it is only in the last part of the discharge, from 0.3 to 0.1 V, where the more significant intensity is seen in the voltage profile, indicating that this voltage window is where most of the Na is inserted. In contrast, the voltage profile of ordered graphite indicates a very large peak at 0.3 V, suggesting that, in this case, a different type of sodiation mechanism is occurring; i.e., intercalation. Thus, sample NaC-3, which was only discharged to 0.3 V is expected to indicate a greater ratio of intercalated vs. trapped Na, while sample NaC-1, which was discharged all the way to 0.01 V, would exhibit more adsorption. This is well reflected in the NMR data. All previous

23

Na studies of Na in disordered carbons have assigned the peak

intensity at ~ +5 ppm to sodium weakly adsorbed in the spaces between misaligned graphene sheets and/or in closed nanopores.35–37 A peak ~ +5 ppm peak is seen in both samples, and in correspondence with the electrochemical data, in sample NaC-3 it comprises only 5 %, while in NaC-1, it increases to 13 %. As expected, due to the restriction of motion of these ions, this line indicates a significantly increased line width (more than five times) from that of the free electrolyte however is still narrow enough to indicate significant mobility and migration capability under an electric potential i.e. between partial and full discharge. Their relatively small percentages are expected for highly ordered graphitic material. The

23

Na NMR shift of sodium intercalated into ordered graphite has not been previously

reported. Hence, by deduction, the remaining intensity in each sample (comprising 65 % and 45 %, respectively, for sample NaC-3 and NaC-1) is attributed to diglyme-coordinated, intercalated Na species. The shifts of these peaks are quite similar to those of the free electrolyte indicating that the susceptibility effect on the chemical shift is negligible at the center of the Na+-(diglyme)2 complex. This is not surprising since the immediate environment of the Na ions complexed to the diglyme solvent will not change very much upon intercalation. The relatively small increase in


29


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linewidth (from ~ 100 to ~ 200 Hz) indicates that the symmetric electronic environment around the ion is preserved also inside the graphite galleries and that restriction of motion is associated with this change. The percentages of liquid electrolyte seen in the

23

Na NMR data for sample

NaC-1 is consistent with the amount measured from the 1H NMR data (4 % in both), confirming that a negligible amount of diglyme coordinated to Na remains as free liquid electrolyte-solvent complex at full discharge. Table 1: Summary of the peaks seen in the line deconvolutions of the spectra in Figure 8, the 23

Na data of the three samples NaC-1, NaC-2 and NaC-3, listed according to their assignments

(deconvolutions are shown in Figure S5)

Sample

NaC-2 (desodiated)

NaC-3 (partially sodiated, discharged to 0.3 V) NaC-1 (fully sodiated, discharged to 0.01 V)

Free Electrolyte

Surface Film from Electrolyte Breakdown

-8.6 ppm

-10.8 ppm

97 Hz

2.3 kHz

52 %

48 %

-8 ppm

-9.8 ppm

5.2 ppm

-7.5 ppm

100 Hz

2.2 kHz

560 Hz

213 Hz

4%

27 %

5%

64 %

-9 ppm

-12.4 ppm

6.4 ppm

-8.2 ppm

100 Hz

1.8 kHz

614 Hz

324 Hz

3%

24 %

13 %

60 %

Reversibly Adsorbed Electrolyte

Intercalated Electrolyte

--

--


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7

Li single-pulse MAS NMR results from samples LiC-1 (fully lithiated), LiC-2 (fully de-

lithiated) and the liquid LiTFSI/diglyme electrolyte are shown in Figure 9. The 7Li NMR spectrum of the delithiated sample, LiC-2, indicates a signal corresponding to the free electrolyte

Figure 9: 7Li single-pulse MAS NMR spectra of cycled graphite samples: LiC-1 (lithiated) (red), LiC-2 (de-lithiated) (orange) and 1M LiTFSI in diglyme (yellow). The spectra were recorded using a 3 s recycle delay, a pulse width of 1.75 µs, 16 transients, at 10 kHz spinning

rate. Spectra are normalized to approximate sample mass and number of scans. Inset shows the full spectrum from sample LiC-1 to show the complete sideband pattern. at -1.5 ppm, in addition to a negatively shifted broad signal, attributed to electrolyte irreversibly


31


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bound to graphite surfaces or trapped inside collapsed graphene sheets. In addition to these two species, the spectrum of the LiC-1 (fully lithiated graphite) indicates two additional resonances at approximately 0.5 ppm and 1.5 ppm, each comprising ~ 40 % of the total spectral intensity (see line deconvolutions of all samples in Figures S6a and S6b). The 1.5 ppm line (FWHM 1260 Hz) is associated with Li ions co-intercalating with the diglyme species. This is consistent with a previous 7Li NMR study of lithiated (t)-GIC,57 in which the intercalated Li resonance is close to 0 ppm, indicating that the Li preserves its ionic character in the ternary complex. This implies that susceptibility effects on the central ion can be neglected here as well.48 Indeed this is consistent with computational predictions which show that the alkali metal loses nearly its entire valence electron density to the graphite, and is then shielded by the intervening solvent molecules, which both lose and gain density.39 The 0.5 ppm line (FWHM = 250 Hz) is associated with uncomplexed, intercalated Li ions.29 A separate peak corresponding to an adsorbed species is not discernible, but likely is subsumed under the large, relatively broad peak from the intercalated species. Figure 9 (inset) shows the full spectrum of the intercalated sample, in which spinning sideband manifolds can be seen. Fitting these sideband manifolds for the 0.5 ppm and 1.5 ppm peaks, using a quadrupolar coupling interaction, one obtains η = 0.00 in both cases and respective Cq values of 13 kHz and 50 kHz. The significant quadrupolar interactions58 imply that the Li environment for the Li ions intercalated either alone or with diglyme is non-symmetric and is more distorted for the ion in complex with the diglyme intercalated inside the graphitic galleries. This finding is in line with reduced motion and significant dipolar interactions observed from the other measurements.


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Additional insight into the metal-solvent interactions in the Li case can be inferred from the 1

H-7Li CP NMR results. The CP spectrum shows only the 1.5 ppm line, confirming the

assignment of this peak to the diglyme-complexed Li, with its associated reduced mobility. The intensity from the broad negative peak (associated with irreversible/trapped Li) was too weak to measure its buildup. The 1H – 7Li CP build-up curve of LiC-1 (Figure 10) was fitted using Eq. (3), to yield TCP = 0.48 ms and T!" = 34.0 ms. The TCP value can be compared to the value measured for the methylene carbon in the same material noting that the dipolar coupling

Figure 10: 1H-7Li CP build-up curve from sample LiC-1, indicating peak intensity of the

single detected peak as a function of contact time, from 0.5 to 8 ms. Additional experimental data provided in Table S1, in the ESI.


33


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strength, :# , in Eq. (4) is dependent on the gyromagnetic ratio of the nucleus polarized by the

@A protons. The ratio of the two times, / , when scaled by gyromagnetic ratio squared, gives

an effective distance ratio of ~0.96 to the protons, suggesting that the Li ions and adjacent methylene carbons in the intercalated Li-diglyme complex are sensing similar proton environment. For a rigid molecular complex, one would expect the polarization of the -CH2 carbons to build much faster than the Li ions. However, the buildup time for the methylene carbons indicated significant motions of the diglyme molecules so as to extend their TCP from an order of ~100 µs to ~1 ms. It is, therefore, quite reasonable that motions of the entire complex will support a similar cumulative 1H dipolar field around the central ion as on the carbons in the molecule. Owing to the large quadrupolar interactions measured for the co-intercalated Li species, it is noteworthy that a CP build-up simulation should account for the contributions of such large interaction. Since this is absent in the model giving rise to Eq. (3), we have simulated a simplistic 1H-7Li spin pair CP build-up accounting for the quadrupolar coupling and using a dipolar coupling of 2000 Hz corresponding to a 1H-7Li distance of 2.858 Å. The resultant buildup curve maximized at 2.0 ms, which is crudely consistent with the duration for maximal intensity observed here in Figure 10. The NMR data of the alkali ions indicates a transfer of charge from the Na and Li ions to the graphite in both LiC-1 and NaC-1 samples as judged by the absence of Knight shifted lines in both the Na and the Li cases. This was rationalized with computational predictions which showed that the alkali metal loses nearly entirely its valence electron density to the graphite, and is then shielded by the intervening solvent molecules.48,39


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The analogous 1H-23Na CP experiments were attempted, however, owing to the large quadrupolar coupling of the

23

Na ions, cross-polarization was rendered inefficient, leading to

negligible detectable signal.

3.7 Evidence of Drastic Changes During Sample Preparations (Drying)

Given the high volatility of the diglyme electrolyte, it was decided that the cycled anode materials would be investigated without rinsing or drying, and instead would be immediately packed into the NMR rotors upon opening the cycled pouch cells. This was so as to preclude any changes to the samples due to a rinsing and/or drying procedure. It was also done in order to investigate the graphite electrode before excessive evaporation of the diglyme took place, in case some of the co-intercalated diglyme may also evaporate. Additional insight into the consequences of this latter concern was provided by NMR measurements of the ‘dry’ samples. As expected, the spectra from the ‘dry’ samples (Figures S7-S9, a-c) indicate a loss of the liquid or liquid-like electrolyte signal, including the narrow peaks in the diglyme region in the

13

C

spectra, the narrow peaks in the 3-4 ppm region in the 1H spectra, the -8 to -9 ppm region in the 23

Na spectra, and the narrow peaks at -1.7 and -2.0 ppm in the 7Li spectra. However more drastic

changes in the spectra are noted upon drying, suggesting more extreme changes in the samples. More detailed discussion is provided as part of the Supplementary Information. The retainment of electrolyte solvent is shown to be crucial for determining the solvent locations and roles in the migration of ions in and out of the electrode. The data showed that even a moderate drying procedure appears to have a significant effect on the sample, inducing the evaporation not just of the residual surface solvent, but also causing the departure of some of


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the electrolyte solvent material from within the intercalated graphite layers, leaving behind nonsolvated and/or partially solvated alkali ions in the graphite structure. Thus, it may be advisable, when studying ex-situ samples, to forgo a rinsing and/or drying procedure, especially when the signal from the solvent is not overwhelmingly larger than other components of the electrochemical cell. The advantage is that the NMR analyses, which can differentiate solid from liquid signals, is then able to capture the realistic state of the charge carriers, either dissolved or bound within a solid matrix.

4.0 CONCLUSIONS The differences in electrochemical performance of graphite as a host for intercalating Na vs. Li in diglyme has drawn much attention, particularly with the ongoing search for an appropriate anode for SIB. The evidence for the formation of ternary complexes inside the graphite galleries was demonstrated experimentally here for both alkali ion systems. In the Li (t)-GIC electrode case, both the charge carrier and graphite were shown to be proximate to diglyme molecules as gleaned from cross polarization of both the host carbons and Li ion by diglyme protons. CP dynamics measurements have vividly demonstrated that within the graphite interstices, the Li(diglyme)2 complex is held more tightly in interaction with the graphene sheets than the Na(diglyme)2 complex. This is in accordance with the stronger interactions calculated for the Li ion with the π - conjugated orbitals of the graphite, even in ternary complex with coordinating diglyme molecules. This interaction acts to reduce the mobility of directly bound diglyme molecules in the ternary complex. Other intercalated diglyme molecules, not coordinated to alkali ions, are observed. These molecules experience a much weaker effect of the graphite


36

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electrons, i.e. their protons experience no downfield shift and concurrently they undergo higher mobility. The dynamics of intercalated Na-(diglyme)2 has some translational character, in accordance with lateral diffusivity computed for the complex and weaker binding to the graphite electrons. The Li-(diglyme)2 complex undergoes lower mobility which mostly rotational in character. The different forms of motions in the two intercalated complexes, as gathered collectively from the different measurements, are summarized in Figure 11. The weaker interactions and higher mobility of the Na complex permits facile intercalationdeintercalation cycles with minimal and reversible changes to the stacking of the graphene sheets. The Li-(diglyme)2 moieties inside the galleries undergo some rotational motions,

Figure 11: Proposed models of the motions of the Na(diglyme)2 and Li(diglyme)2 complexes inside the graphite.


37


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however are more restricted due to their stronger binding to the graphite (see Figure 11). Shuttling the complexes out and in of the structure can therefore be more detrimental to the host structure. The Li intercalated complexes also suffer from distortion of the coordination sphere around the Li ion inside the galleries. To infer the exact rotational motions of the solvent molecules in the ternary complex requires further studies to measure accurately the distance of the alkali ions to the graphite carbons. Recent work suggested that at high temperatures Na is coordinated through two Na+-O bonds to the pair of diglyme molecules allowing for rotational motions of the entire solvent molecule15. The current measurements cannot support such kind of motion, mainly due to the observed polarization of graphite by diglyme protons in the Na-(diglyme)2 -graphite compound, but preparation of the material was different than here which may explain discrepancy between the measurements.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: One table indicating the acquisition and processing parameters used for all of the NMR experiments; nine figures showing the full 13C single-pulse spectrum of uncycled graphite, line deconvolutions of 13C and 1H single pulse spectra of samples NaC-1, LiC-1, NaC-2 & LiC-2, line deconvolutions of 1H - 13C INEPT spectra of samples NaC-1 & LiC-1, line deconvolutions of 23Na single pulse spectra of samples NaC-1, NaC-2 & Na-C-3, line deconvolutions of 7Li single pulse spectra of samples LiC-1 & LiC-2, and 13C, 1H and 23Na single-pulse spectra of


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samples NaC-1, NaC-1-D & NaC-1-VD; a second table quantitatively summarizing the deconvolution data from figures S2, S3 a-d & S4a & b. AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgments The authors wish to thank Shani Hazan for help with the CP build-up curves and molecular modeling. There are no external funding sources to report. REFERENCES (1)

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NMR Detected Insights into the Dynamics of Sodium Co-Intercalation

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