of 10
Selective formation of pyridinic-type nitrogen-
doped graphene and its application in lithium-ion
battery anodes
Jacob D. Bagley,
a
Deepan Kishore Kumar,
b
Kimberly A. See
a
and Nai-Chang Yeh
*
c
We report a high-yield single-step method for synthesizing nitrogen-doped graphene nanostripes (N-
GNSPs) with an unprecedentedly high percentage of pyridinic-type doping (>86% of the nitrogen sites),
and investigate the performance of the resulting N-GNSPs as a lithium-ion battery (LIB) anode material.
The as-grown N-GNSPs are compared with undoped GNSPs using scanning electron microscopy (SEM),
Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), helium ion-beam microscopy (HIM), and
electrochemical methods. As an anode material we
fi
nd that pyridinic-type N-GNSPs perform similarly to
undoped GNSPs, suggesting that pyridinic sites alone are not responsible for the enhanced performance
of nitrogen-doped graphene observed in previous studies, which contradicts common conjectures. In
addition, post-mortem XPS measurements of nitrogen-doped graphene cycled as a lithium-ion battery
anode are conducted for the
fi
rst time, which reveal direct evidence for irreversible chemical changes at
the nitrogen sites during cycling. These
fi
ndings therefore provide new insights into the mechanistic
models of doped graphene as LIB anodes, which are important in improving the anode designs for
better LIB performance.
1. Introduction
Increasing the energy density of lithium-ion batteries (LIBs) is
an important issue in energy research in order to meet growing
energy demands and long-term sustainability.
1
The energy
density of LIBs depends on the charge storage capacities and
potentials of the battery electrode materials. Therefore, one
approach to improve LIB energy density is to increase the
capacity of the anode,
i.e.
, the amount of lithium that the anode
can reversibly accommodate. The current industry standard LIB
anode material is graphite, and the processing of graphite for
LIBs has been developed to the point that its practical perfor-
mance is approaching its theoretical capacity.
2,3
Therefore, new
materials are being explored to develop next-generation high
energy density LIB anodes.
3
Among them, doping graphene
with heteroatoms (
e.g.
, nitrogen, boron,
etc.
) as the anode
material is an appealing approach because doped graphene LIB
anodes have demonstrated reversible capacities greater than
1000 mA h g

1
(167% higher than graphite) with good lifetimes
(>500 cycles).
4
10
Additionally, doped graphene is chemically
similar to graphite so that it is compatible with current LIB
assemblies (
e.g.
, electrolyte compatibility).
Although doped graphene anodes have demonstrated good
performance, the e
ff
ects of dopant type and dopant con
gura-
tion on LIB performance is not well understood. This gap in
knowledge is in part due to the di
ffi
culty in preparing graphene
with a single dopant type in a speci
c con
guration, such that
measurements to date generally involve convoluted e
ff
ects from
multiple dopants and/or di
ff
erent dopant con
gurations.
Nitrogen is the most studied graphene dopant, and nitrogen
can substitute into the graphene lattice in three di
ff
erent
con
gurations that are termed as graphitic, pyrrolic, and pyr-
idinic, as illustrated in Fig. 1.
11,12
At the graphitic sites, nitrogen
bonds to three carbon atoms and preserves the graphene
honeycomb lattice. At the pyrrolic sites, nitrogen is adjacent to
a vacancy defect and bonds to two carbons that are part of a
ve-
membered ring. At the pyridinic sites, nitrogen is adjacent to
a vacancy defect and bonds to two carbons that are part of a six-
membered ring. In addition to their structural characteristics,
these sites di
ff
er electronically so that the speci
c sites can be
identi
ed and quanti
ed by X-ray photoelectron spectroscopy
(XPS).
11
Among di
ff
erent types of nitrogen-doped graphene
sites, the pyridinic type has been conjectured to yield the
highest lithium storage capacity based on empirical results,
although LIB applications of nitrogen-doped graphene with
a
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA, 91125, USA
b
Department of Electrical Engineering, California Institute of Technology, Pasadena,
CA, 91125, USA
c
Department of Physics, California Institute of Technology, Pasadena, CA, 91125, USA.
E-mail: ncyeh@caltech.edu
Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06199a
Cite this:
RSCAdv.
, 2020,
10
, 39562
Received 16th July 2020
Accepted 19th October 2020
DOI: 10.1039/d0ra06199a
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purely pyridinic-sites has never been accomplished so that the
conjecture cannot be directly veri
ed.
10
14
Thus, the purpose of
this work is to examine the validity of current conjectures by
studying the performance of nitrogen-doped graphene nano-
materials with predominant pyridinic-type doping in a LIB
anode con
guration.
Herein, we report the synthesis of nitrogen-doped graphene
nanostripes (N-GNSPs) with predominantly pyridinic sites and
the application of such N-GNSPs as the anode material in LIBs.
In addition, we report for the
rst time post-mortem XPS
characterization that reveals direct evidences for chemical
changes at the nitrogen doped sites during the LIB operation.
Although our N-GNSPs have a similar nitrogen content (

8%)
as other reports and an unprecedentedly high percentage of
pyridinic content (>86% of the nitrogen sites), these materials
only demonstrate limited enhancement (

13%) in gravimetric
lithium storage capacity when compared to undoped graphene
nanostripes (GNSPs). We also observe irreversible chemical
changes at the nitrogen sites
via
XPS during solid electrolyte
interphase (SEI) formation,
i.e.
, the electronically insulating
and Li
+
-conducting interface that forms on LIB electrodes due
to solvent decomposition during the
rst cycle. These
ndings
therefore imply that pyridinic sites alone are unlikely respon-
sible for the enhanced performance of nitrogen-doped gra-
phene observed in previous studies, and further investigations
are necessary to understand the e
ff
ects of nitrogen doping on
the performance of LIB anodes.
2. Experimental
2.1 Synthesis of graphene materials
N-GNSPs were synthesized by modifying the plasma enhanced
chemical vapour deposition (PECVD) synthesis for GNSPs that
was previously presented by Hsu
et al.
15
Speci
cally, a micro-
wave induced hydrogen/methane plasma with traces 1,2-
dichlorobenzene (Alfa Aesar, 99%) or 3-chloropyridine (Alfa
Aesar, 99%) yielded GNSPs or N-GNSPs on Cu-substrates,
respectively. The PECVD growth system was custom built and
consisted of eight parallel deposition chambers. Each chamber
included a 0.75 cm

1.25 cm copper foil (McMaster-Carr,
99.9%) in a
1/2
outer diameter glass tube
tted with an
Evenson cavity (Opthos Instruments Inc., Frederick, MD, USA)
excited by a 2.45 GHz microwave power source (ENS 4

200 W
CPS, SAIREM, D
́
ecines-Charpieu, France). All chambers simul-
taneously received 70 W of microwave power, which created
a plasma volume of

1cm
3
.H
2
(99.999%) and CH
4
(99.999%)
gases were introduced to the chamber by mass
ow controllers
(MC series, Alicat Scienti
c, Tuscon, AZ, USA), and traces of 1,2-
dichlorobenzne (3-chloropyridine) were introduced to the
chamber
via
a leak valve from a vacuum sealed vial of 1,2-
dichlorobenzne (3-chloropyridine). The pressure in the
chamber before splitting into eight chambers was held at 3.8
Torr, the total
ow rates of H
2
and CH
4
were 48 sccm and 5
sccm, respectively, and the ratio of CH
4
to 1,2-dichlorobenzene
(3-chloropyridine) was

2 : 1 as measured by a residual gas
analyser (RGA; XT300M, Extorr Inc., New Kensington, PA, USA)
placed upstream of the deposition chamber and connected
via
a capillary, as detailed in ESI Note 1 and schematically shown in
Fig. S1.
The plasma was maintained for

3 hours to synthesize
su
ffi
cient graphene material for use as LIB anodes. We note that
the synthesis yield was found to be linear with the growth time
at a rate of

6mgcm

2
hour per chamber on copper substrates,
as described in ESI Note 2 and demonstrated in Fig. S2.
For
each LIB electrode slurry (described in Section 2.3), we used
100
150 mg graphene. By employing all eight PECVD chambers
in parallel for graphene growth, we were able to collect su
ffi
-
cient material in

3 hours to make an electrode slurry which
could make anodes for several coin cells.
2.2 Characterization of fabricated graphene materials
Raman spectroscopy was performed in a Renishaw M-1000
Micro-Raman (Renishaw, Gloucestershire, UK) spectrometer
operating with a 514.5 nm argon ion laser with a spectral
resolution of 1 cm

1
and a spot size of

20
m
m. A dual-wedge
polarization scrambler was inserted to depolarize the laser.
Scanning electron microscopy (SEM) was performed in a Hita-
chi S-4100 (Hitachi, Tokyo, Japan) with an accelerating voltage
of 5 kV. Helium-ion beam microscopy (HIM) imaging was per-
formed in a ZEISS ORION NanoFab (Pleasanton, CA, USA) with
an accelerating voltage of 30 kV, a beam current of 1.2 pA and
a working distance of 8.021 mm. XPS data were collected using
a Kratos AXIS Ultra spectrometer (Kratos Analytical, Man-
chester, UK). The instrument was equipped with a hybrid
magnetic and electrostatic electron lens system, a delay-line
detector and a monochromatic Al K
a
X-ray source (1486.7 eV).
Data were collected at a pressure of

5

10

9
Torr with
photoelectrons collected at 0

with respect to the sample. For
the survey spectra the analyser pass energy was 80 eV and the
step size was 1 eV, and for all other spectra the analyser pass
energy was 10 eV and the step size was 0.025 eV. The instrument
energy scale and work function were calibrated using clean Au,
Fig. 1
Con
fi
guration of nitrogen dopants in graphene and precursor
molecules 1,2-dichlorobenzene and 3-chloropyridine.
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Ag and Cu standards. The instrument was operated by Vision
Manager so
ware v. 2.2.10 revision 5. The data were analysed
using CasaXPS so
ware (CASA So
ware Ltd). The carbon sp
2
peaks were
t as asymmetric Gaussian
Lorentzian, and all
other peaks were
t as symmetric Gaussian
Lorentzian. In
addition, the carbon sp
3
and
p
p
*
satellite peaks were con-
strained to be 0.8 eV and 6.4 eV higher binding energy than the
sp
2
peak, respectively. For post mortem XPS characterizations
the data were boxcar averaged using a width of eleven points to
improve the signal-to-noise ratio.
2.3 Coin cell preparation
Working electrodes, which consisted of graphene material
(GNSP or N-GNSP), carbon black (Super-P, Alfa Aesar, >99%)
and polyvinylidene
uoride (PVDF, MTI corporation, >99.5%)
binder in a 7 : 2 : 1 ratio, were mixed in
N
-methyl-2-
pyrrolidinone (NMP, Sigma-Aldrich, 99.5%) in a centrifugal
mixer (AR-100 Thinky USA, Inc., Laguna Hills, CA, USA) at
5000 rpm for 10 minutes. The resulting slurry was spread across
copper foil (McMaster-Carr, 99.9%) using a doctor blade with
a thickness of
0.2
and dried at 120

C in vacuum for 16 hours

7/16
diameter circular electrodes were cut from the dried
slurry/copper foil. Separately, as-deposited N-GNSPs on the
copper growth substrate were used as working electrodes. Two-
electrode 2032 coin cells (MTI) were assembled in an argon
lled glove box (with O
2
< 0.1 ppm and H
2
O < 0.1 ppm). The
counter/reference electrodes were lithium foil (Sigma Aldrich,
99.9%, 0.75 mm, mechanically cleansed immediately before cell
assembly), and the electrolyte was 1 M LiPFX6 (Sigma Aldrich,
$
99.99%) in ethylene carbonate/dimethyl carbonate (1 : 1
mixture by volume, both Sigma Aldrich,
$
99%). Dimethyl
carbonate was stored over molecular sieves (3
̊
A, Beantown
Chemical) prior to use, and the electrolyte was mixed in a dried
HDPE bottle. A polypropylene separator (Celgard 2400) was
used and

8 drops of electrolyte were used in each coin cell.
Details of the coin cell fabrication procedure are given in ESI
Note 3 and schematically shown in Fig. S3.
A
er electro-
chemical cycling, coin cells were disassembled in an argon
l-
led glovebox, and the working electrodes were rinsed with
dimethyl carbonate and transferred in an argon
lled container
directly to the XPS ultrahigh vacuum chamber for post mortem
XPS characterization.
2.4 Electrochemical characterization
All electrochemical measurements were performed on a refer-
ence 600 (Gamry Instruments, Warminster, PA, USA). Galva-
nostatic charge
discharge measurements were taken at
indicated current densities within a voltage range of 3 V to
0.01 V.
3. Results and discussion
3.1 Pyridinic N-GNSP synthesis rationale and comparison to
previous syntheses
We accomplished synthesis of predominantly pyridinic type
N-GNSPsbymodifyingprecursorsusedinourprevious
synthesis of undoped GNSPs.
15
In the original synthesis, trace
content of 1,2-dichlorobenzene (structure shown in Fig. 1) in
a hydrogen/methane plasma resulted in growth of high
quality (in terms of the chemical purity and crystallinity)
vertically oriented graphene on a copper substrate. We note
that vertically oriented graphe
ne refers to a class of graphene
nanomaterials wherein graphene
grows vertically with respect
to the growth substrate forming wall-like nanostructures on
the substrate.
16
We believe that in the synthesis of GNSPs, 1,2-
dichlorobenzene forms benzene radicals in the plasma due to
the weak C
Cl bonds, which then seed and propagate the
graphene structure along with methane radicals assisting the
growth and hydrogen radicals etching away defects. The role
of methane and hydrogen was proposed by other researchers
studying the growth of vertically oriented graphene,
16
and the
role of 1,2-dichlorobenzene is corroborated by our previous
ndings of the RGA data showing substantial increase in C
6
and C
6
H
6
during the growth of GNSPs in the presence of 1,2-
dichlorobenzene.
15
For the synthesis of pyridinic type N-
GNSPs, we replace 1,2-dichlorobenzene with 3-chloropyr-
idine (see Fig. 1). Similarly, we
conjecture that 3-chloropyr-
idine yields pyridine radicals in the plasma due to the weak C
Cl bond, which then seed the N-GNSP structures. We further
conjecture that the use of 3-chloropyridine for graphene
growth yields predominantly pyridinic-type N-GNSPs (see
Section 3.2) because the bonding con
guration of nitrogen in
3-chloropyridine resembles the bonding con
guration of
nitrogen in pyridinic graphene sites.
As demonstrated in Section 3.2, we
nd that this method
indeed produces N-GNSPs that are >86% pyridinic-type doping
for the nitrogen-doped sites. To the best of our knowledge, this
is the highest fraction of pyridinic-type doping that has been
accomplished with high yield and good crystallinity. For
comparison, we brie
y summarize here previous reports of
related studies: Bang
et al.
,
17
Mombeshora
et al.
,
18
and Yasuda
et al.
19
reported selective synthesis of pyridinic type nitrogen-
doped graphene. However, the Raman peaks of each of these
materials were relatively wide, indicating structural disorder.
20
Yang
et al.
reported a synthesis of nitrogen-doped graphene
with fairly narrow Raman peaks, but the pyridinic doping
content was only

50%.
21
Wisitsoraat
et al.
reported a nitrogen-
doped graphene synthesis with narrow Raman peaks, but the
doping type was completely pyrrolic.
22
Finally, Luo
et al.
23
re-
ported a selective synthesis of pyridinic type nitrogen-doped
graphene with narrow Raman peaks, but the material was
monolayer graphene, and thus the yield was insu
ffi
cient for
applications in LIB.
3.2 Characterization of the as-grown material
Graphene nanomaterials fabricated by the aforementioned
PECVD methods were characterized by SEM, helium-ion beam
microscopy (HIM), Raman spectroscopy, XPS and electro-
chemical methods. Normal incidence SEM images of as grown
GNSPs and N-GNSPs are shown in Fig. 2A and B, respectively.
The top edge of the vertical graphene sheets are visible as bright
lines in the images. Pores between vertical graphene sheets are
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darker in the image because electrons cannot escape to the
detector from the deep pores. Sheets that do not stand perfectly
vertical are visible as medium contrast points in the image.
Visually, the GNSPs and N-GNSPs are structurally similar,
i.e.
,
they have similar pore and sheet sizes. A cross sectional image is
shown in Fig. 2C, where the height of the vertical graphene
nanomaterial measures at

20
m
m. To further di
ff
erentiate the
microscopic structures of GNSPs and N-GNSPs, we employed
HIM imaging, which can resolve features as small as

3nm
without damaging graphene under correct experimental
conditions.
24
Fig. 2D displays a HIM image of N-GNSPs which
reveals perforations in the N-GNSPs sheets, whereas Fig. 2E
does not show perforations in the GNSPs sheets. Further,
previous transmission element microscopy studies on GNSPs
do not reveal perforations in GNSPs sheets.
15
We conjecture that
pyridinic sites are present at the edges of the perforations, and
the perforations are the result of vacancies associated with the
pyridinic sites.
The Raman spectrum (Fig. 2F) of GNSPs and N-GNSPs
nanomaterials con
rms the growth of graphene with the char-
acteristic D (1361 cm

1
), G (1589 cm

1
) and 2D (2704 cm

1
)
peaks.
25
These materials also exhibit the D
0
(1609 cm

1
) and D +
D
0
(2945 cm

1
) peaks which together with the G and D
0
peaks
being well resolved classify the samples as nanocrystalline
graphene material according to the three stage defect model.
20,25
We acknowledge that the D-peak is very intense in these gra-
phene samples, which is in part due to the abundant edges.
15
Comparing the GNSP and N-GNSP spectra, we
nd that the
intensity ratios and the full-width-half-maximum (FWHM) of
the D and G peaks are similar, whereas the 2D peak is much
suppressed in N-GNSPs relative to the undoped GNSPs, which is
consistent with previous observation of N-doped graphene
materials and may be attributed to the presence of vacancies
associated with the N-doped sites.
11,26
The absence of residual 3-
chloropyridine on the sample is con
rmed by the absence of
a peak in the Raman spectrum at 1034 cm

1
(marked by an
asterisk), which is the position of a very strong 3-chloropyridine
Raman signal.
27
We conducted XPS studies to quantitatively determine the
chemical composition and doping type in our materials (see
Table 1 for a compositional analysis of the N-GNSPs). The survey
spectrum of the undoped GNSP material (Fig. 3A) shows a pure
carbon material. The C-1s spectrum of the same undoped GNSP
material (Fig. 3B) was
t with three peaks corresponding to sp
2
hybridized C
C bonds (284.9 eV), sp
3
hybridized C
C bonds
(285.7 eV), and a
p
p
*
satellite (291.4 eV). The strong sp
2
peak
and the presence of the
p
p
*
peak verify the graphene crystal-
linity,
i.e.
, domains with delocalized p-orbitals.
28,29
The N-GNSP
survey spectrum (Fig. 3C) shows the material is composed of
89.4% carbon, 7.5% nitrogen and 3.1% chlorine. Notably, there
is no oxygen peak in our N-GNSPs (even though the sample was
exposed to air), which is in contrast to the typical presence of
oxygen in other doped graphene materials made at scales
appropriate for LIB application.
4
10
The C-1s high resolution spectrum of N-GNSPs (Fig. 3D) is
analysed by considering
ve components that correspond to sp
2
hybridized C
C bonds (284.9 eV), sp
3
hybridized C
C bonds
(285.7 eV), C
N bonds (285.8 eV), C
Cl bonds (287.1 eV), and
Fig. 2
(A) Normal incidence SEM image of the as-grown GNSPs. Scale bar: 2
m
m. (B) Normal incidence SEM image of the as-grown N-GNSPs.
Scale bar: 2
m
m. (C) Cross sectional SEM image of the as-grown N-GNSPs. Scale bar: 20
m
m. (D) Normal incidence HIM image of the as-grown N-
GNSPs, showing perforations at the edges, which we conjecture to result from the vacancies associated with the pyridinic sites. Scale bar: 1
m
m.
(E) Normal incidence HIM image of the as-grown GNSPs, showing no perforations at the edges. Scale bar: 200 nm. (F) Raman spectra of the
undoped GNSPs and N-GNSPs, showing signi
fi
cantly suppressed 2D-band in N-GNSPs, which may be attributed to the substantial presence of
vacancies due to N-doping. The absence of a peak at 1034 cm

1
(marked by an asterisk) demonstrates residual 3-chloropyridine is not present on
the sample.
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a
p
p
*
satellite (291.9 eV). The C
C bonds and the satellite peak
assignments are justi
ed by the literature (
i.e.
, the peak posi-
tions and relative spacings are consistent with previous
reports),
30
and the C
Cl and C
N peak assignments are justi
ed
by comparing the relative peak intensities of carbon, nitrogen
and chlorine components (Table 1). We acknowledge here that
the sp
3
-hybridized C-component in the N-GNSP sample is
relatively large (30.6% of total composition), which initially
seems incongruent with our claim of having a graphene nano-
material, which should have consisted of purely sp
2
-hybridized
C. However, noting that XPS is a surface sensitive technique,
31
we attribute the measured sp
3
signals to the edge structure of
the GNSPs and N-GNSPs exposed due to the vertical growth,
i.e.
,
the sp
3
signal is arti
cially high because the edges are more
exposed than the basal plane. Additionally, the N-GNSPs sample
likely has a larger sp
3
component due to more sp
3
defects in the
sample, as evidenced by a slightly wider D peak and larger D
0
peak in the Raman spectrum.
For the N-1s high resolution spectrum, we followed previ-
ously reported assignments and
t the data to a superposition
of two peaks that corresponded to contributions from the pyr-
idinic sites (399.5 eV) and pyrrolic sites (401.5 eV).
11
We found
that the pyridinic sites accounted for

86% of the nitrogen
content. This is an unprecedentedly large ratio of pyridinic type
nitrogen doping sites, particularly considering the relatively
narrow Raman peaks (Fig. 2F) and a relatively high yield. In the
case of the Cl-2p high resolution spectrum (Fig. 3F), we
considered the contributions of two components (with four
peaks due to the 2p
1/2
/2p
3/2
splitting), which corresponded to C
Cl bonds (201.9/200.3 eV) and C
Cl
x
bonds (197.2/196.1 eV).
32
3.3 Electrochemical characterization
The fabricated graphene nanomaterials (both GNSPs and N-
GNSPs) were tested for LIB anode application
via
galvano-
static charging/discharging (Fig. 4A
C) and rate performance
analysis (Fig. 4D) by assembling with conductive additive and
Table 1
Compositional analysis of the N-GNSP material from XPS
fi
tting. (Slight discrepancies between fractions and sums are due to rounding)
Element
Con
guration
Peak position (eV)
Concentration
(%)
Fraction of total composition (%)
C
89.4
sp
2
289.4
24.0
sp
3
285.7
30.6
C
N
285.8
16.6
C
Cl
287.1
3.1
p
p
*
291.9
17.8
N
7.5
Pyridinic
399.5
6.5
Pyrrolic
401.5
1.0
Cl
3.1
C
Cl
202.5/200.9
2.8
C
Cl
x
197.8/196.7
0.4
Fig. 3
(A) XPS survey spectrum of undoped GNSPs, showing a pure carbon composition. (B) XPS high resolution C-1s spectrum of GNSPs. (C)
XPS survey spectrum of N-GNSPs, showing 89.4% carbon, 7.5% nitrogen and 3.1% chlorine. High resolution XPS spectra of N-GNSPs for (D) C-1s
(E) N-1s and (F) Cl-2p.
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polymer binder in a coin cell con
guration. These data
resemble previously reported graphene LIB anode data in that
the discharge capacity substantially decreased between the
rst
and second cycles, the redox potentials varied substantially with
the state of charge, and the capacity decreased with increasing
charge/discharge rates.
4
10
For both GNSPs and N-GNSPs, SEI
formation during the
rst cycle was apparent as a plateau at
800
700 mV, consistent with the report in literature.
33
A
er the
rst discharge, the charge/discharge curves (Fig. 4A and B)
nearly completely overlapped, indicating that the SEI entirely
forms during the
rst discharge. The reversible capacities of the
GNSP and N-GNSP anodes at 100 mA g

1
are

373 mA h g

1
and

423 mA h g

1
, respectively (Fig. 4A and B), and both anodes
returned to these values a
er cycling at high rates, suggesting
minimal capacity fade (Fig. 4D). At 100 mA g

1
the capacity of N-
GNSPs was 13% higher than the capacity of GNSPs; the impli-
cations of this observation will be discussed in Section 4. To
compare the voltage pro
les of the GNSP and N-GNSP anodes,
we plotted in Fig. 4C the voltage as a function of anode state-of-
charge (SOC). Both the charge and discharge pro
les of N-
GNSPs showed a higher redox potential than the undoped
GNSPs for all SOC, suggesting that the pyridinic nitrogen had
bene
cial e
ff
ects on the reduction kinetics and detrimental
e
ff
ects to the oxidation kinetics.
Additionally, to compare the electrochemically active surface
areas of GNSPs and N-GNSPs, we estimated the double layer
capacitance (which is linearly proportional to the active surface
area) based on the galvanostatic charge/discharge curves, as
detailed in ESI Note 4 and Fig. S4.
The estimated double layer
capacitance of GNSPs and N-GNSPs is 5.6 F g

1
and 10.8 F g

1
,
respectively, suggesting that N-GNSPs have a signi
cantly
higher surface area than GNSPs per unit mass, which is
consistent with the perforations of N-GNSPs as imaged in
Fig. 2D. While the double layer capacitance for the N-GNSPs
electrode is nearly twice of the capacitance of the undoped
GNSPs electrode, the lithium storage capacitance of the N-
GNSPs electrode is only 14% higher than the GNSPs electrode.
This discrepancy suggests that although the double layer
capacitance and the lithium storage capacitance follow the
same trend between these two materials, they are not neces-
sarily related. That is, the lithium storage capacitance is due to
faradaic charge storage rather than non-faradaic charge storage.
3.4 Post mortem XPS characterization of N-GNSP LIB anodes
To study the chemical changes that occur at dopant sites during
the LIB operation, we performed XPS characterization on N-
GNSP anodes a
er lithium cycling. For these experiments,
Fig. 4
(A) First three cycles of galvanostatic charge/discharge for GNSP anode. Rate: 100 mA g

1
. Note: the second discharge is eclipsed by the
third discharge. (B) First three cycles of galvanostatic charge/discharge for N-GNSP anode. Rate: 100 mA g

1
. Note: the second discharge is
eclipsed by the third discharge. (C) Galvanostatic charge/discharge of the
fi
fth cycle of both the GNSP and N-GNSP anodes normalized by state
of charge. Rate: 100 mA g

1
. (D) Rate performance of GNSPs and N-GNSPs with the rate of each cycle (in mA g

1
) indicated above the data. All
voltages are referenced to Li/Li
+
.
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samples were prepared without polymer binders and conduc-
tive additives to simplify data interpretation and to avoid
contamination from polymers of the XPS ultrahigh vacuum
environment. Rather, because our N-GNSP material grew
directly on a copper foil, the as-grown sample with its copper
substrate was assembled directly into a coin cell, as shown in
the right panel of Fig. S3.
Three samples were prepared this
way, each with an open-circuit voltage of

3V
vs.
Li/Li
+
, and
each sample was subjected to di
ff
erent conditions. The
rst,
labelled
rest
, was kept at open circuit for

3 days. The second,
labelled
800 mV
, was cycled between 3 V (open-circuit voltage)
and 800 mV
ve times, leaving the cell charged at 3 V. The third,
labelled
10 mV
, was cycled between 3 V (open-circuit voltage)
and 10 mV
ve times, leaving the cell charged at 3 V. Coin cells
were cycled
ve times to ensure the relevant chemical processes
proceeded to completion. Then, each coin cell was de-crimped
in an argon-
lled glovebox, rinsed with dimethyl carbonate,
and dried in a vacuum chamber without being exposed to air.
The samples were subsequently transferred to an argon-
lled
air-tight suitcase directly to the XPS load lock chamber. In
addition, the top layers of sample
10 mV
were removed by
gently scraping the sample with a spatula in the glovebox to
reduce interference from the SEI. In addition, because these
samples were very porous (see Fig. 2B), the electrolyte salt was
not e
ff
ectively removed from the surface even a
er rinsing the
samples with dimethyl carbonate. The resulting XPS spectra
displayed a strong signal from the salt and a weak (and noisy)
signal from the graphene material. Therefore, we smoothed the
data using boxcar averaging with a width of eleven points. This
width was justi
ed as the step size was 0.025 eV, so a width of
eleven points was 0.25 eV, which was much smaller than the
width of the XPS peaks (see Fig. 5). For completeness, we
provide the raw data for Fig. 5 in Fig. S5.
XPS analysis of these samples revealed that the nitrogen sites
(Fig. 5A) became chemically altered during cycling. We attribute
the peak at

402.1 eV to lithiation of the nitrogen sites (
e.g.
,N
Li
+
) and the peak at 404.4 eV to solvent decomposition that
resulted in either oxygen functionalization (N
O) or carbon-
ation (N
CO
2
R) of the nitrogen sites. These assignments may
be justi
ed as follows. Given the composition of the battery cell
contents, the only possible chemical functionalization would
involve carbon, lithium, oxygen, phosphorus, and
uorine. We
can rule out phosphorus and
uorine functionalization because
the N-1s binding energies for N
P and N
F bonds were reported
in the range 397.3 eV to 400.3 eV and 417.1 eV, respectively,
according to the XPS database of National Institute of Standards
and Technology (NIST),
34
which di
ff
ered from our measured
peak positions. Therefore, the peak shi
sa
er lithium cycling
were likely due to lithiation and carbon/oxygen interactions. We
assign N
Li
+
(lithiation) and N
O/N
CO
2
R (solvent decompo-
sition) to the peaks at 402.1 eV and 404.4 eV, respectively,
because solvent decomposition reactions on nitrogen sites
likely occurred during the SEI formation (which was, in fact,
solvent and salt decomposition
33
), but the solvent decomposi-
tion peak could only appear a
er the electrode was cycled to
potentials beyond the SEI formation potential.
The chlorine 2p peak also shi
ed in the samples assembled
into battery cells a
er lithium cycling (Fig. 5B). However, the
peak shi
ed to a lower binding energy and becomes convoluted
with the phosphorus 2s peak, the latter resulted from the LiPF
6
salt physisorbed to the surface. Therefore, detailed analysis of
the chlorine 2p peak was di
ffi
cult, although it is reasonable to
Fig. 5
(A) Evolution of the N-1s XPS peak during LIB cycling. (B) Evolution of the Cl-2p XPS peak during LIB cycling.
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assign the peak to Cl
Li
+
bond,
34
and some forms of solvent
decomposition also cannot be ruled out.
Given the aforementioned peak assignments, we may
describe the evolution of the nitrogen sites during the LIB
charging/discharging cycles as follows. Even in the absence of
any applied bias, nitrogen sites will bind Li
+
once assembled in
a battery, as illustrated by the
rest
spectrum in Fig. 5A. This N
Li
+
interaction persists when the battery is cycled between 3 V
and 0.8 V, as shown by the
800 mV
spectrum in Fig. 5A. If the
battery is cycled below 800 mV, nitrogen sites become irre-
versibly functionalized due to solvent decomposition, as
demonstrated by the
10 mV
spectrum in Fig. 5A. We believe
that these
ndings can serve as useful input for computation
studies of LIB anode reaction mechanisms, which have not yet
considered di
ff
erent chemical shi
s of electrode components
under di
ff
erent reactions.
14
4. Discussion
Our experimental results suggest that selectively pyridinic-type
nitrogen-doped graphene nanomaterials do not signi
cantly
increase the capacity of graphene nanomaterials. We also
suggest that this lack of capacity enhancement is not due to the
chlorine dopants as chlorine doping has also been demon-
strated to increase the capacity of graphene nanomaterials,
i.e.
,
chlorine does not have a deleterious e
ff
ect on lithium storage
capacity.
8
To understand the origin of improved LIB perfor-
mance previously reported for pyridinic-type nitrogen-doped
graphene materials, we provide in Table 2 a brief summary of
the characterization and performance of doped graphitic
materials reported in several studies.
For instance, the material used in study 1 of Table 2 has fairly
broad D and G peaks in the Raman spectra, contains signi
cant
amounts of other dopants, has a large surface area and
demonstrates a moderately high reversible capacity of

1000 mA h g

1
. Study 2 reports a colossally high reversible
capacity of 3525 mA h g

1
, and the nitrogen-doped graphene in
this study displays a Raman spectrum with D and G peaks so
broad that they overlap (suggesting strong disorder), has a large
surface area, and contains additional sulfur and oxygen
dopants. On the other hand, the materials in studies 4 and 5
demonstrate smaller reversible capacities and display moder-
ately broad peaks in the Raman spectra, smaller surface areas
and fewer dopants than the materials in studies 1, 2 and 3. Our
work (study 6 in Table 2) displays the narrowest Raman peaks,
purest nitrogen-doping, and the smallest reversible capacity.
While the comparison in Table 2 is by no means exhaustive,
it reveals a trend that better performing doped graphene
materials generally contain signi
cant crystalline disorder (as
demonstrated by the broad peaks in their Raman spectra) and
have larger surface areas. Additionally, graphene materials
containing multiple dopants appear to perform better, although
this trend cannot be well established based on limited
comparison in Table 2. On the other hand, researchers have
recently substantiated that incorporating multiple dopants in
graphene nanomaterials can improve the electrocatalytic e
ff
ect
through the synergism among di
ff
erent heteroatoms or
nitrogen sites.
35
37
Such synergetic e
ff
ects may also take place in
the case of LIB applications, although there are insu
ffi
cient
experimental data to draw this conclusion at present.
The phenomenon of high capacity in disordered carbon
materials has been studied for decades, and several models
have been proposed to explain it (
e.g.
, see Section 2.3.3 in ref.
38). While many recent studies have focused on incorporating
dopants to enhance lithium storage, it is possible that the
dopants themselves have little e
ff
ect on the lithium storage
capacity. Rather, structural disorder that is coincident with
doping may be the essential factor a
ff
ecting the lithium storage
capacity.
5. Conclusion
We have demonstrated a novel method of synthesizing
nitrogen-doped graphene nanostripes with an unprecedentedly
Table 2
Brief comparison of reported reversible capacities, doping and Raman spectra pro
fi
les of several studies of graphene LIB anodes. Values
in blue are estimated from
fi
gures in references cited
Study
% nitrogen
(% pyridinic)
Other dopants
Raman spectra peaks FWHM (cm

1
)
Surface area
(m
2
g

1
)
Reversible capacity
(mA h g

1
)
Reference
1
1.5 (33)
15% S
D peak: 130
624
1110
4
14% O
G peak: 140
1% H
2
1.8 (25)
6% S
D and G peaks are convoluted
906
3525
9
5.2% O
3
2 (57)
O
a
D peak: 80
a
900
10
G peak: 70
4
3.1 (65)
3.1% O
D peak: 110
290
500
6
G peak: 80
5
0 (0)
1.8% P
D peak: 100
a
450
7
3.25% O
G peak: 70
6
7.5 (87)
3.1% Cl
D peak: 65
423
This work
G peak: 76
a
Authors did not specify the amount.
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high percentage of pyridinic-type doping (>86% of the nitrogen
sites) and good crystallinity, performed studies of such selec-
tively pyridinic type nitrogen-doped graphene as LIB anode
materials, and provided experimental evidences for changes in
the chemical state of nitrogen sites during LIB operation for the
rst time
via
XPS studies as a function of the cycling voltage.
Our
ndings reveal that pyridinic-type nitrogen-doping alone
does not signi
cantly enhance the LIB anode performance
when compared to an undoped graphene sample, suggesting
that pyridinic sites may not be responsible for the enhanced
performance of nitrogen-doped graphene materials observed in
previous studies. We hypothesize that strong crystalline
disorder, high surface area and possibly multiple types of
dopants in the LIB anode material may be important to
increasing the reversible capacity. Additionally, post mortem
XPS characterization of the N-GNSP LIB anodes further reveals
immediate lithiation of the nitrogen sites upon contact with
lithium electrolyte and functionalization of nitrogen sites by
solvent decomposition and coincident SEI formation. These
ndings thus provide useful insights into more intelligent
design and mechanistic understanding of doped graphene
anodes for better LIB performance.
Author contributions
J. D. Bagley developed the 8-chamber PECVD growth system,
synthesized N-GNSPs, carried out Raman spectroscopic char-
acterization, SEM surface characterization, XPS studies, and
fabrications of LIB anodes and coin cells. D. Kishore Kumar
carried out the HIM measurement and synthesized GNSPs. K.
See contributed to the LIB studies and provided the facilities for
the coin cell fabrications. N.-C. Yeh coordinated the research
project and data analysis, and wrote the manuscript together
with J. D. Bagley.
Con
fl
icts of interest
There are no con
icts to declare.
Acknowledgements
This research was jointly supported by the United Advanced
Materials and the National Science Foundation under the
Institute for Quantum Information and Matter (IQIM) at Cal-
tech (Award #1733907). We thank Professor George R. Rossman
for allowing our access to his Raman spectroscopic facilities,
and also acknowledge the use of XPS/UPS at the Beckman
Institute and HIM at the Kavli Nanoscience Institute at Caltech.
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