ZnO-Embedded N-Doped Porous Carbon Nanocomposite as a Superior Anode Material for Lithium-Ion Batteries

Article abstract of DOI:10.1016/j.electacta.2017.09.079

A nano-sized ZnO-loaded/N-doped carbon composite (ZnO/N-CF) was prepared by anchoring the zeolitic imidazolate framework, ZIF-8, onto a porous carbon foam (CF) with water vapor-assisted method, followed by the controlled two-step heat treatments. Utilization of ZIF-8 as a Zn source facilitated the impregnation of Zn species into the mesopores of the CF matrix, establishing the beneficial conductive networks between ZnO particles and preventing the growth and aggregation of ZnO particles during the prolonged cycles. The ZnO/N-CF sample exhibited stable discharge capacities of more than 1177 mA h g^?1 after 100 cycles with a current density of 98 mA g^?1 between 0.01 and 3.0 V. It also showed excellent rate capabilities compared to bare ZnO and previously reported other ZnO-based electrodes.

Full text of DOI:10.1016/j.electacta.2017.09.079

Accepted Manuscript
Title: ZnO-Embedded N-Doped Porous Carbon
Nanocomposite as a Superior Anode Material for Lithium-Ion
Batteries
Authors: Taegyeong Kim, Hanah Kim, Jung-Min Han,
Jongsik Kim
PII:
S0013-4686(17)31950-3
DOI:
Reference:
http://dx.doi.org/10.1016/j.electacta.2017.09.079
EA 30278
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Electrochimica Acta
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Accepted date:
30-5-2017
14-9-2017
14-9-2017
Please cite this article as: Taegyeong Kim, Hanah Kim, Jung-Min Han,
Jongsik Kim, ZnO-Embedded N-Doped Porous Carbon Nanocomposite
as a Superior Anode Material for Lithium-Ion Batteries, Electrochimica
Actahttp://dx.doi.org/10.1016/j.electacta.2017.09.079
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ZnO-Embedded N-Doped Porous Carbon
Nanocomposite as a Superior Anode Material for
Lithium-Ion Batteries
Taegyeong Kim,a Hanah Kim,a Jung-Min Han,b and Jongsik Kim*a
aDepartment of Chemistry, Dong-A University, Busan 604-714, South Korea
bMaterial R&D Center, Samsung SDI, Suwon, Gyunggido 16677, South Korea
Corresponding author’s contact information
*E-mail: jskimm@dau.ac.kr; Fax: +82 51 200 7259; Tel: +82 51 200 7253
Highlights
1

(1) ZnO impregnated N-doped carbon composite (ZnO/N-CF) is fabricated by using ZIF-8 and
mesoporous carbon foam (CF).
(2) Utilization of ZIF-8 significantly improves the impregnation process due to the
hydrophobic interaction with CF matrix.
(3) The ZnO/N-CF sample shows superior electrochemical properties to those of bare ZnO.
ABSTRACT. A nano-sized ZnO-loaded/N-doped carbon composite (ZnO/N-CF) was prepared
by anchoring the zeolitic imidazolate framework, ZIF-8, onto a porous carbon foam (CF) with
water vapor-assisted method, followed by the controlled two-step heat treatments. Utilization
of ZIF-8 as a Zn source facilitated the impregnation of Zn species into the mesopores of the
CF matrix, establishing the beneficial conductive networks between ZnO particles and
preventing the growth and aggregation of ZnO particles during the prolonged cycles. The
ZnO/N-CF sample exhibited stable discharge capacities of more than 1177 mA h g-1 after 100
cycles with a current density of 98 mA g-1 between 0.01 and 3.0 V. It also showed excellent
rate capabilities compared to bare ZnO and previously reported other ZnO-based electrodes.
KEYWORDS. ZnO, ZIF-8, N-doped porous carbon, anode material, lithium-ion batteries
1. Introduction
Over recent decades, lithium-ion batteries (LIBs) have become major energy supplies for
portable electronic devices and electric vehicles.[1, 2] The increased use of LIBs has facilitated
the development of high performance LIBs, leading to the further exploration of novel anode
2

materials to replace the most commonly used anode material, graphite, because of its limited
theoretical capacity (372 mA h g-1).[3, 4] Until recently, several transition metal oxides, such
as NiO, Co3O4, CuO, Fe2O3, and Fe3O4, have been investigated as alternative anode materials
to graphite due to their high safety and large theoretical capacities of about 674  922 mA h g-
1.[5-9] Among them, zinc oxide (ZnO) has also been an attractive anode material because it
has a large theoretical capacity of about 978 mA h g-1 through the conversion reaction of ZnO
to Li2O (ZnO + 2Li  Zn + Li2O) and additional alloying reactions (Zn + Li  LiZn).[10-12]
However, the cycling properties of ZnO have not been satisfactory due to its low electrical
conductivity and large volume changes during prolonged cycles.[10-13]
To address these issues, various experimental approaches have been reported to fabricate
nanostructured ZnO,[13-15] metal-modified ZnO particles,[10, 16-22] and ZnO/carbons[11,
12, 23-26]. Among them, the ZnO/carbon electrodes have been most frequently synthesized
with reduced graphene,[24-26] organic polymers such as dopamine,[27, 28] and porous
carbons[11, 12] to improve the electrochemical properties of ZnO. In particular, the utilization
of ZnO/organic matrix composites as precursors for ZnO/carbon composites has been an
effective pathway to establish an intimate contact between ZnO particles and the resulting
conductive carbon matrices after the reductive calcination processes.[12, 27, 28] Recently,
metal-organic frameworks (MOFs), such as MOF-5 and ZIF-8, have also been exploited as
another version of sacrificial Zn-containing organic precursors to prepare carbon-coated ZnO
electrodes through simple pyrolysis processes.[12, 15, 16, 29, 30] However, these synthetic
methods have the difficulties to increase the calcination temperatures greater than about 700
C due to the undesirable growth of ZnO particles and the vaporization of Zn species although
the higher calcination temperatures are beneficial to improve the electrical conductivities of
3

the resulting carbon matrices.[30, 31] Therefore, impregnation of ZnO particles into the
preformed conductive, mesoporous carbon has been considered as one of the favorable
synthetic strategies to establish the conductive networks between the ZnO and the carbon
matrices.[11, 26] However, it is still very challenging to avoid the formation of ZnO particles
outside the mesopores and the aggregation of the ZnO particles apart from the porous
carbon.[11]
In an effort to improve the shortcomings of the previous methods, we report a facile approach
for the synthesis of a nanoscaled ZnO/N-CF composite using ZIF-8 as Zn and N precursors,
and mesoporous carbon foam (CF) as a conductive hard template, respectively. Herein, the
imidazole moiety of ZIF-8 is predicted to enable ZIF-8 to have stronger hydrophobic
interactions with the unsaturated carbon species of CF than other common Zn sources such as
Zn(NO3)26H2O, ZnCl2, and Zn(CH3COO)22H2O, thus facilitating the impregnation of ZnO
into the mesopores of CF. In addition, the calcination temperatures were below 450 C during
the synthesis, providing the merits to prevent the growth of the ZnO particles and the
vaporization of Zn(0) species. Briefly, ZIF-8 crystals were first anchored onto the mesoporous
CF by water-vapor assistance in a hydrothermal reactor as shown in Figure 1. Subsequently,
the ZIF-8/CF sample was annealed at 350 °C in air, and then 450 °C under Ar, in order to
decompose ZIF-8 to nano-sized ZnO particles. During the calcination process above the
melting point (420 °C) of Zn(0),[32] Zn2+ ions of ZIF-8 are most likely reduced to Zn(0), as an
intermediate phase, by the nearby carbon moieties under the inert Ar atmosphere. Then, the
formed Zn(0) species melts into the pores of the CF and crystallizes to ZnO nanoparticles. The
small sizes of ZnO particles were attributed to both the large difference between ZIF-8 and
ZnO in density (0.95 vs. 5.61 g/cm3, respectively)[33] and the N-doped carbon layers formed
4

by thermal decomposition of ZIF-8.[12, 29] The fabricated ZnO/N-CF electrode exhibited
excellent electrochemical performance as an anode material for LIBs.
2. Experimental Section
2.1. Sample Preparation
2.1.1. Bare ZnO[34, 35]
Bare ZnO nanoparticles were synthesized by adding an aqueous 1 M NH4OH solution
dropwise to 30 mL of a 0.1 M Zn(NO3)26H2O (Aldrich, 98%) solution until the pH of the
solution reached 8.5. The ZnO precipitate was filtered, washed with deionized (DI) H2O, and
dried at 80 °C for 12 h.
2.1.2. Carbon foam (CF)[36, 37]
Firstly, 2.13 g of SiO2 (15 nm diameter, Aldrich) was dispersed in 30 mL of DI H2O by
ultrasonication. Then, 1.38 mL of formaldehyde solution (37 wt%, Aldrich), 0.92 g of
resorcinol (≥ 99.0%, Aldrich), and 0.82 mL of 0.2 M NaCO3 (≥ 99.95, Aldrich) were added
into the dispersed SiO2 solution with stirring for 1 h. Subsequently, the solution was heated at
90 °C for 6 h (oil bath) and aged at 90 °C for 12 h in a drying oven. The orange gel was filtered,
washed with 200 mL of DI H2O, 200 mL of isopropanol (Aldrich), and dried at 80 °C for 12 h
in a drying oven. The resulting powder was calcined to 800 °C for 2 h at a heating rate of 10
°C/min under an argon (Ar) flow (50 mL/min). The carbonized powder was washed in 20 mL
of 15 wt% HF solution for 2 h, filtered, and dried at 80 °C for 12 h, to afford carbon foam (CF)
as a black powder.
5

2.1.3. Nanosized ZnO-loaded/N-doped carbon (ZnO/N-CF) composites
Firstly, 2.3 mL of a 0.24 M zinc acetate dihydrate (Zn(CH3COO)22H2O, Aldrich, 99.99%)
solution was dropped onto 0.025 g of CF powder and the mixture stirred for 48 h at room
temperature. The impregnated CF was filtered and dried at 80 °C in a drying oven. The obtained
powder was uniformly mixed with 0.41 g of 2-methylimidazole (Aldrich, 99%). The mixed
powder was then transferred into a 20 mL Teflon beaker, and the beaker was placed in a 50
mL stainless steel hydrothermal reactor containing 2.0 mL of DI H2O at the bottom. The reactor
was then heated at 120 °C for 24 h to form ZIF-8.[38] Finally, the resulting ZIF-8/CF was
calcined to 350 °C for 1 h (5 °C/min) in air, and subsequently at 450 °C for 1 h (5 °C/min)
under an Ar flow (50 mL/min). The obtained powder sample was named as ZnO/N-CF.
For comparison, ZnO/CF sample was also prepared in a similar method to the ZnO/N-CF
sample, but without using 2-methylimidazole. Etched N-CF sample was prepared by dissolving
out ZnO of the ZnO/N-CF sample in 25 mL of 0.1 M HCl solution.
2.2. Material Characterization
The XRD patterns of the samples were measured on Rigaku benchtop X-ray diffractometer
(Cu Kα radiation,  = 1.5418 Å). The morphologies and particle sizes were determined by
TEM (transmission electron microscopy, JEOL JEM-2011) and FESEM (field emission
scanning electron microscopy, JEOL JSM-6700F). TEM images of the ZnO/N-CF samples
after the 100th cycle were acquired by disassembling the coin cells after cycling. The electrodes
were washed with dimethyl carbonate (DMC) and dried in an Ar-filled glove box prior to TEM.
Thermogravimetric analyses (TGA) were performed from 20 to 800 °C at a heating rate of 5
6

°C/min in air, on a Shimadzu DTG-60H instrument. The carbon (C) and nitrogen (N)
compositions were quantified by elemental analysis (Thermo Fisher Flash EA 1112). FT-IR
spectra were recorded on a JASCO FT/IR-4600 over 400  4000 cm-1. X-ray photoelectron
spectroscopy (XPS) was conducted using a Thermo Fisher Theta Probe AR-XPS system, and
the spectra were analyzed using XPSPeak 4.1 software. The surface areas and pore volumes of
the samples were estimated on a Micromeritics ASAP 2010 gas sorption analyzer, using N2
gas. The pore-size distributions were determined by the Barrett−Joyner−Halenda (BJH)
method.
2.3. Electrochemical Measurements
Electrode slurries were fabricated by mixing the active material, acetylene black (Denka
Black, Denki Kagaku Kogyo), and poly(vinyl difluoride) (PVDF, Aldrich) binder, in a weight
ratio of 7:2:1, in N-methylpyrrolidone (NMP, Aldrich). When acetylene black was not used,
the active material and the binder were mixed in a weight ratio of 8:2. The slurries were then
pasted onto Cu foil, dried at 80 °C for 2 h under vacuum, and punched into circular shaped
electrodes with diameters of 14 mm. The loading mass of the electrode film was about 1.82 mg
cm-2. Coin cells (CR2032) for the electrochemical experiments were assembled in an Ar-filled
glove box using a polypropylene membrane (Celgard 2400) as the separator, lithium metal as
the reference electrode, and a non-aqueous electrolyte (1.0 M LiPF6 in 1:1 v/v ethylene
carbonate (EC) : dimethyl carbonate (DMC); Panaxetec, Korea). Cycling performance and rate
capabilities were examined on a battery tester (WonATech WBCS3000), in galvanostatic mode
at various current densities, over the 0.01  3.0 V voltage range. Electrochemical impedance
7

spectra (EIS) were measured for samples charged to 3.0 V over 10-2 to 105 Hz, with a 10 mV
AC signal, using an impedance analyzer (WonATech ZIVE SP2). Nyquist plots (Z” vs. Z’)
were analyzed using ZMAN software (Version 2.2 f3, WonATech). Cyclic voltammetry (CV)
curves were obtained at a scan rate of 0.2 mV s-1 on the above-mentioned battery test system.
Specific capacities of the samples were estimated on the basis of the total masses of the
electrodes (ZnO and carbons). All electrochemical experiments were performed at a constant
temperature of 25 °C.
3. Results and Discussion
3.1 Structural and Morphological Characterization
Figure 2 shows the XRD patterns of the bare ZnO, ZIF-8/CF, and ZnO/N-CF samples. All the
observed reflections of bare ZnO and ZnO/N-CF are clearly indexed to a wurtzite structured
ZnO (JCPDS No. 36-1451) in Figures 2(a) and (c), showing high purity of the samples.[23, 39]
The sharp and strong peaks observed for the ZIF-8/CF sample are well matched with those of
ZIF-8, indicating that high crystalline ZIF-8 was formed.[31, 40] The broad features at about
24 – 26 were ascribed to CF in Figure 2(b) and (c).[41] The average grain sizes of ZnO in the
ZnO/N-CF and bare ZnO samples were estimated to be about 7.6 and 24.8 nm, respectively,
from their (100), (002), and (101) reflections, using the Debye-Scherrer equation. These results
show that ZIF-8 was successfully synthesized and converted to ZnO during the controlled two-
step heat treatments in the presence of CF. When Zn(CH3COO)22H2O was used in place of
ZIF-8 as the Zn2+ source for the ZnO/CF sample, the diameters of the ZnO particles were
increased to about 13.8 nm in Figure S1(a). This indicates that utilization of ZIF-8 and CF as
8

a Zn2+ source and a carbon template, respectively, is an effective approach to downsize the
formed ZnO particles.
The surface areas of the CF and ZnO/N-CF samples were estimated by BET measurements,
as shown in Figure 3 and Table 1. The BET curves for the CF and ZnO/N-CF samples show a
typical type-IV N2-adsorption/desorption isotherm behavior with the H3-type hysteresis loops,
revealing the existence of mesoporous structures in Figure 3(a).[42] The CF sample has a large
surface area of about 717.2 m²/g with the total pore volume of about 0.826 cm3/g and the pore
size of about 3.83 nm as depicted in Figure 3(b).[36, 37] These values were decreased to about
636.5 m²/g, 0.795 cm3/g and 3.68 nm, respectively, for the ZnO/N-CF sample. This indicates
that ZnO particles are most likely to be impregnated into the pores of the CF. In contrast, the
bare ZnO sample has a significantly less porosity with the surface area of about 18.1 m²/g
compared to the ZnO/N-CF sample as shown in Table 1.[43, 44]
The FESEM image of the CF sample shows a sponge-like morphology with pores of about 1
– 3 m in diameter, which probably resulted from the aggregates of silica nanoparticle (about
15 nm in diameter), as depicted in Figure 4(a). In addition, the mesoporous structure was also
observed in the magnified TEM image in Figure 4(c).[36, 37] The bare ZnO sample was
composed of irregularly shaped aggregates of ZnO particles with lengths of about 50 – 200 nm
and diameters of 10 – 100 nmin FESEM and TEM images, respectively (Figures 4(b) and (d)).
The lattice fringes were clearly observable with an interlayer spacing of about 0.28 nm in the
inset image of Figure 4(d), which corresponds to the (100) plane of the ZnO wurtzite structure
(JCPDS No. 36-1451).[45, 46]The TEM image of ZIF-8/CF in Figure 4(e) shows hexagonally
shaped ZIF-8 crystals with diameters of about 100 – 200 nm, which were anchored onto the
surface of the CF because of their larger particle sizes than the mesopores of the CF. It is worth
9

noting that the ZIF-8 crystals grow up to about 2 – 6 m in diameter in the absence of the CF
matrix in Figure S2(a), indicating that CF played a role in limiting the growth of ZIF-8 crystals
in the ZIF-8/CF sample. The enlarged TEM image of the ZnO/N-CF sample shows the
impregnated ZnO nanoparticles with diameters of about 5-7 nm into CF in Figure 4(f). In
contrast, ZnO/CF sample has the aggregates of ZnO particles apart from CF as shown in Figure
S1(b), demonstrating that ZIF-8 is a more suitable precursor reagent than Zn(CH3COO)22H2O
for the impregnation process. The significant decrease of the particle sizes was observed during
the thermal conversion of the ZIF-8 crystals to the ZnO nanoparticles as shown in Figure 4(e)-
(f), which was mainly ascribed to the large difference in densities between ZnO and ZIF-8.[33]
Similar to the bare ZnO, the lattice fringes corresponding to the (100) crystal plane were also
obviously observed for the ZnO/N-CF sample.[45, 46] The crystallinity and lattice fringes were
further characterized by fast Fourier transform (FFT) and Fourier-filter magnified high-
resolution TEM (HRTEM) images in Figure 4(g) and (f), revealing the high crystallinity of
ZnO particles in the ZnO/N-CF sample. The FESEM image, EDS (Energy Dispersive X-ray
Spectrometry) spectrum, and EDS mapping image of the ZnO/N-CF sample exhibited that the
carbon and nitrogen are intermixed well with the zinc and oxygen as shown in Figures S2(b)-
(d). It also indicates that nitrogen remains in the sample during the decomposition of ZIF-8 by
the two-step heat treatment.
Figure 5(a) shows the TGA curves of bare ZnO and ZnO/N-CF. No significant weight loss
was observed for the bare ZnO sample during heating from about 20 to 800 °C. However, the
ZnO/N-CF sample exhibited a weight loss of about 2.9 wt% between about 20 and 140 °C due
to the evaporation of adsorbed water on the surface of the ZnO/N-CF composite.[11] The
increased amount of the surface water compared to bare ZnO is probably due to the increased
10

surface area of the ZnO/N-CF composite. In addition, a weight loss of about 44.6 wt% is clearly
observed above 300 °C, which was attributed to the decomposition of carbon-containing
moieties. The carbon and nitrogen contents of the ZnO/N-CF sample were also determined by
elemental analysis to be about 42 and 1.4 wt%, respectively, which are consistent with values
obtained by TGA measurement. This indicates that the ZnO/N-CF sample is composed of about
55.4 wt% of ZnO.
FT-IR spectra of bare ZnO and ZnO/N-CF samples showed broad peaks at about 430 – 486
and 906 cm-1, which are typically observed at ZnO (Figure 5(b)).[47] Both samples also
exhibited two peaks at about 1635 and 3400 cm-1, which were ascribed to O-H bending and
stretching vibrations of the surface water molecules, respectively.[47, 48] For the bare ZnO
sample, the peak at about 1398 cm-1 was assigned to the vibrations of surface adsorbed
CO2.[49] On the other hand, several additional peaks corresponding to carbon moieties were
observed for the ZnO/N-CF sample. For example, the broad peak at about 1165 cm-1 was
ascribed to C-C, C-O, and C-N stretching vibrations.[48, 50] The two other peaks, at about
1545 and 1702 cm-1, were attributed to the stretching modes of C=C and C=O bonds,
respectively.[48, 50] The symmetric and asymmetric stretching vibrations of C-H bonds were
also observed at about 2853 and 2919 cm-1, respectively.[48] These results indicate that the
carbon matrix in the ZnO/N-CF sample contains C and N elements derived from the thermally
decomposed imidazole moiety of ZIF-8.
The surface chemical compositions of the bare ZnO and ZnO/N-CF samples were
characterized by XPS in Figure 6. Each full XPS spectrum reveals that the main chemical
compositions are Zn and O elements (Figure 6(a)). However, a large additional C1s peak and
a small N1s peak are clearly visible at about 285 and 400 eV, respectively, in the spectrum of
11

the ZnO/N-CF sample, indicating the presence of C and N elements in agreement with the
elemental and IR analyses. The C1s peak of the ZnO/N-CF sample was deconvoluted into five
peaks at about 284.5, 285.1, 285.6, 286.8, and 289.1 eV, which were assigned to C=C, C-C, C-
N, C-O-Zn, and C=O, respectively (Figure 6(b)).[51-53] The ratio of the relative peak
intensities based on the deconvoluted data indicate that most carbons are present as C=C
species. In addition, it is clear that carbons are chemically bound to ZnO. A broad peak as
observed related in the N region (Figure 6(c)), and it was deconvoluted into two peaks at about
398.8 and 400.5 eV with similar intensities each other, corresponding to the binding energies
of C=N and C-N bonds, respectively.[25] This result indicates that nitrogen was introduced
into the ZnO/N-CF sample during the thermal transformation of ZIF-8 into ZnO. It is worth
noting that the presence of pyridinic and pyrrolic nitrogens is often desirable to improve the
electrical conductivities of metal-oxide/carbon composite materials.[25, 29] Figure 6(d) shows
the four main peaks in the O1s spectral region, signifying the existence of four different kinds
of oxygen species. The strongest peak was observed at 531.4 eV, and was ascribed to C-O-Zn
chemical bonds because of its higher shift value compared to that of a typical Zn-O bond in the
bare ZnO sample ( 530 eV), as shown in Figure S3.[51] This indicates the establishment of
intimate contact between the ZnO nanoparticles and the N-doped conductive-carbon matrix.
The peak at 532.2 eV is attributed to oxygen atoms near an oxygen deficient region of the ZnO
matrix.[54, 55] The coexistence of C-O-Zn and oxygen vacancies can have a positive effect on
improving the electrical and ionic conductivities of ZnO-based electrodes.[51, 54, 55] The
other peaks at about 532.9 and 533.7 eV are assigned to oxygen atoms of O-H and COOH
groups on the surface of the ZnO/N-CF composite.[51, 54, 55]
3.2 Electrochemical Performance
12

The electrochemical properties of bare ZnO, ZnO/CF, and ZnO/N-CF samples were examined
over the 0.01  3.0 V voltage range, as depicted in Figure 7. Figure 7(a) shows CV curves for
the ZnO/N-CF sample at the initial three cycles. A strong cathodic peak was observed at about
0.67 V at the first cycle and disappeared during subsequent cycles. Therefore, it is reasonable
to attribute the peak to the formation of SEI layers.[11, 56] Another weak peak was observed
at about 0.25 V and is probably due to the formation of a lithium-zinc alloy, such as LiZn.[57,
58] In the following reductive cycle, an additional peak is clearly probed at about 0.83 V, and
is assigned to the conversion reaction of ZnO and Li ions to form Zn(0) and Li2O.[56, 57] In
addition, the peak below 0.12 V was attributed to lithiation of the carbon matrix.[20] Anodic
cycles exhibited multiple peaks between about 0.72 and 1.69 V that correspond to the
delithiation reactions of the LiZn alloy and the formation of ZnO from the reaction of Zn(0)
and Li2O, respectively.[11, 12, 28] All of the cathodic and anodic peaks were constantly
observed, without any significant change, except for the peak at 0.67 V at the first cathodic
cycle, indicating that the ZnO/NCF sample undergoes redox reactions reversibly with lithium
ions. The broader peaks observed for the ZnO/NCF sample over those observed for the bare
ZnO (Figure S4) imply an increased involvement of the surface and interface of the ZnO/N-
CF sample for lithium ion storage,[28] due to the few nano-sized ZnO particles and large
surface area of ZnO/NCF, as depicted in Figures 3 and 4.
The initial discharge profile of the ZnO/N-CF sample (Figure 7(b)) shows two main plateaus
at about 0.27 and 0.7 V due to the reductive lithiation of ZnO and the formation of solid-
electrolyte interphase (SEI) layers, respectively, which are consistent with the phenomena
observed at the voltage profiles in Figure 7(a).[11, 15, 51, 56] For the ZnO/N-CF sample, the
plateaus are less obvious, but observed at slightly higher voltages than those of the bare ZnO
13

sample. The voltage profiles of the ZnO/N-CF sample were changed from the shape with the
plateaus at the first cycle to the sloped curves with the similar shapes during the subsequent
cycles, indicating its excellent cycling reversibility. The large initial discharge capacity of
about 1922 mA h g-1 of ZnO/N-CF was attributed to the conversion and alloying reactions of
ZnO to Zn and LiZn and the SEI formation.[11, 12, 15, 51] The reversible capacity of about
1128 mA h g-1 was measured at the second cycle, showing the large irreversible capacity loss
mainly due to the formation of SEI layers, which are resulting from reductive decomposition
of the electrolyte on the surface of the electrode.[12, 30] The initial coulombic efficiency (CE)
of the ZnO/N-CF sample was observed to be about 57%, which is higher than that of bare ZnO
(about 49%), because of its protective carbon matrix.[11, 12, 56] However, this CE value is
often lower than those of other metal oxides such as Fe2O3, Fe3O4, etc.[59-62] To increase the
applicability the ZnO/N-CF sample in real batteries, its initial CE needs to be further improved.
Its larger reversible capacities were well maintained during the subsequent cycles, discharging
a capacity of about 1030 mA h g-1 at the 50th cycle with the CE value close to 97%, as shown
in Figure 7(c). These values are larger than those of bare ZnO, which showed the reversible
capacities of about 979 and 305 mA h g-1 at the first and 50th cycles, respectively. The ZnO/N-
CF sample exhibited an outstanding cycling retention with the capacity of about 1177 mA h g-
1 during the long term cycling up to 100th cycles. For the ZnO/N-CF sample, gradual increases
were found in discharge capacity after the 50th cycle. A similar phenomenon has been observed
for other metal oxide based anode materials,[57, 63-65] and is probably due to the formation
of a polymeric/gel-like film on the electrode surface, through the decomposition of the
electrolyte, and the slow activation of electrode particles that are enclosed by the carbon
matrix.[57, 64, 66] In addition, the discharge capacities of the ZnO/N-CF sample exceeded the
theoretical capacity of ZnO/N-CF (about 788 mA h g-1), which was calculated based on the
14

reversible capacities of about 574 and 282 mA h g-1 for the etched N-CF carbon and acetylene
black, respectively, at the 50th cycle in Figure S5. The large extra capacities are tentatively
attributed to the synergistic effect between the ZnO particles and N-CF matrix and the
interfacial storage of lithium ions at the surface and voids due to the large surface area of the
ZnO/N-CF sample, as has been previously observed for other porous electrode materials.[12,
51, 56, 67, 68]
Figure 7(d) displays the rate capabilities of bare ZnO and ZnO/N-CF at various current
densities. The ZnO/N-CF sample discharged with capacities of about 1047, 866, 697, 573, 473,
371, and 296 mA h g-1 at 98, 245, 489, 978, 1956, 4890, and 9780 mA g-1, respectively, which
are larger than those of bare ZnO. (Here, the discharge capacity at 98 mA g-1 was referred to
the capacity measured at the second cycle.) It is noted that the reversible capacity of the ZnO/N-
CF sample at 9780 mA g-1 is still higher than that of bare ZnO at 245 mA g-1. When the current
density was reduced back to 98 mA g-1, about 64.0% of the original capacity of the bare ZnO
sample was retained. On the other hand, the recovery ofthe ZnO/N-CF sample reached to about
92.3%, indicating its improved rate performances compared to bare ZnO.
To investigate the cycling performance at a high current density, the ZnO/N-CF sample was
further examined at a current density of about 489 mA g-1, discharging about 720 mA h g-1 at
the 200th cycle in Figure 7(e), which is still larger than that of bare ZnO measured with 98 mA
g-1 at the 50th cycle. When the ZnO/N-CF sample was electrochemically tested without
acetylene black as a conductive additive, it still exhibited a reversible capacity of about 647
mA h g-1 at the 50th cycle, at 98 mA g-1, as shown in Figure 7(f). These results are superior to
those of bare ZnO and other previously reported ZnO-based electrodes, as compared in Table
S1.[11-13, 15, 16, 23, 24, 30]
15

The improved electrochemical performance of the ZnO/N-CF sample is ascribed mainly to
its porous structure and the downsized ZnO particles wrapped by the conductive N-doped
carbon matrix, providing the reduced diffusion paths of the lithium ions and the increased
electrical conductivity to the ZnO/N-CF sample.[11, 24, 26] In addition, the carbon matrix also
acts as a physical barrier to effectively protect the ZnO nanoparticles from severe particle
aggregation and adverse side reactions with the electrolyte during prolonged cycling.[24, 29]
Figure S6 shows that that ZnO nanoparticles in the ZnO/N-CF sample still retain their original
particles sizes without any significant aggregation, preserving intimate contact with the carbon
matrix after 100 cycles.
The electrochemical kinetics of bare ZnO and ZnO/N-CF samples were further analyzed by
measuring their EIS spectra (Figure 8). Both samples display similar Nyquist plots, with
depressed semicircles and slanted straight lines at the high and low frequency regions,
respectively. The diameter of the semicircle is determined by the charge transfer impedance
(Rct) at the electrode/electrolyte interface.[26, 29] The tilted line is indicative of the Warburg
resistance (Wo) for the lithium diffusion process in the solid electrode.[11, 26, 29] The Rct
values of the bare ZnO and ZnO/N-CF samples were estimated to be about 202 and 123 ,
respectively, by fitting the measured impedance curves with the equivalent circuit shown
Figure 8(a). The other circuit parameters are summarized in Table S2. In addition, the diffusion
coefficients for Li ions (DLi) for the bare ZnO and ZnO/N-CF samples were calculated by
applying Eqs. (1) and (2) to the Warburg regions of Figure 8(b)[69, 70];
ꢉ
ꢁꢂ
DLi = 0.5 ꢀꢃꢄ ꢆ  ꢇꢈ
(Eq. 1)
ꢅ
ꢅ

16

Zreal = Rs + Rct + -1/2
(Eq. 2)
(Note that R is the gas constant, T is temperature (K), A is the surface area of the electrode, n
is the number of electrons related to the redox reactions, F is the Faraday constant,  is the
Warburg coefficient, and C is the concentration of lithium ions). The diffusion coefficient of
the ZnO/N-CF samples was estimated to be about 8.03  10-12 cm/s, which is almost six times
larger than that of bare ZnO (1.32  10-12 cm/s), highlighting the improved electrochemical
kinetics of the ZnO/N-CF sample.
4. Conclusions
In this study, the nano-sized ZnO-loaded N-doped carbon composites were successfully
synthesized using water-vapor assistance followed by the controlled thermal treatments. By
simply choosing ZIF-8 as a precursor reagent, it was possible to facilitate the impregnation
process of ZnO particles into the conductive N-doped CF matrix and to improve an intimate
contact between ZnO particles and N-CF matrix. It leaded to the superior reversible capacities,
cycling stabilities, and rate capabilities of ZnO/N-CF as a result of improving electrical and
ionic conductivities of ZnO, achieving the small size of the ZnO particle, and preventi
ng their aggregation and severe volume changes during the long term cycling. Therefore, this
methodology can be exploited to enhance the electrochemical properties of other
nonconductive solid materials through the impregnation technique.
5. Acknowledgements
17

This research was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF), funded by the Ministry of Education
(2015R1D1A1A01057813 and 2016R1A6A1A03012812).
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22

FIGURES
Figure 1. The preparation of the nano-sized ZnO/N-CF composite.
23

Figure 2. XRD patterns of (a) bare ZnO, (b) ZIF-8/CF, and (c) ZnO/N-CF. The asterisks
indicate carbon.
24

Figure 3. (a) BET curves for N2 adsorption/desorption isotherms, and (b) BJH pore-size
distribution plots of CF, ZnO/N-CF, and bare ZnO.
25

Figure 4. FESEM images of (a) CF and (b) bare ZnO; TEM images of (c) CF, (d) bare ZnO,
(e) ZIF-8/CF, (f) ZnO/N-CF, (g) FFT of ZnO/N-CF, and (h) Fourier-filtered images of (g).
Inset in (d): the magnified image of (d).
26

Figure 5. (a) TGA curves and (b) FT-IR spectra of bare ZnO (black line) and (b) ZnO/N-CF
(red line).
27

Figure 6. (a) XPS spectra of bare ZnO and ZnO/N-CF, and (b) C1s, (c) N1s, and (d) O1s
regions of ZnO/N-CF.
28

Figure 7. (a) CV curves of ZnO/N-CF; (b) discharge/charge profiles of bare ZnO (dashed
lines) and ZnO/N-CF (solid lines); (c) cycling performance of bare ZnO and ZnO/N-CF at 98
mA g-1; (d) discharge rate performance of bare ZnO and ZnO/N-CF as functions of current
densities; cycling performances of (e) ZnO/N-CF at a rate of about 489 mA g-1; and (f) the
29