Ultrafast Solvent-Assisted Sodium Ion Intercalation into HighlyCrystalline Few-Layered GrapheneAdam P. Cohn,† Keith Share,‡ Rachel Carter,† Landon Oakes,‡ and Cary L. Pint*,†,‡

†Department of Mechanical Engineering and ‡Interdisciplinary Materials Science Program, Vanderbilt University, Nashville,Tennessee 37235, United States

*S Supporting Information

ABSTRACT: A maximum sodium capacity of ∼35 mAh/ghas hampered the use of crystalline carbon nanostructures forsodium ion battery anodes. We demonstrate that a diglymesolvent shell encapsulating a sodium ion acts as a “nonstick”coating to facilitate rapid ion insertion into crystalline few-layergraphene and bypass slow desolvation kinetics. This yieldsstorage capacities above 150 mAh/g, cycling performance withnegligible capacity fade over 8000 cycles, and ∼100 mAh/gcapacities maintained at currents of 30 A/g (∼12 s charge).Raman spectroscopy elucidates the ordered, but nondestruc-tive cointercalation mechanism that differs from desolvated ionintercalation processes. In situ Raman measurements identifythe Na+ staging sequence and isolates Fermi energies for the first and second stage ternary intercalation compounds at ∼0.8 eVand ∼1.2 eV.

KEYWORDS: Graphene, sodium ion batteries, in situ Raman spectroscopy, solvent cointercalation, graphene intercalation compounds,anode, Na+

Graphite and more recently graphene, which are crystallineforms of carbon, have been pivotal in the development of

lithium-ion batteries. Despite years of research on alternativeanode materials, carbons remain the paramount choice forbattery manufacturing due to low cost, excellent stability withdiverse electrolytes, and high capacity. Whereas sodium ionbatteries present a cost and manufacturing landscape that couldpotentially revolutionize low-cost secondary storage applica-tions, such as grid-scale storage, a major hurdle has been thatcrystalline carbon materials are a poor host for sodium ions,leading to a maximum capacity of <35 mAh/g.1 As a result,researchers have strived to discover alternative anodematerials2,3 with recent notable advances such as the develop-ment of a high-performance graphene-phosphorene hybridanode4 and significant progress toward stable Na metalanodes.5 However, a major arm of sodium anode researchremains focused on a class of disordered or nongraphitizedcarbons. In these materials, the disordered stacking of thecarbon prevents a staging reaction, and the Na ions areproposed to react with defect sites and fill microporous voids inthe carbon.6 To overcome higher resistance and surfacemediated storage processes, expanded graphite,7 hollownanowires,8 nanospheres,9 and nanosheets10 have been ex-plored. Despite the range of defective carbon materialsconsidered, long operational life spans and high rate capabilityremain elusive.On another front, the ability to leverage the cointercalation

of a solvent shell to assist metal ion storage has opened new

routes to explore combinations of ion species and materials thatare otherwise impossible. Recently, Lin et al. showed thataluminum ion batteries based on graphitic electrodes can beproduced with specific capacity of ∼70 mAh/g by facilitatingintercalation of the Al ions through a chloroaluminate ionspecies formed in the electrolyte.11 These authors reportminimal capacity fade over 7500 cycles and rate capabilities upto 4 A/g. Such ideas have also recently been explored for Na-ion batteries using commercially purchased bulk graphitematerials to achieve capacities of ∼110 mAh/g by usingglyme-based electrolytes.12−15 Overall, the ability to bypass theslow desolvation step16 has been proven to yield higher ratecapabilities than traditional intercalation processes.13 Whereasnanoscale materials such as few-layered graphene exhibitinherent large electrode−electrolyte interface areas that resultin ion storage near the electrode−electrolyte interface, suchmaterials should be ideally suited to optimize cointercalationstorage, though no studies so far have investigated nano-structured electrodes for sodium cointercalation.A key challenge for next-generation batteries is to

simultaneously improve multiple metrics over state-of-the-artdevices to enable wide use in emerging applications. Forexample, solar-storage integrated systems require lifetimesmatching solar cells (30 years), electric vehicles require a

Received: October 14, 2015Revised: November 23, 2015Published: November 30, 2015

Letter

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© 2015 American Chemical Society 543 DOI: 10.1021/acs.nanolett.5b04187Nano Lett. 2016, 16, 543−548

high power and capacity, and grid storage requires an extremelow cost. As we demonstrate in this work, few-layered graphenematerials may enable sodium-ion batteries as a storage platformwhich brings simultaneous promise for all of these applications.Here we demonstrate the first successful use of crystalline

few-layered graphene material for sodium ion storage byleveraging solvent assisted cointercalation. Extraordinary ratecapability for sodium storage is observed, with ∼100 mAh/gstorage capacity at 30 A/g currentsa rate currently onlypossible using lower-capacity electrochemical supercapacitors.

Further, this performance is maintained over 8000 cycles withvirtually no capacity fade. Raman spectroscopy supports ahighly ordered staging process with cointercalated solventmediating the ion−lattice interaction to prevent irreversibledamage to the electrode and lead to highly pristine materialsand invariant performance after 8000 consecutive cycles.Few-layer graphene foam was grown using chemical vapor

deposition (CVD) on a nickel foam substrate17 (110 ppi fromMTI) using a C2H2 precursor.

18 The surface of the graphenefoam is shown in Figure 1a, with inset showing the 3D

Figure 1. (a) SEM image showing the surface of the few-layer graphene foam; scale bar, 20 μm. Inset, SEM image showing 3D foam; scale bar, 400μm. (b) Representative Raman spectra acquired using 2.33 eV laser. (c) TEM characterization of the thickness of graphenic sheets: scale bars, 5 nm.(d) Distributions of Raman spectra acquired over ∼50 μm × 50 μm region (225 spectra) with respect to the relative 2D peak intensity.

Figure 2. (a) First five galvanostatic charge−discharge profiles at current density of 0.2 A/g. (b) Galvanostatic charge−discharge profiles at currentdensities ranging from 1 A/g to 30 A/g with the corresponding cycling performance (c). Inset shows the linear relation between specific capacity andcurrent density. (d) Extended cycling performed at current density of 12 A/g over 8000 cycles with selected Galvanostatic charge−discharge profiles(e). Inset, the decreasing overpotential with cycling.

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structure. To characterize the properties of the graphenematerial grown on the Ni foam, a micro-Raman map wasperformed covering a ∼ 50 μm × 50 μm region, collecting 225spectra. Representative Raman spectra are presented in Figure1b, with the three characteristic Raman peaks labeled. The Dpeak (∼1350 cm−1) arises from defect-activated in-planebreathing modes and corresponds to sp3 carbon bonding, theG peak (∼1580 cm−1) arises from in-plane optical phononmodes at the Γ point and corresponds to sp2 carbon bonding,and the 2D peak (∼2700 cm−1) arises from a two-phononprocess that is sensitive to the electronic band structure.19,20

The line shape, position, and relative intensity of the 2D peakcan be used to approximate the layer thickness, and the D/Grelative intensity ratio is used as a measure of carbon quality. Inthis manner, the presented spectra correspond to high-quality,crystalline, few-layer graphene typical of previous reports ofgraphene grown on nickel.17,21 High-resolution transmissionelectron microscopy (TEM) (Figure 1c) indicates most sheetsto consist of 2−10 graphene layers. Raman spectroscopic mapsindicate the majority of few-layered graphene materials toexhibit a 2D/G ratio of ∼0.5−1, consistent with layer thicknessmeasured in TEM.To evaluate the electrochemical performance of the FLG

foam, coin cells were assembled using Na metal as the referenceelectrode, few-layered graphene foam as the working electrode,and a diglyme electrolyte containing 1 M NaPF6. Galvanostaticcycling was carried out at varying rates in the potential range of0.01−2.0 V vs Na/Na+. The first five cycles performed at 0.2 A/g presented in Figure 2A (corresponding dQ/dV plots shownin Figure S1) show stable cycling after initial Na+ insertion witha reversible capacity of ∼150 mAh/g, suggesting a stoichiom-etry we propose to be approximately Na(Diglyme)xC15, whichis in agreement with previous reports on chemically derivedstage 1 Na+ ternary graphite intercalation compounds(GICs).22 We attribute the initial irreversible capacity to thepartial reductive decomposition of the electrolyte and theformation of a solid-electrolyte interphase (SEI) layer. Whereasthe overall shape of the charge−discharge profiles closelymatches previously reported curves for diglyme cointercalationinto commercially purchased graphite,12,13 testing the FLGmaterial at higher rates (Figure 2b and c) demonstrates a ratecapability that is unmatched by any other known carbon-basedsodium anode material that we are aware of. Remarkably, theFLG foam electrode maintains ∼125 mAh/g (∼80% maximumcapacity) at a rate of 10 A/g and ∼100 mAh/g (∼65%maximum capacity) at a rate of 30 A/g (corresponding to a∼12 s charge). In comparison, Lin et al. utilizes solventcointercalation for Al-ion batteries and report up to 4 A/g ratecapability (∼60 s charge) with 50% capacity drop, and Kim etal. report ∼50% capacity drop at 10 A/g for Na-ioncointercalation into graphite.11,13 In order to characterize thediffusion properties of the Na/diglyme in the FLG host, weperformed galvanostatic intermittent titration technique(GITT) measurements23 as well as rate-dependent cyclicvoltammetry24 and calculated diffusion coefficients ranging upto ∼2 × 10−7 cm2/s in the heavily sodiated state and ∼2 × 10−8

cm2/s during the prominent reaction at ∼0.7 V (Figure S2 andS3). Accordingly, we attribute the superior rate capability to fastdiffusion through the electrode, likely due to both materialintegrity under large volumetric expansion14 as well as themitigation of desolvation through solvent cointercalation.16

In order to evaluate the operation life span, extended cyclingwas performed (Figure 2d). Over a span of 8000 cycles, the

FLG electrode retained 96% of its initial capacity, suggestingvirtually no capacity fade through cycling with averageCoulombic efficiencies of 99.2%, which indicates a stable SEIlayer. 8000 cycles was chosen due to a lifetime exceeding over20 years assuming one cycle per daya benchmark for solarcells. This cycling performance indicates that improved ratecapability can be achieved without sacrificing structural stability(similar trends observed at slower cycling rates shown in FigureS2). This cycling stability of a half-cell is likely due in part tothe stable electrolyte-electrode interface formed between Nametal and NaPF6/diglyme.

5 We also observed a decreasingoverpotential with cycling (Figure 2e), which may be due to agradual weakening of the interlayer interaction betweengraphene sheets. However, the near-perfect capacity retentiondemonstrates that exfoliation25 was not an issue. Thisperformance is extraordinary in the field of sodium-ion anodes,with rate capability and cycling stability comparable or betterthan the best electrochemical supercapacitors26 while storingmany times more charge.To examine the impact of cycling on the carbon structure, a

second micro-Raman map was performed covering a ∼50 μm ×50 μm region, collecting 225 spectra from a FLG electrode after8000 cycles. Figure 3 presents distributions of the relative D

peak intensities found in the pristine and the postcycling FLG.We see that, even after 8000 cycles, the distribution of ID/IGratios remains centered <0.05. This demonstrates that a highdegree of crystallinity is preserved through cycling and explainsthe near-perfect capacity retention. In contrast, the ID/IG ratioin graphene has been reported to increase to >1.0 after only fivelithiation cycles.27 We attribute the retention of crystallinity toweaker ion−host lattice interactions due to solvent screening.This is in comparison to intercalation occurring afterdesolvation at the electrode−electrolyte interface, wherestronger interactions between ions and the crystalline carbons(e.g., LiC6) yields enhanced electrode degradation andirreversibility over successive cycling. To further understandthe mechanisms associated with this fast and highly stablereaction, we performed in situ Raman spectroscopy to opticallyprobe the FLG material during electrochemical testing(experimental setup is shown in Figure S5).One striking feature from in situ optical microscopy is the

vibrant color changes that occur during the sodiation and

Figure 3. Distributions of 225 Raman spectra (acquired over ∼50 μm× 50 μm region using 2.33 eV laser) with respect to relative D peakintensity prior to testing (above) and after 8000 galvanostatic charge−discharge cycles (below) showing minimal cycling-induced degrada-tion. Inset, individual spectrum acquired after cycling with D and Gcomponents fitted with Lorentzian peaks.

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desodiation processes. By optically monitoring the reaction inreal time,28 we can correlate color changes in the material to theelectrochemical potential as shown by the images of the FLGfoam (Figure 4A). A video (Video S1) is included showing this

color change over four successive cycles in full entirety. TheFLG foam initially appears gray/silver with the color darkeningto black as the potential reaches the start of the pronounced

plateau. Then, individual grains begin switching to red, withmost of the material appearing red/orange by the end of theplateau. Finally, near full insertion capacity, the color graduallytransitions to gold. Upon sodium removal, these color changesrepeat in reverse order, and successive cycles show the samecolor transitions. Notably, insertion of Na ions into FLG withcapacity of 150 mAh/g corresponds to an electron concen-tration of ∼2.5 × 1014 cm−2 for each graphene layer, which ismuch higher than that achievable using a top-gate method.29−31

As a result, the Fermi level shift is sufficient enough to blockoptical interband absorption and increase the transparency32,33

for photons with ℏω < 2EF, with greater description of thisphenomenon for Li intercalated ultrathin graphite described byBao et al.32 With this in mind, we attribute the red/orange andgold colors to the increased transmittance of the graphenematerial for low energy photons and the subsequent reflectionof the transmitted photons into the microscope objective.To gain more insight, in situ Raman measurements were

conducted. Figure 4c presents intensity plots comprised of 40spectra (with 20 s exposure times) for both 1.58 eV (785 nm)and 2.33 eV (532 nm) laser excitations acquired during theelectrochemical intercalation (at ∼0.6 A/g) of FLG shown inFigure 4B. For 1.58 eV excitations, a single G peak (∼1580cm−1) is initially observed which is denoted as GUC,representing the G mode of an uncharged graphene layer.After ∼100 s, a second, blue-shifted G peak emerges at ∼1600cm−1, which is denoted as GC, as it corresponds to the G modeof a charged graphene layer. This G peak splitting ischaracteristic of graphite staging reactions,32,34−37 arisingfrom the presence of both charged graphene layers in contactwith an intercalant layer and uncharged graphene layers that areshielded. Accordingly, the appearance of the GC peak signifiesthe beginning of the staging process. As the reaction continues,the intensity of the GC peak begins to drastically increase as theGUC peak red-shifts and disappears, all coinciding with the timewhen the pronounced voltage plateau is reached electrochemi-cally (a normalized intensity plot is shown in Figure S8). Thedisappearance of the GUC layer indicates the absence ofuncharged layers, which takes place when the reaction reaches astage 2 compound (where the stage number corresponds to thenumber of graphene layers in between each intercalant layer).

Figure 4. (a) Selected microscope images from the Video (S1)showing the vibrant color change in the FLG during intercalation;scale bar, 20 μm. (b) Galvanostatic discharge (∼0.6 A/g) profilerecorded during in situ Raman measurements with band illustrationsshowing corresponding Fermi levels. (c) In situ Raman intensity plotsnormalized to the initial G peak intensity acquired using 1.58 eV laser(top) and 2.33 eV laser (bottom) consisting of 40 spectra each withschematics depicting the setup shown on the right.

Figure 5. (a) In situ Raman spectra (normalized) of FLG showing the highly ordered staging reaction as measured using a 1.58 eV laser with (b)selected spectra and Lorentzian fits of GC (blue line) and GUC (red line) components. (c) Tracking the positions of the Raman G peak components(GC shown in blue and GUC shown in red) measured in situ with the 1.58 eV laser (triangles) and the 2.33 eV laser (circles) during theelectrochemical intercalation reaction with the corresponding Galvanostatic discharge (∼0.2 A/g) profile shown with respect to right y-axis (blackline).

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At this point, we see a rapid enhancement of the Gc peakintensity, reaching 12× the initial GUC peak intensity. Thisdramatic change in the G peak intensity can be attributed to thePauli blocking of destructive interference Raman pathways31

that takes place when the Fermi level approaches half theexcitation laser energy. Accordingly, we can use this under-standing to estimate the Fermi level of the stage 2 compound tobe ∼0.8 eV, which corresponds to a work function of ∼3.8 eV.As the reaction continues to progress, the GC peak intensityfades and then is completely suppressed by 800 s, which can beattributed to Pauli blocking of all of the G-peak Ramanpathways. For this reason, we chose to switch to a 2.33 eV laserto better probe the later portion of the reaction. For the 2.33eV excitations, G peak splitting similarly occurs, but the rapidGC peak enhancement is delayed until after the pronouncedplateau reaction is completed. At this point, which correspondsto a stage one compound, we observe 22× enhancement andcan again estimate the Fermi level as ∼1.2 eV, corresponding toa work function of ∼3.4 eV. In comparison to our results, thestage 1 LiC6 compound has been reported with EF ∼ 1.5 eV.38

Other Raman studies have reported stage 1 FeCl3 intercalatedFLG to exhibit EF ∼ 0.9 eV, and stage 2 and stage 1 ammoniumpersulfate/sulfuric acid intercalation compounds to exhibit EF ∼1.0 eV and EF ∼ 1.2 eV, respectively.35,39 However, our work isthe first to utilize G peak enhancement during in situelectrochemical testing to monitor the Fermi level of aprogressing intercalation reaction.Next, we conducted in situ Raman measurements using a

slower rate to accurately monitor staging processes. Consec-utive Raman spectra taken during insertion with a 1.58 eV laser(Figure 5A) demonstrates a transition from all unchargedgraphene layers to all charged graphene layers, withrepresentative spectra and Lorentzian peak fits shown in Figure5B. Peak positions for in situ measurements are plotted inFigure 5C for both laser energies with respect to the chargingtime (using a rate of ∼0.2 A/g). The staging process, asobserved through Raman measurements, is distinctly differentfrom the lithiation of FLG32,40 and graphite,37 which are usefulwell-studied benchmarks due to their similarities to our systemand their comparatively limited rate capability and cycling.These lithiation reactions exhibit an initial blue-shift in a singleG-peak corresponding to the formation of a dilute stage 1compound, followed by peak broadening and splitting into twopoorly defined peaks (staging >2). Finally the G peaks evolveinto a broad (fwhm ∼60 cm−1) peak by stage 2 and thendisappear by stage 1.37 In contrast, we do not observe any initialdilute staging, and the progressing spectra show extremelysharp, well-resolved Lorenztian peaks through stage 2formation, indicating a more ordered staging process.Accordingly, these findings demonstrate that minimal in-planedeformation of the lattice occurs during the reaction, which islikely a result of the weak interaction of the ion with the hostand appears to be another key factor facilitating the fast in-plane diffusion and improved cycling stability. Additional in situRaman data showing spectra acquired with the 2.33 eV laser,the deintercalation reaction, the evolution of the 2D peak, andthe methodology used to identify the early stage compoundsare included in Figures S6−11.While the narrowing of the G peak can be simply attributed

to increasing structural order, it has also been ascribed toincreased phonon lifetimes in charge graphenea result ofblocking the decay of G-mode phonons into electron−holepairs that takes place during the Kohn anomaly process.19 The

blocking of this renormalization process also explains thestiffening of the G mode that has been shown to occur duringboth electron-doping and hole-doping.19,29,31,41 Therefore, theG peak is a signature of the charge present on the graphenelayers which can identify the staging processes duringintercalation. In Figure 5C, after the initial blue shift (Δpos∼18 cm−1) in the position of the GC peak, there are twoadditional blue shifts (Δpos ∼4 cm−1 at ∼700 s and Δpos ∼2cm−1 at ∼1700 s) with increasing Na insertion. The initial blueshift (the formation of the GC peak) indicates the start of thestaging reaction; the second blue-shift occurs just before theloss of the GUC peak and the start of the pronouncedelectrochemical plateau. This shift was not anticipated, sinceone would expect the charge on the FLG layers to maintainrelatively consistent through stage 2 formation.34 We attributethis to a reconfiguration of the Na+-solvent intercalated layerthat appears to take place prior to stage 2 formation. Thisdeviation from the conventional staging process can also beseen in the potential profile, which does not exhibit thecharacteristic plateau pattern of a staging reaction.42 The lastblue-shift, which takes place before the GC peak disappears inthe 1.58 eV laser data is attributed to the formation of a stage 1compound where each individual graphene layer is surroundedon both sides by an intercalant layer. The GC peak at this point,as measured using 2.33 eV excitations, displays a sharp, butnoticeably asymmetric, line shape (shown in Figure S10) whichhas been reported to be a signature of stage 1 GICs.43

In conclusion, we demonstrate an ultrafast Na-ion anodeusing crystalline few-layered graphene materials made possiblethrough the highly ordered cointercalation of diglyme solvent,which acts as a “non-stick coating” to facilitate insertion andmitigate desolvation kinetics at the electrode−electrolyteinterface. This leads to exceptional performance with capacityof ∼150 mAh/g at slow charge−discharge rates and ∼100mAh/g capacity retained at rates of 30 A/g, which is a ratecapability that outperforms most state-of-the-art supercapaci-tors. This material is demonstrated to exhibit negligible capacityfade over 8000 cycles enabled through weak ion-host latticeinteractions facilitated by solvent cointercalation. Collectively,this is the best rate capability and cycling performance everreported for a carbon-based Na-ion battery anode to the best ofour knowledge. Utilizing in situ Raman spectroscopy, we reveala highly ordered staging process and determine the Fermienergy of the stage 1 and stage 2 intercalation compounds as1.2 and 0.8 eV, respectively. As sodium ion batteries bringpromise for sustainable and low-cost battery applications tousher in a new era of portable technologies, our work is the firstto demonstrate that crystalline carbon nanomaterials can play apivotal role in these advanced storage platforms.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.5b04187.

(i) Additional experimental details, (ii) dQ/dV differ-ential capacity curves based on galvanostatic data, (iii)GITT and CV measurements used to calculate diffusioncoefficient, (iv) cycling performance measured at 1 A/grates, (v) experimental setup used for in situ Ramanspectroscopy, (vi) full sequence of in situ Ramanspectroscopy scans, with two lasers and correlation

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with corresponding galvanostatic charge−dischargemeasurement, (vii) in situ Raman characterization ofthe 2D mode during staging, and (viii) analysis andmethodology of determining the staging sequences(PDF)Video S1 showing colors observed through opticalmicroscope during in situ intercalation of Na+ into few-layered graphene (AVI)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: cary.l.pint@vanderbilt.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank William Erwin for machine shop assistance, DhirajPrasai, Bradly Baer, Nitin Muralidharan and Anna Douglas foruseful discussions, and Rizia Bardhan for use of Ramanmicroscope critical for this work. This work was supported inpart by National Science Foundation grant EPS 1004083 andVanderbilt start-up funds. A.P.C. and K. S. are supported in partby the National Science Foundation Graduate ResearchFellowship under Grant No. 1445197.

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