Nitrogen/chlorine-doped carbon nanodisk-encapsulated hematite nanoparticles for high-performance lithium-ion storage

Article abstract of DOI:10.1016/j.jallcom.2020.156045

Iron oxide (Fe₂O₃) is an intriguing anode material of electrochemical energy storage systems such as rechargeable batteries. The rational design of its nanostructure at mild condition to cope with the issues of low reversible capacity and sluggish kinetics is required. Herein, an efficient, facile, and potentially large-scale synthesis approach using the precursor of laminated iron oxychloride?polyaniline heterostructure and mild annealing is developed, yielding unique Fe₂O₃?carbon nanocomposites with hematite nanoparticles (～20 nm) that embedded in nitrogen/chlorine-doped carbon nanodisk (N/Cl–C). Given the benefit of abundant active sites, good chemical contact between carbon and Fe₂O₃, and robust composite structure, the as-prepared Fe₂O₃?N/Cl–C anode material delivers competent lithium-ion storage properties, including high reversible capacity of 1010 mAh g^?1 (based on the mass of the as-prepared nanocomposite) at 0.1 A g^?1, decent rate performance upon a rigorous current change, and superior cycling stability with 955 mAh g^?1 after 180 cycles as well as a sustained Coulombic efficiency of about 99%. This structural design may provide a new avenue for achieving efficient iron oxide-based materials in chemical and electrochemical applications such as catalysis, lithium-ion batteries, and sodium-ion batteries.

Full text of DOI:10.1016/j.jallcom.2020.156045

Journal Pre-proof
Nitrogen/chlorine-doped carbon nanodisk-encapsulated hematite nanoparticles for
high-performance lithium-ion storage
Yajuan Li, Tianchen Xia, Tingting Yu, Yingchun Miao, Meng Yang, Xiangyu Zhao,
Liqun Ma
PII:
S0925-8388(20)32409-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.156045
JALCOM 156045
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 2 May 2020
Revised Date: 9 June 2020
Accepted Date: 10 June 2020
Please cite this article as: Y. Li, T. Xia, T. Yu, Y. Miao, M. Yang, X. Zhao, L. Ma, Nitrogen/chlorine-doped
carbon nanodisk-encapsulated hematite nanoparticles for high-performance lithium-ion storage, Journal
of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.156045.
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2020 Published by Elsevier B.V.

Yajuan Li, Data curation; Investigation; Methodology; Writing-original draft;
Tianchen Xia, Investigation;
Tingting Yu, Investigation;
Yingchun Miao, Investigation; Resources
Meng Yang, Methodology; Investigation;
Xiangyu Zhao, Conceptualization; Methodology; Supervision; Writing-review &
editing; Funding acquisition
Liqun Ma, Supervision; Project administration;

Nitrogen/chlorine-doped carbon nanodisk-encapsulated
hematite nanoparticles for high-performance lithium-ion storage
a
a
b
c
a,d
Yajuan Li , Tianchen Xia , Tingting Yu , Yingchun Miao , Meng Yang , Xiangyu
a,d,
a
Zhao *, Liqun Ma
a
College of Materials Science and Engineering, Nanjing Tech University, Nanjing
11816, China
2
b
Department of Chemical and Materials Engineering, Hefei University, Hefei 230601,
China
c
Advanced Analysis and Testing Center, Nanjing Forestry University, Nanjing 210037,
China
d
Jiangsu Collaborative Innovation Center for Advanced Inorganic Functional
Composites, Nanjing Tech University, Nanjing 211816, China
*
Corresponding author.
Email: xiangyu.zhao@njtech.edu.cn
1

ABSTRACT
Iron oxide (Fe O ) is an intriguing anode material of electrochemical energy storage
2
3
systems such as rechargeable batteries. The rational design of its nanostructure at mild
condition to cope with the issues of low reversible capacity and sluggish kinetics is
required. Herein, an efficient, facile, and potentially large-scale synthesis approach
using the precursor of laminated iron oxychloride@polyaniline heterostructure and
mild annealing is developed, yielding unique Fe O @carbon nanocomposites with
2
3
hematite nanoparticles (~ 20 nm) that embedded in nitrogen/chlorine-doped carbon
nanodisk (N/Cl-C). Given the benefit of abundant active sites, good chemical contact
between carbon and Fe O , and robust composite structure, the as-prepared
2
3
Fe O @N/Cl-C anode material delivers competent lithium-ion storage properties,
2
3
-1
including high reversible capacity of 1010 mAh g (based on the mass of the
-1
as-prepared nanocomposite) at 0.1 A g , decent rate performance upon a rigorous
-1
current change, and superior cycling stability with 955 mAh g after 180 cycles as
well as a sustained Coulombic efficiency of about 99%. This structural design may
provide a new avenue for achieving efficient iron oxide-based materials in chemical
and electrochemical applications such as catalysis, lithium-ion batteries, and
sodium-ion batteries.
Keywords: Iron oxide; nitrogen/chlorine-doped carbon; rechargeable batteries; iron
oxychloride; polyaniline
2

1
. Introduction
Versatile iron oxides such as Fe O and Fe O have been developed for diverse
2
3
3
4
[
1]
[2]
[3]
applications such as catalysis , biomedicine , chemical adsorption , and gas
[
4]
sensing . Considering their high chemical stability, abundant elemental resources,
nontoxicity, and high theoretical capacity, iron oxides are also regarded as intriguing
anode materials for rechargeable batteries such as lithium-ion and sodium-ion
[
5,6]
batteries
. For instance, Fe O , as a naturally occurring material, shows an
2 3
-1
impressive theoretical capacity of about 1007 mAh g that is 2.7 times larger than that
of graphite. However, its application was hindered by sluggish electronic and ionic
transport, large volume change upon ion storage, and severe agglomeration of active
material during cycling, which led to poor cycling performance and low reversible
[
7,8]
capacity of Fe O electrodes
.
2
3
To cope with the above issues, two kinds of effective strategies, including design of
[7-12]
nanostructures and preparation of composite materials, have been developed
. The
formation of nanoscale materials could facilitate the reversible ion storage of Fe O
2
3
because of abundant active surface, the decrease in the ion diffusion length, and the
[
13-15]
alleviation of lattice strain by volume change
, thereby achieving enhanced
reversible capacity. The combination of nanoscale Fe O with a second phase or a
2
3
ductile and conductive matrix such as carbon material or conducting polymer could
[
16-18]
further improve its electrochemical properties
. For instance, Zhou el al. used a
template-engaged redox reaction and a subsequent annealing to fabricate a ladder-like
3

α-Fe O nanostructure at a high temperature of 800 °C for 5 h, which showed good
2
3
[15]
structural stability against repeated lithiation
. Ni et al. reported that surface
sulfurization of electrochemically anodized Fe O nanotube arrays (about 1.4 μm in
2
3
thickness) or a low-valent iron and nitrogen dual doping of Fe O nanorods on carbon
2
3
cloth achieved significant enhanced charge carrier transport and reaction activity, thus
[
19,20]
enabling large reversible capacity and outstanding rate capability
. Kang et al.
synthesized hollow Fe O nanospheres embedded in an amorphous carbon matrix
2
3
through a multistep heating of an electrospun precursor. This unique hollow structure
[
21]
could accommodate the volume variation upon cycling
. Zhang et al. designed a
Fe O @carbon material with robust yolk-shell structure, which was formed by
2
3
etching the SiO layer of Fe O @SiO @carbon and allowed the lithiation induced
2
2
3
2
[
17,22]
expansion of the Fe O core, and thus contributed to a superior cycling stability
.
2
3
The SiO layer can also avoid the reduction reaction of Fe O during the
2
2
3
high-temperature (600 °C, 3 h) carbon coating process. Similar design concept has
been used to synthesize the multishelled Fe O @SnO @carbon material with an
2
3
2
[
5]
active intermediate SnO layer . Although some achievements on the Fe O anode
2
2
3
materials have been obtained, the rational structural design and scalable preparation of
high-performance Fe O -based materials without high processing cost and complex
2
3
technologies are still highly desired.
Herein, we report a new facile, efficient, and low-cost approach using a
FeOCl@polyaniline precursor with a laminated heterosturcture to synthesize a series
of hematite@carbon nanocomposites, which consist of nitrogen/chlorine-doped
4

carbon nanodisk (N/Cl-C) and hematite nanoparticles that are uniformly embedded in
the amorphous carbon nanodisk. The evolution of structure, composition, and
morphology of the precursor upon annealing has been systematically investigated.
The as-prepared Fe O @N/Cl-C shows good contact between the carbon matrix and
2
3
Fe O , robust composite structure during lithium-ion storage, and abundant active
2
3
sites, thereby achieving superior electrochemical performance with a high reversible
-1
-1
capacity of 955 mAh g after 180 cycles at 0.1 Ah g . The relationship between the
structure and electrochemical properties of the as-prepared Fe O @N/Cl-C materials
2
3
has been elucidated.
2
. Experimental
2
.1 Synthesis of materials
Iron oxychloride (FeOCl) was fabricated using a microwave heating method. The
FeCl ·6H O (Aladdin, ACS) was loaded in a glass bottle and heated in a microwave
3
2
oven (700 W) for 3 min. The obtained product of FeOCl was washed using acetone
Lingfeng, AR) to remove the residue FeCl species and dried at 60 °C for 10 h under
(
3
vacuum. The as-prepared FeOCl material (2 g) was added into the purified aniline
solution (2 ml, 98%, Alfa Aesar) with constant magnetic stirring at 50 °C for 18-36 h
under argon atmosphere, yielding the FeOCl@polyaniline (FeOCl@PANI) materials.
Then, this precursor was heated at 200 °C for 1 h under argon atmosphere, followed
by a further annealing at 300, 350, 400, 450, or 500 °C for 5 min (heating rate: 5 °C
-1
min ) to prepare the iron oxide/carbon materials. Finally, the product was washed
with ethanol, and then dried overnight under vacuum at 60 °C.
5

2
.2 Materials characterization
X-ray diffraction (XRD) data were recorded by using a Rigaku SmartLab
diffractometer (Cu-Kα radiation). Elemental (vario EL III, USA) and thermal
(NETZSCH 449 F3) analyses were carried out to measure the nitrogen and carbon
contents in the as-prepared materials. The Fourier transform infrared spectroscopy
(FTIR) spectra were collected on a Thermo Nicolet Nexus 670 Spectrometer (400 to
-1
4
000 cm wave number range). Raman spectra were obtained from a Raman
spectroscopy (Labram HR800, 532 nm laser beam). The morphology and
microstructure were investigated using the field-emission scanning electron
microscopy (FE-SEM, Ultra55) and transition electron microscopy (TEM, Tecnai G2
F30 S-Twin). X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD) was
conducted to characterize the bonding state and elemental components.
2
.3 Electrochemical measurements
The electrodes were fabricated using a common slurry coating approach. The active
material, carbon black, and polyvinylidene fluoride (PVDF) binder (weight ratio,
8
:1:1) were mixed in the N-methyl-2-pyrrolidone (NMP) solvent to make a slurry,
which was coated on a clean and dried Cu foil and then dried overnight under vacuum
at 80 °C. Electrochemical half-cells (CR2032) were assembled using the working
electrode of the as-prepared electrode, the separator of Celgard 2400 film, and the
lithium metal foil anode. The electrolyte was a 1 M LiPF in ethylene carbonate (EC),
6
dimethyl carbonate (DMC), and diethyl carbonate (DEC) (volume ratio, 1 1: 1).
The charge and discharge testing were performed using a Neware battery system in a
6

+
voltage range of 0.01-3 V versus Li /Li at room temperature. Cyclic voltammetry (CV)
-1
tests at a scan rate of 0.05 mV s were conducted using a Bio-logic (VMP3)
+
electrochemical workstation in a voltage range of 0.01-3 V versus Li /Li.
3
. Results and Discussion
Fig. 1 shows the schematic preparation process of hematite@nitrogen/chlorine-doped
carbon (Fe O @N/Cl-C) materials and XRD patterns of FeOCl, FeOCl@PANI, and
2
3
Fe O @N/Cl-C materials. The high-efficiency transformation (~91%) from
2
3
FeCl ·6H O to FeOCl was achieved by the microwave heating for only 3 min. This
3
2
approach is much convenient for producing FeOCl compared to the chemical vapor
o
transport (CVT) at 370 C for several days and the thermal decomposition at 180-250
o
[23,24]
C for several hours
. Upon the aniline treatment for 18 h, the as-prepared FeOCl
material showed a large interlayer expansion and the fully intercalation by polyaniline,
as demonstrated by the disappearance of the (010) main diffraction peak at 11.2° (7.9
[
25,26]
Å) and the formation of a new diffraction peak at 6.4° (13.8 Å)
. This
intercalation occurred because of the interaction between the aniline monomer (−NH−)
and the chlorine in the interlayers of FeOCl by the formation of the hydrogen bond
(H−Cl). The polyaniline layer was subsequently formed in the interlayers through the
3
+
oxidative polymerization by the Fe (Fe−Cl) of FeOCl, thereby forming a laminated
[
25,27]
heterostructure consisting of FeOCl and polyaniline monolayers
.
The elemental analysis (Supplementary EA) manifested that the as-prepared
laminated FeOCl@PANI (18 h) possessed a polyaniline component of about 15.1 wt%
(20.5 at%) on the basis of the detected carbon and nitrogen contents. The prolonged
7

aniline treatment to 36 h kept the phase structure of FeOCl@PANI, and led to a slight
increase in the polyaniline content to 16 wt% (22 at%), yielding a formula of
PANI0.22FeOCl. This content is much higher than 17 at% of the previously reported
[
28]
polyaniline-intercalated FeOCl , in which the FeOCl material prepared by the CVT
method showed a large particle size and required a very long time of six days for the
full polyaniline intercalation. This further indicates the facile and highly efficient
preparation of FeOCl by the microwave heating. Moreover, a higher polyaniline
content is beneficial to the formation of a more integrated polyaniline layer in the
interlayers of FeOCl. Upon a further treatment to 48 h, the layered structure was
3
+
destroyed because of the excess decrement of the Fe (Fe-Cl) by the polymerization
reaction; instead, the ultrafine α-Fe O phase (PDF card No. 72-469) was formed (Fig.
2
3
[28]
S1)
, indicating the formation of the Fe O @PANI material, although a small
2 3
fraction of the FeOCl@PANI material still existed.
The as-prepared laminated FeOCl@PANI material (36 h) was annealed under argon
atmosphere to fabricate the iron oxide/carbon materials at different temperatures,
which were selected according to the results of the thermal analysis (Fig. S2a). A
continuous weight loss occurred during the heating process. The initial weight loss in
the thermogravimetric (TG) curve at about 150 °C was dominated by the release of
the adsorbed water, followed by the loss of anionic chloride species from PANI at
[
29,30]
about 200 °C
. The following weight loss is due to the decomposition of FeOCl
and polyaniline. Fig. 1c shows the XRD patterns of the annealed samples. Upon the
annealing at 300 °C, the α-Fe O with a small grain size of about 19 nm (based on the
2
3
8

Scherrer's formula) was formed, indicating the decomposition of FeOCl component in
the laminated FeOCl@PANI material. The byproduct of iron chloride species was
removed during the washing process using organic solvent. Meanwhile, the
crosslinking of the PANI chains and the reorganization of carbon and nitrogen bonds
[
31]
occurred . The decomposition of FeOCl was completed at about 350 °C according
to the formation of an endothermic reaction at this temperature and a plateau region in
the range of 350-400 °C in the differential scanning calorimetry (DSC) curve (Fig.
S2). The samples treated at higher temperatures of 350 and 400 °C also showed the
reflections of α-Fe O materials with increased grain sizes of about 22 and 29 nm,
2
3
respectively. The EA results demonstrated that the carbon and nitrogen contents in the
samples decreased from 15.7 to 15.2 wt% and increased from 2.5 to 2.8 wt%,
respectively, upon an annealing temperature increase from 350 to 400 °C. The
chlorine content in the as-prepared materials will be analyzed later. Note that the (110)
and (104) peak ratio increased after the 400 °C annealing, which may be due to the
formation of few Fe O from the reduction reaction of Fe O by the amorphous
3
4
2
3
carbon that was produced from the decomposition of PANI. This reaction was
confirmed by a prolonged annealing for 15 min at the same temperature that the
distinct diffraction peaks of Fe O were formed in the XRD pattern (Fig. S3). The
3
4
further increase in the annealing temperature to 450 °C also led to the formation of a
mixture of Fe O and Fe O (PDF card No. 72-2303) (Fig. S3), indicating the obvious
2
3
3
4
reaction between Fe O and carbon. The pure Fe O was formed at 500 °C.
2
3
3
4
Fig. 2 shows the electron microscopy results of the as-prepared materials. The
9

distinct expanded interlayers of FeOCl by the intercalation of PANI were observed
from the High-resolution TEM (HRTEM) images (Fig. 2a-d). This confirms the peak
shift in the XRD patterns. A distinct lattice distortion was also observed after this
intercalation. The SEM image (Fig. S4) showed that the polyaniline was also coated
on the surface of the as-prepared FeOCl nanoflakes, indicating the excellent
combination between FeOCl and polyaniline from interlayer to surface layer. The
following annealing resulted in the formation of a distinct and uniform cookies-like
morphology of nanodisks embedded with ultrafine nanoparticles (Fig. 1a and Fig.
2
g,h), which showed an increase in the particle size as the annealing temperature
increased. These nanoparticles showing a particle/grain size of ~20 nm are determined
to α-Fe O phase by TEM, HRTEM, and the corresponding fast Fourier transform
2
3
(FFT) pattern (Fig. 2e,f). This is consistent with XRD result for the annealed materials.
The annealing at 400 °C led to the severe agglomeration of these nanoparticles into an
integrated nanoplate in the amorphous carbon nanodisk matrix (Fig. 2i). This
nanodisk morphology was destroyed upon the annealing at a higher temperature of
4
50 °C because of the reaction between α-Fe O and the carbon matrix, which also
2 3
caused the formation of Fe O polyhedrons at 500 °C (Fig. S4). The EDS analysis for
3
4
the as-prepared samples (Fig. S5) indicated that the nitrogen element was introduced
into the sample by the PANI intercalation, which also led to a decrease in the chlorine
content due to the polymerization of aniline. A further decrease in the chlorine content
occurred by the annealing at 300 °C because of the decomposition of FeOCl, as
confirmed by the decrease of the chlorine to iron atomic ratio (Cl/Fe: from 0.6 to
1
0

0
.36), which tended to be stable at 350-400 °C (Cl/Fe: ~ 0.15). This also demonstrated
the full decomposition of FeOCl that was observed in the DSC curve. The TG
analysis (Fig. S2b) for the as-prepared Fe O /carbon nanocomposites derived at 350
2
3
and 400 °C indicated that their total carbon, nitrogen, and chlorine contents are 23.7
and 22.6 wt%, respectively. Their chlorine contents can be calculated to be 5.5 and 4.8
wt%, respectively, by considering the carbon and nitrogen contents derived from the
EA results. The explicit elemental composition of the as-prepared Fe O @N/Cl-C
2
3
materials are therefore obtained.
Fig. 3 shows the FTIR and Raman curves of the as-prepared materials. The FeOCl
-1
material showed a characteristic absorption band at around 500 cm that is ascribed to
[27]
the Fe-O stretching vibration
, which was kept after the PANI intercalation that
-1
induced the formation of evident PANI signals in the range of 600-1600 cm (Table
S1). For instance, the quinonoid ring and C–C aromatic ring stretching vibrations
-1
+
-1
appeared at 1569 and 1493 cm , respectively. The AnH signal at 745 and 687 cm ,
-1
the aromatic C–H in-plane bending modes between 1010 and 1170 cm , and the C-N
-1
[28,31]
stretching mode at 1315 and 1250 cm were also observed
. The following
-1
annealing led to the formation of dominant signals located at 549 and 467 cm , which
[11,32]
are related to the Fe-O stretching vibration in α-Fe O
. The carbon-based signals
2
3
are weak because of its low content in the as-prepared nanocomposites; however,
some evident changes upon annealing can still be observed. The disappearance of the
-1
bands at about 687, 745, and 1140 cm suggested the full deprotonation of PANI by
−
1
annealing. The signals of the benzene ring at 1493 cm and aquinonoid ring at 1569
1
1

-1
cm were weakened and shifted to higher positions. This may be ascribed to the
crosslinking of carbon-based chains and rearrangement of carbon and nitrogen bonds.
-1
The peak of the C-N in-plane stretching modes at 1303 and 1250 cm moved toward
[
31,33-35]
-1
a lower position
. Furthermore, the bands at 1164 and 826 cm may be
-1
correlated with C-Cl bonds. The band at 1164 cm shifted toward a higher positon at
[
36]
-1
a higher annealing temperature
. A new signal appeared at 756 cm may be
attributed to the formation of phenazine units by the bond breaking and cross-linking.
The as-prepared FeOCl material shows characteristic Raman peaks of Fe-O
-1
‒1
stretching vibrations at 334, 370, and 638 cm . The band at 200 cm may be related
[
37,38]
to the Fe-Cl bond
. For the as-prepared FeOCl@PANI material, the distinct
signals of PANI are detected (Table S2). For instance, the C-C stretching vibrations
-1
+∙
were reflected at about 1559 and 1618 cm . The C-N (charge delocalization)
-1
vibrations were correlated to the band at about 1340 cm . A broad region around 1190
-1
cm was ascribed to the C-N stretching vibrations and/or C-H bending vibration of
−
1
quinonoid rings. The several weak bands between 700 and 900 cm were attributed
[
31,35]
to deformations of aromatic rings
. The Fe-O signal was not shown in the Raman
spectrum because the surface of the FeOCl@PANI material was covered by PANI
[38]
(Fig. S4)
. When the laminated FeOCl@PANI was annealed at 300 °C, the five
-1
[39,40]
bands between 200 and 600 cm appeared because of the formation of α-Fe O
.
2
3
Moreover, the characteristic Raman peaks of PANI disappeared; instead, the weak
signal corresponding to carbonaceous material was observed. The evident G and D
-1
-1 [31,41]
bands at 1559 (a weak shoulder at 1410 cm ) and 1295 cm
, respectively, was
1
2

shown by the annealing at 350 °C. The increased ratio of I /I implicated an enhanced
G
D
o
ordered carbon structure. However, this ratio decreased after the annealing at 400 C.
[39]
This may be caused by the slight reduction reaction between carbon and α-Fe O
.
2
3
Fig. 4 shows the XPS spectra of the C 1s, N 1s, and Cl 2p for the FeOCl@PANI
and Fe O @N/Cl-C materials. The C 1s spectrum collected from the surface of
2
3
FeOCl@PANI exhibits deconvoluted peaks of C-C/C-H (284.5 eV), C-N/C=N/C-H
+
+
[42,43]
(
285.1 eV), C-N /C=N /C-Cl (286.0 eV), and C=O/C-O (288.9 eV)
. After the
annealing at 350 °C, a strong signal at 284.8 eV related to carbonaceous material and
[
43,45]
a secondary peak at 286.0 eV (C-N/C-O/C-Cl) were observed (Fig. 4a)
. This
indicates the formation of N/Cl-doped carbon material. In addition, the minor peak at
[
46]
2
88.9 eV may be attributed to the C=O/C(O)O , which could hardly be detected for
o
the samples annealed at 450 and 500 C (Fig. S6). This bond may indicate the well
chemical combination or interaction between the α-Fe O nanoparticles and carbon
2
3
nanodisk matrix; however, it was weakened by the higher temperature annealing
because of the reaction between α-Fe O and carbon to form Fe O . For the N 1s
2
3
3
4
region spectra (Fig. 4b), three deconvoluted components centered at 399.5 (-NH-),
00.4 (polaron), and 401.9 eV (bipolaron) are obtained for the as-prepared
4
[28]
FeOCl@PANI material . The corresponding pyridinic nitrogen (N-6) at 399.5 eV
and pyrrole or pyridone nitrogen (N-5) at 400.5 eV were produced in the as-prepared
o
[47]
Fe O @N/Cl-C material (350 C)
. These nitrogen species were also formed in
2
3
other carbon materials prepared by pyrolyzing polymers and were beneficial to
modify the polarity and electron distribution of carbon, thereby improving ion
1
3

[
48]
o
diffusion . A new peak at 401.4 eV appeared by the annealing at 400 C, which led
[
47]
to the breaking of N-5 bond to form a quaternary nitrogen species (N-Q) . However,
upon the further increase of the annealing temperature, the content of N-6 and N-Q
decreased compared to that of N-5. This may be due to that the N-6 and N-Q were
consumed by the aforementioned internal reaction of the as-prepared Fe O @N/Cl-C
2
3
o
[49]
material. Consequently, N-Q was depleted at 500 C . The Cl 2p signals did not
show evident variation before and after annealing. A distinct pair of Cl 2p peaks was
[
50,51]
located at 198.5 and 200.1 eV for the as-prepared FeOCl@PANI material
. This
Cl 2p signal was derived from the PANI coating because the pure FeOCl exhibited the
[
52]
Cl 2p signal at 198.8 and 200.4 eV
. Similar Cl 2p peaks were obtained in the
as-prepared Fe O @N/Cl-C materials, although the PANI component has been
2
3
pyrolyzed to carbon after the annealing. The chlorine doping in the carbon material
[
53-55]
was considered to be helpful to enhance the electrochemical active sites
.
Fig. 5 shows the electrochemical properties of the Fe O @N/Cl-C electrodes for
2
3
lithium storage. The CV profiles in the initial cycles of the Fe O @N/Cl-C-300,
2
3
Fe O @N/Cl-C-350, and Fe O @N/Cl-C-400 electrodes exhibited explicit redox
2
3
2
3
peaks. The irreversible cathodic reactions in the first cycle may be associated with the
+
reduction of some oxidative groups of carbon material (above 1.5 V vs Li/Li ) and the
+
[15]
formation of solid-electrolyte interphase (SEI) layer (below 1.0 V vs Li/Li )
,
which caused capacity loss and low initial Coulombic efficiency (CE). The redox
reactions in the following cycles showed good reversibility, as confirmed by the well
overlapped CV curves. The high reversibility also demonstrates that the formation of
1
4

the SEI layer was completed in the first cycle, thereby contributing to a decreased
electrochemical polarization and thus the emergence of distinct cathodic peak at
higher potential in the following cycles. This cathodic peak is ascribed to the lithiation
[
22,56]
of Fe O to form Fe and Li O by a two-step transformation reaction
. The
2
3
2
0
3+
corresponding multi-step oxidation reaction from Fe to Fe
can be well
+
distinguished from the broad anodic region of 1.3-2.2 V vs Li/Li . Although the
as-prepared three Fe O -based electrodes present similar CV profiles, the
2
3
discrepancies can still be observed. The Fe O @N/Cl-C-300 showed the largest
2
3
cathodic current drop, which may be caused by the residue oxychloride according to
the high chlorine content and also some unstable organics that were not carbonized.
The Fe O @N/Cl-C-350 electrode demonstrated an evident improved reversibility.
2
3
Moreover, it had a lower charge and discharge voltage gap than the
Fe O @N/Cl-C-400 electrode due to that the small Fe O nanoparticles were kept
2
3
2
3
without agglomeration in the carbon matrix at a lower annealing temperature.
Fig. 5d-f show the discharge and charge profiles of the Fe O @N/Cl-C electrodes
2
3
-1
at 0.1A g in the 1st, 2nd, 5th, and 10th cycles. The specific capacity was calculated
on the basis of the total weight of the as-prepared Fe O @N/Cl-C material in the
2
3
electrode. The Fe O @N/Cl-C-300, Fe O @N/Cl-C-350, and Fe O @N/Cl-C-400
2
3
2
3
2
3
-1
electrodes deliver discharge capacities of 1246.1, 1363.6 and 1271.5 mAh g at the
first cycle, respectively. The corresponding Coulombic efficiencies are 62.3%, 74.1%
and 65.0%, respectively. The Fe O @N/Cl-C-350 electrode possesses the best
2
3
reversibility at the first cycle. This is consistent with the CV result. In the following
1
5

cycles, the median voltages of the discharge and charge curves are very close to the
voltages of cathodic and anodic peaks in the CV patterns, respectively. The evident
voltage plateaus were observed during discharge, as reflected by the distinct cathodic
peaks in the CV patterns. The high reversible capacity is kept for the
Fe O @N/Cl-C-350 electrode. For instance, the Fe O @N/Cl-C-350 electrode has a
2
3
2
3
-
1
reversible discharge capacity of about 980 mAh g at the 10th cycle. This capacity is
-1
much higher than 621 and 755 mAh g of the Fe O @N/Cl-C-300 and
2
3
Fe O @N/Cl-C-400 electrodes, respectively. The higher capacity of the
2
3
Fe O @N/Cl-C-350 electrode may be ascribed to the efficient use of smaller and well
2
3
distributed Fe O nanoparticles in the carbon nanodisk with more active lithium
2
3
storage sites such as N-5 and C-Cl species. The low reversible capacity of the
Fe O @N/Cl-C-300 electrode may be caused by the aforementioned insufficient
2
3
carbonization of PANI and decomposition of FeOCl, although the
Fe O @N/Cl-C-300 material has a uniform morphology of nanodisk embedded with
2
3
fine nanoparticles (Fig. 2g). The aggregation of Fe O particles (Fig. 2i) at a higher
2
3
o
annealing temperature of 400 C led to a reduced reversible capacity of the
Fe O @N/Cl-C-400 electrode. The result of rate performance (Fig. 5g) showed that
2
3
the Fe O @N/Cl-C-300 electrode also exhibited depressed rate capability with very
2
3
-1
-1
-1
-1
low discharge capacities of 256 mAh g at 1 A g and 60 mAh g at 2 A g . In
contrast, the Fe O @N/Cl-C-350 electrode achieved high discharge capacities of 764
2
3
-1
and 576 mAh g at the same current densities, which are 73.1% and 56.4% of the
-1
reversible capacity at 0.1 A g in the second cycle. These values are also high than
1
6

-1
4
59 (54.7%) and 343 mAh g (40.8%) of the Fe O @N/Cl-C-400 electrode. When
2 3
-1
the current rate was switched to 0.1 A g , high discharge capacities of 1060.4 and 840
-1
mAh g was recovered for the Fe O @N/Cl-C-300 and Fe O @N/Cl-C-400
2
3
2
3
electrodes, respectively, indicating their admirable rate capability upon large current
rate change.
-1
The long-term cycling (Fig. 5h) at the low current rate of 0.1 A g was carried out
to investigate the cycling stability of the Fe O @N/Cl-C electrodes. The
2
3
Fe O @N/Cl-C-300 electrode suffered from a large capacity decay upon cycling, and
2
3
-1
only 359.9 mAh g was produced after 180 cycles. This capacity decay may be
related to the dissolution of residue chloride species in the Fe O @N/Cl-C-300
2
3
material. Both the Fe O @N/Cl-C-350 and Fe O @N/Cl-C-400 electrodes exhibited
2
3
2
3
good cycling stability with a high Coulombic efficiency of about 99% and discharge
-1
capacities of 955 and 762 mAh g , respectively, after 180 cycles. The prolonged
cycling further demonstrated the good cycling stability of the Fe O @N/Cl-C-350
2
3
-1
electrode, which maintained a high stable discharge capacity of 995 mAh g after 500
-1
cycles at 0.5 A g (Fig. S7). This performance is superior or comparable to those of
some typical previously reported Fe O -based materials (Table S3) such as Cu-doped
2
3
-1
[57]
-1
[58]
Fe O (841 mAh g /80 cycles)
Fe O @NiMoO -S (1001 mAh g /100 cycles)
,
2
3
,
2
3
4
-1
[59]
3
D netlike FeO /C (851.3 mAh g /50 cycles)
, yolk-shell Fe O @C (1013 mAh
2 3
x
-1
[17]
-1
[56]
g /80 cycles)
, MWCNT/γ-Fe O (723 mAh g /310 cycles)
, hierarchical
2
3
-1
[60]
hollow TiO @Fe O (530 mAh g /200 cycles) , and Fe O -carbon nanofibers (820
2
2
3
2
3
-1
[61]
mAh g /100 cycles) . Moreover, their preparation approaches such as a long time
1
7

hydrothermal synthesis with high pressure, special template or matrix, and
high-temperature treatment are not required for our case, in which a new efficient,
facile, and potentially scalable preparation strategy is developed. The FE-SEM and
-1
TEM images of the Fe O @N/Cl-C-350 material after a long cycling at 0.1 A g
2
3
showed that the agglomeration of the Fe O nanoparticles was not observed (Fig. 6);
2
3
instead, the particle size of Fe O in the carbon nanodisk was refined by the repeated
2
3
conversion reactions. Importantly, the pristine morphology of the nanodisk embedded
with Fe O nanoparticles was maintained, indicating the robust nanocomposite
2
3
structure of the as-prepared Fe O @N/Cl-C-350 material and thus the superior
2
3
cycling stability.
. Conclusions
In summary, we have designed unique α-Fe O @cabon nanocomposites composed of
4
2
3
N/Cl-doped carbon nanodisk and hematite nanoparticles via a new approach that
annealed the precursor of laminated FeOCl@PANI heterostructure at a mild condition.
This precursor was prepared by an efficient microwave synthesis of FeOCl and a
subsequent facile polyaniline intercalation. The hematite nanoparticles were
uniformly embedded in the dual-doped amorphous carbon nanodisk matrix with good
contact through both physical and chemical interactions. The as-prepared composite
structure can provide abundant active sites for effective ion storage and afford the
volume variation during cycling. Consequently, the as-prepared Fe O @N/Cl-C-350
2
3
-1
electrode achieved a high reversible capacity above 1000 mAh g and maintained 955
-1
-1
mAh g after a long cycling of 180 cycles at 0.1 Ah g , indicating its superior
1
8

lithium-ion storage ability and cycling stability. This high reversible capacity can also
-1
be recovered after a rigorous current rate change in the range of 0.1~2 Ah g ,
suggesting the decent rate performance. Our proposed material design as well as the
facile, efficient, and potentially scalable synthesis strategy thereof are expected to
produce high-performance iron oxide-based anode materials for various
electrochemical energy storage systems. They may also be extended to other
applications such as catalysis and chemical adsorption.
Acknowledgements
The support from the National Natural Science Foundation of China (grant no.
5
1674147), QingLan Project of Jiangsu Province, the Natural Science Foundation of
the Jiangsu Higher Education Institutions (grant no. 16KJA430001), and the Priority
Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is
acknowledged.
1
9

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