Letter
pubs.acs.org/NanoLett
Ultrafast Solvent-Assisted Sodium Ion Intercalation into Highly
Crystalline Few-Layered Graphene
Adam 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/g
has hampered the use of crystalline carbon nanostructures for
sodium ion battery anodes. We demonstrate that a diglyme
solvent shell encapsulating a sodium ion acts as a “nonstick”
coating to facilitate rapid ion insertion into crystalline few-layer
graphene and bypass slow desolvation kinetics. This yields
storage capacities above 150 mAh/g, cycling performance with
negligible capacity fade over 8000 cycles, and ∼100 mAh/g
capacities maintained at currents of 30 A/g (∼12 s charge).
Raman spectroscopy elucidates the ordered, but nondestructive cointercalation mechanism that differs from desolvated ion
intercalation processes. In situ Raman measurements identify
the Na+ staging sequence and isolates Fermi energies for the first and second stage ternary intercalation compounds at ∼0.8 eV
and ∼1.2 eV.
KEYWORDS: Graphene, sodium ion batteries, in situ Raman spectroscopy, solvent cointercalation, graphene intercalation compounds,
anode, Na+
G
routes to explore combinations of ion species and materials that
are otherwise impossible. Recently, Lin et al. showed that
aluminum ion batteries based on graphitic electrodes can be
produced with specific capacity of ∼70 mAh/g by facilitating
intercalation of the Al ions through a chloroaluminate ion
species formed in the electrolyte.11 These authors report
minimal capacity fade over 7500 cycles and rate capabilities up
to 4 A/g. Such ideas have also recently been explored for Naion batteries using commercially purchased bulk graphite
materials to achieve capacities of ∼110 mAh/g by using
glyme-based electrolytes.12−15 Overall, the ability to bypass the
slow desolvation step16 has been proven to yield higher rate
capabilities than traditional intercalation processes.13 Whereas
nanoscale materials such as few-layered graphene exhibit
inherent large electrode−electrolyte interface areas that result
in ion storage near the electrode−electrolyte interface, such
materials should be ideally suited to optimize cointercalation
storage, though no studies so far have investigated nanostructured electrodes for sodium cointercalation.
A key challenge for next-generation batteries is to
simultaneously improve multiple metrics over state-of-the-art
devices to enable wide use in emerging applications. For
example, solar-storage integrated systems require lifetimes
matching solar cells (30 years), electric vehicles require a
raphite and more recently graphene, which are crystalline
forms of carbon, have been pivotal in the development of
lithium-ion batteries. Despite years of research on alternative
anode materials, carbons remain the paramount choice for
battery manufacturing due to low cost, excellent stability with
diverse electrolytes, and high capacity. Whereas sodium ion
batteries present a cost and manufacturing landscape that could
potentially revolutionize low-cost secondary storage applications, such as grid-scale storage, a major hurdle has been that
crystalline 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 anode
materials2,3 with recent notable advances such as the development of a high-performance graphene-phosphorene hybrid
anode4 and significant progress toward stable Na metal
anodes.5 However, a major arm of sodium anode research
remains focused on a class of disordered or nongraphitized
carbons. In these materials, the disordered stacking of the
carbon prevents a staging reaction, and the Na ions are
proposed to react with defect sites and fill microporous voids in
the carbon.6 To overcome higher resistance and surface
mediated storage processes, expanded graphite,7 hollow
nanowires,8 nanospheres,9 and nanosheets10 have been explored. Despite the range of defective carbon materials
considered, long operational life spans and high rate capability
remain elusive.
On another front, the ability to leverage the cointercalation
of a solvent shell to assist metal ion storage has opened new
© 2015 American Chemical Society
Received: October 14, 2015
Revised: November 23, 2015
Published: November 30, 2015
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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, 400
μm. (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 current
densities ranging from 1 A/g to 30 A/g with the corresponding cycling performance (c). Inset shows the linear relation between specific capacity and
current 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.
high power and capacity, and grid storage requires an extreme
low cost. As we demonstrate in this work, few-layered graphene
materials may enable sodium-ion batteries as a storage platform
which 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 by
leveraging solvent assisted cointercalation. Extraordinary rate
capability for sodium storage is observed, with ∼100 mAh/g
storage capacity at 30 A/g currentsa rate currently only
possible using lower-capacity electrochemical supercapacitors.
Further, this performance is maintained over 8000 cycles with
virtually no capacity fade. Raman spectroscopy supports a
highly ordered staging process with cointercalated solvent
mediating the ion−lattice interaction to prevent irreversible
damage to the electrode and lead to highly pristine materials
and invariant performance after 8000 consecutive cycles.
Few-layer graphene foam was grown using chemical vapor
deposition (CVD) on a nickel foam substrate17 (110 ppi from
MTI) using a C2H2 precursor.18 The surface of the graphene
foam is shown in Figure 1a, with inset showing the 3D
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FLG electrode retained 96% of its initial capacity, suggesting
virtually no capacity fade through cycling with average
Coulombic efficiencies of 99.2%, which indicates a stable SEI
layer. 8000 cycles was chosen due to a lifetime exceeding over
20 years assuming one cycle per daya benchmark for solar
cells. This cycling performance indicates that improved rate
capability can be achieved without sacrificing structural stability
(similar trends observed at slower cycling rates shown in Figure
S2). This cycling stability of a half-cell is likely due in part to
the stable electrolyte-electrode interface formed between Na
metal and NaPF6/diglyme.5 We also observed a decreasing
overpotential with cycling (Figure 2e), which may be due to a
gradual weakening of the interlayer interaction between
graphene sheets. However, the near-perfect capacity retention
demonstrates that exfoliation25 was not an issue. This
performance is extraordinary in the field of sodium-ion anodes,
with rate capability and cycling stability comparable or better
than the best electrochemical supercapacitors26 while storing
many 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 after
8000 cycles. Figure 3 presents distributions of the relative D
structure. To characterize the properties of the graphene
material grown on the Ni foam, a micro-Raman map was
performed covering a ∼ 50 μm × 50 μm region, collecting 225
spectra. Representative Raman spectra are presented in Figure
1b, with the three characteristic Raman peaks labeled. The D
peak (∼1350 cm−1) arises from defect-activated in-plane
breathing modes and corresponds to sp3 carbon bonding, the
G peak (∼1580 cm−1) arises from in-plane optical phonon
modes at the Γ point and corresponds to sp2 carbon bonding,
and the 2D peak (∼2700 cm−1) arises from a two-phonon
process that is sensitive to the electronic band structure.19,20
The line shape, position, and relative intensity of the 2D peak
can be used to approximate the layer thickness, and the D/G
relative intensity ratio is used as a measure of carbon quality. In
this manner, the presented spectra correspond to high-quality,
crystalline, few-layer graphene typical of previous reports of
graphene grown on nickel.17,21 High-resolution transmission
electron microscopy (TEM) (Figure 1c) indicates most sheets
to consist of 2−10 graphene layers. Raman spectroscopic maps
indicate the majority of few-layered graphene materials to
exhibit a 2D/G ratio of ∼0.5−1, consistent with layer thickness
measured in TEM.
To evaluate the electrochemical performance of the FLG
foam, coin cells were assembled using Na metal as the reference
electrode, few-layered graphene foam as the working electrode,
and a diglyme electrolyte containing 1 M NaPF6. Galvanostatic
cycling was carried out at varying rates in the potential range of
0.01−2.0 V vs Na/Na+. The first five cycles performed at 0.2 A/
g presented in Figure 2A (corresponding dQ/dV plots shown
in Figure S1) show stable cycling after initial Na+ insertion with
a reversible capacity of ∼150 mAh/g, suggesting a stoichiometry we propose to be approximately Na(Diglyme)xC15, which
is in agreement with previous reports on chemically derived
stage 1 Na+ ternary graphite intercalation compounds
(GICs).22 We attribute the initial irreversible capacity to the
partial reductive decomposition of the electrolyte and the
formation of a solid-electrolyte interphase (SEI) layer. Whereas
the overall shape of the charge−discharge profiles closely
matches previously reported curves for diglyme cointercalation
into commercially purchased graphite,12,13 testing the FLG
material at higher rates (Figure 2b and c) demonstrates a rate
capability that is unmatched by any other known carbon-based
sodium anode material that we are aware of. Remarkably, the
FLG foam electrode maintains ∼125 mAh/g (∼80% maximum
capacity) 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 solvent
cointercalation for Al-ion batteries and report up to 4 A/g rate
capability (∼60 s charge) with 50% capacity drop, and Kim et
al. report ∼50% capacity drop at 10 A/g for Na-ion
cointercalation into graphite.11,13 In order to characterize the
diffusion properties of the Na/diglyme in the FLG host, we
performed galvanostatic intermittent titration technique
(GITT) measurements23 as well as rate-dependent cyclic
voltammetry24 and calculated diffusion coefficients ranging up
to ∼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 and
S3). Accordingly, we attribute the superior rate capability to fast
diffusion through the electrode, likely due to both material
integrity under large volumetric expansion14 as well as the
mitigation of desolvation through solvent cointercalation.16
In order to evaluate the operation life span, extended cycling
was performed (Figure 2d). Over a span of 8000 cycles, the
Figure 3. Distributions of 225 Raman spectra (acquired over ∼50 μm
× 50 μm region using 2.33 eV laser) with respect to relative D peak
intensity prior to testing (above) and after 8000 galvanostatic charge−
discharge cycles (below) showing minimal cycling-induced degradation. Inset, individual spectrum acquired after cycling with D and G
components fitted with Lorentzian peaks.
peak intensities found in the pristine and the postcycling FLG.
We see that, even after 8000 cycles, the distribution of ID/IG
ratios remains centered <0.05. This demonstrates that a high
degree of crystallinity is preserved through cycling and explains
the near-perfect capacity retention. In contrast, the ID/IG ratio
in graphene has been reported to increase to >1.0 after only five
lithiation cycles.27 We attribute the retention of crystallinity to
weaker ion−host lattice interactions due to solvent screening.
This is in comparison to intercalation occurring after
desolvation at the electrode−electrolyte interface, where
stronger interactions between ions and the crystalline carbons
(e.g., LiC6) yields enhanced electrode degradation and
irreversibility over successive cycling. To further understand
the mechanisms associated with this fast and highly stable
reaction, we performed in situ Raman spectroscopy to optically
probe 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
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plateau. Then, individual grains begin switching to red, with
most of the material appearing red/orange by the end of the
plateau. Finally, near full insertion capacity, the color gradually
transitions to gold. Upon sodium removal, these color changes
repeat in reverse order, and successive cycles show the same
color transitions. Notably, insertion of Na ions into FLG with
capacity of 150 mAh/g corresponds to an electron concentration of ∼2.5 × 1014 cm−2 for each graphene layer, which is
much higher than that achievable using a top-gate method.29−31
As a result, the Fermi level shift is sufficient enough to block
optical interband absorption and increase the transparency32,33
for photons with ℏω < 2EF, with greater description of this
phenomenon for Li intercalated ultrathin graphite described by
Bao et al.32 With this in mind, we attribute the red/orange and
gold colors to the increased transmittance of the graphene
material for low energy photons and the subsequent reflection
of the transmitted photons into the microscope objective.
To gain more insight, in situ Raman measurements were
conducted. Figure 4c presents intensity plots comprised of 40
spectra (with 20 s exposure times) for both 1.58 eV (785 nm)
and 2.33 eV (532 nm) laser excitations acquired during the
electrochemical intercalation (at ∼0.6 A/g) of FLG shown in
Figure 4B. For 1.58 eV excitations, a single G peak (∼1580
cm−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 ∼1600
cm−1, which is denoted as GC, as it corresponds to the G mode
of a charged graphene layer. This G peak splitting is
characteristic of graphite staging reactions,32,34−37 arising
from the presence of both charged graphene layers in contact
with an intercalant layer and uncharged graphene layers that are
shielded. Accordingly, the appearance of the GC peak signifies
the beginning of the staging process. As the reaction continues,
the intensity of the GC peak begins to drastically increase as the
GUC peak red-shifts and disappears, all coinciding with the time
when the pronounced voltage plateau is reached electrochemically (a normalized intensity plot is shown in Figure S8). The
disappearance of the GUC layer indicates the absence of
uncharged layers, which takes place when the reaction reaches a
stage 2 compound (where the stage number corresponds to the
number of graphene layers in between each intercalant layer).
desodiation processes. By optically monitoring the reaction in
real time,28 we can correlate color changes in the material to the
electrochemical potential as shown by the images of the FLG
foam (Figure 4A). A video (Video S1) is included showing this
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) profile
recorded during in situ Raman measurements with band illustrations
showing corresponding Fermi levels. (c) In situ Raman intensity plots
normalized to the initial G peak intensity acquired using 1.58 eV laser
(top) and 2.33 eV laser (bottom) consisting of 40 spectra each with
schematics depicting the setup shown on the right.
color change over four successive cycles in full entirety. The
FLG foam initially appears gray/silver with the color darkening
to black as the potential reaches the start of the pronounced
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 the
electrochemical intercalation reaction with the corresponding Galvanostatic discharge (∼0.2 A/g) profile shown with respect to right y-axis (black
line).
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blocking of this renormalization process also explains the
stiffening of the G mode that has been shown to occur during
both electron-doping and hole-doping.19,29,31,41 Therefore, the
G peak is a signature of the charge present on the graphene
layers which can identify the staging processes during
intercalation. In Figure 5C, after the initial blue shift (Δpos
∼18 cm−1) in the position of the GC peak, there are two
additional blue shifts (Δpos ∼4 cm−1 at ∼700 s and Δpos ∼2
cm−1 at ∼1700 s) with increasing Na insertion. The initial blue
shift (the formation of the GC peak) indicates the start of the
staging reaction; the second blue-shift occurs just before the
loss of the GUC peak and the start of the pronounced
electrochemical plateau. This shift was not anticipated, since
one would expect the charge on the FLG layers to maintain
relatively consistent through stage 2 formation.34 We attribute
this to a reconfiguration of the Na+-solvent intercalated layer
that appears to take place prior to stage 2 formation. This
deviation from the conventional staging process can also be
seen in the potential profile, which does not exhibit the
characteristic plateau pattern of a staging reaction.42 The last
blue-shift, which takes place before the GC peak disappears in
the 1.58 eV laser data is attributed to the formation of a stage 1
compound where each individual graphene layer is surrounded
on both sides by an intercalant layer. The GC peak at this point,
as measured using 2.33 eV excitations, displays a sharp, but
noticeably asymmetric, line shape (shown in Figure S10) which
has been reported to be a signature of stage 1 GICs.43
In conclusion, we demonstrate an ultrafast Na-ion anode
using crystalline few-layered graphene materials made possible
through the highly ordered cointercalation of diglyme solvent,
which acts as a “non-stick coating” to facilitate insertion and
mitigate desolvation kinetics at the electrode−electrolyte
interface. This leads to exceptional performance with capacity
of ∼150 mAh/g at slow charge−discharge rates and ∼100
mAh/g capacity retained at rates of 30 A/g, which is a rate
capability that outperforms most state-of-the-art supercapacitors. This material is demonstrated to exhibit negligible capacity
fade over 8000 cycles enabled through weak ion-host lattice
interactions facilitated by solvent cointercalation. Collectively,
this is the best rate capability and cycling performance ever
reported for a carbon-based Na-ion battery anode to the best of
our knowledge. Utilizing in situ Raman spectroscopy, we reveal
a highly ordered staging process and determine the Fermi
energy of the stage 1 and stage 2 intercalation compounds as
1.2 and 0.8 eV, respectively. As sodium ion batteries bring
promise for sustainable and low-cost battery applications to
usher in a new era of portable technologies, our work is the first
to demonstrate that crystalline carbon nanomaterials can play a
pivotal role in these advanced storage platforms.
At this point, we see a rapid enhancement of the Gc peak
intensity, reaching 12× the initial GUC peak intensity. This
dramatic change in the G peak intensity can be attributed to the
Pauli blocking of destructive interference Raman pathways31
that takes place when the Fermi level approaches half the
excitation laser energy. Accordingly, we can use this understanding to estimate the Fermi level of the stage 2 compound to
be ∼0.8 eV, which corresponds to a work function of ∼3.8 eV.
As the reaction continues to progress, the GC peak intensity
fades and then is completely suppressed by 800 s, which can be
attributed to Pauli blocking of all of the G-peak Raman
pathways. For this reason, we chose to switch to a 2.33 eV laser
to better probe the later portion of the reaction. For the 2.33
eV excitations, G peak splitting similarly occurs, but the rapid
GC peak enhancement is delayed until after the pronounced
plateau reaction is completed. At this point, which corresponds
to a stage one compound, we observe 22× enhancement and
can again estimate the Fermi level as ∼1.2 eV, corresponding to
a work function of ∼3.4 eV. In comparison to our results, the
stage 1 LiC6 compound has been reported with EF ∼ 1.5 eV.38
Other Raman studies have reported stage 1 FeCl3 intercalated
FLG to exhibit EF ∼ 0.9 eV, and stage 2 and stage 1 ammonium
persulfate/sulfuric acid intercalation compounds to exhibit EF ∼
1.0 eV and EF ∼ 1.2 eV, respectively.35,39 However, our work is
the first to utilize G peak enhancement during in situ
electrochemical testing to monitor the Fermi level of a
progressing intercalation reaction.
Next, we conducted in situ Raman measurements using a
slower rate to accurately monitor staging processes. Consecutive Raman spectra taken during insertion with a 1.58 eV laser
(Figure 5A) demonstrates a transition from all uncharged
graphene layers to all charged graphene layers, with
representative spectra and Lorentzian peak fits shown in Figure
5B. Peak positions for in situ measurements are plotted in
Figure 5C for both laser energies with respect to the charging
time (using a rate of ∼0.2 A/g). The staging process, as
observed through Raman measurements, is distinctly different
from the lithiation of FLG32,40 and graphite,37 which are useful
well-studied benchmarks due to their similarities to our system
and their comparatively limited rate capability and cycling.
These lithiation reactions exhibit an initial blue-shift in a single
G-peak corresponding to the formation of a dilute stage 1
compound, followed by peak broadening and splitting into two
poorly defined peaks (staging >2). Finally the G peaks evolve
into a broad (fwhm ∼60 cm−1) peak by stage 2 and then
disappear by stage 1.37 In contrast, we do not observe any initial
dilute staging, and the progressing spectra show extremely
sharp, well-resolved Lorenztian peaks through stage 2
formation, indicating a more ordered staging process.
Accordingly, these findings demonstrate that minimal in-plane
deformation of the lattice occurs during the reaction, which is
likely a result of the weak interaction of the ion with the host
and appears to be another key factor facilitating the fast inplane diffusion and improved cycling stability. Additional in situ
Raman data showing spectra acquired with the 2.33 eV laser,
the deintercalation reaction, the evolution of the 2D peak, and
the methodology used to identify the early stage compounds
are 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 to
increased phonon lifetimes in charge graphenea result of
blocking the decay of G-mode phonons into electron−hole
pairs that takes place during the Kohn anomaly process.19 The
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.nanolett.5b04187.
(i) Additional experimental details, (ii) dQ/dV differential capacity curves based on galvanostatic data, (iii)
GITT and CV measurements used to calculate diffusion
coefficient, (iv) cycling performance measured at 1 A/g
rates, (v) experimental setup used for in situ Raman
spectroscopy, (vi) full sequence of in situ Raman
spectroscopy scans, with two lasers and correlation
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with corresponding galvanostatic charge−discharge
measurement, (vii) in situ Raman characterization of
the 2D mode during staging, and (viii) analysis and
methodology of determining the staging sequences
(PDF)
Video S1 showing colors observed through optical
microscope during in situ intercalation of Na+ into fewlayered graphene (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank William Erwin for machine shop assistance, Dhiraj
Prasai, Bradly Baer, Nitin Muralidharan and Anna Douglas for
useful discussions, and Rizia Bardhan for use of Raman
microscope critical for this work. This work was supported in
part by National Science Foundation grant EPS 1004083 and
Vanderbilt start-up funds. A.P.C. and K. S. are supported in part
by the National Science Foundation Graduate Research
Fellowship under Grant No. 1445197.
■
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