Deep eutectic-solvothermal synthesis of nanostructured Fe3S4 for electrochemical N2 fixation under ambient conditions

Article abstract of DOI:10.1039/c8cc08045c

We report the synthesis of nanostructured Fe₃S₄ from deep eutectic solvents via a one-step solvothermal method. The as-obtained Fe₃S₄ catalyst was capable of electrochemically reducing N₂ to NH₃ under ambient conditions, and exhibits a high NH₃ yield (75.4 μg h^?1 mg^?1_cat.) and faradaic efficiency (6.45%) at ?0.4 V vs. a reversible hydrogen electrode.

Full text of DOI:10.1039/c8cc08045c

Journal of
Materials Chemistry A
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PAPER
Sandwiched porous C/ZnO/porous C nanosheet
battery anodes with a stable solid-electrolyte
interphase for fast and long cycling†
Cite this: J. Mater. Chem. A, 2018, 6,
2870
2
a
a
a
a
a
Yuting Zhao, Gaoshan Huang, * Dingrun Wang, Yan Ma, Zhongyong Fan,
b
a
Zhihao Bao
and Yongfeng Mei
*
Producing carbon/transition metal oxide composites is a promising strategy for improving the stability of
high energy alkali-ion batteries. However, fracture of the thin carbon layer by enormous volume change
of the oxide and sluggish ion transport in the composite lead to premature cell failure. In this study, we
show that anodes consisting of an active ZnO nanosheet sandwiched between two layers of porous
thick carbon coating can cycle over 1650 times without obvious capacity decay. The outer thick carbon
coating with mechanical robustness suppresses the expansion of the inner oxides without cracks,
+
resulting in a stable SEI layer. Its porous structure and conductive nature also facilitate Li /electron
transportation into the inner oxides. The battery containing composite nanosheets shows a fast and high
Received 13th August 2018
Accepted 15th October 2018
ꢀ1
ꢀ1
capacity of 330 mA h g at 5000 mA g . We notice that a critical thickness ratio of ZnO to the carbon
layer may exist, and the carbon layer is prone to fracture in the case of large thickness ratio. The results
have important applications in designing carbon/metal oxide composites for high performance anodes
of Li-ion batteries.
DOI: 10.1039/c8ta07848c
rsc.li/materials-a
long life anodes made from materials with large volume
changes.
Introduction
Transition metal oxides have emerged as the most promising
candidate anode for high energy density electrodes due to introduced as matrices or surface coating layers to constrain the
11–15
To overcome these obstacles, carbonaceous materials are
1,2
their high theoretical capacity. Recently, the exponentially volume change and control the growth of the SEI layer.
increased research studies on two-dimensional (2D) metal Among them, sandwich-like hybrid nanosheets in the form of
oxides have offered an unprecedented opportunity for their use carbon/oxides/carbon, with a conductive carbon coating on
3
–5
in high rate capacity lithium ion batteries. The well-designed both sides of oxides, have manifested a signicant improve-
10,16–20
2
D nanostructures have demonstrated their superiority for fast ment in both rate capacity and cycling stability.
Compared
and long-lifespan lithium storage by providing a shortened ion- with a sandwiched structure with oxides standing on the
3
–5
diffusion length and releasing the volume change strain.
surfaces of the carbon layer (oxide/carbon/oxide), the carbon
However, the destruction of the desirable morphologies of 2D coating as the outer surface helps to constrain the volume
metal oxides could not be truly avoided during repeated change and prevents the oxides from reacting with the electro-
6
16–19
expansion and contraction of the lattice. Lithiation induced lyte, thus enabling an enhanced lifespan.
nanocracks have been observed, and the exposure of fresh oxide a capacity decay of 70% was observed for the G@SnO
to the electrolyte causes further growth of the solid-electrolyte composite because of the agglomeration and pulverization of
interphase (SEI). The SEI becomes thicker with each charge/ SnO during cycling. Aer the structure was coated with
For example,
2
2
discharge cycle and eventually leads to severe capacity decay a uniform carbon layer, the G@SnO
@C composite demon-
2
+
17
because of the consumption of electrolyte/Li and its electrically strated a much improved cycling stability.
7
–10
insulating nature.
Thus it is critical to get a stable SEI for
However, a thin layer of surface carbon coating is prone to
fracture since excessive stress is developed in the carbon
21,22
coating during cycling by inner oxide expansion.
Such frac-
a
Department of Materials Science, Fudan University, Shanghai 200433, P. R. China. ture of the carbon coating, upon lithiation, would immediately
E-mail: gshuang@fudan.edu.cn; yfm@fudan.edu.cn
b
propagate into the oxide core due to the elevated driving force
Shanghai Key Laboratory of Special Articial Microstructure Materials and
caused by the material inhomogeneity between the coating and
Technology, School of Physical Science and Engineering, Tongji University, Shanghai
22
the core. The exposed fresh core material reacts with the
electrolyte and causes further growth of the SEI. Thus a capacity
decay of 25% in 60 cycles was observed in graphene-conned Sn
2
†
1
00092, P. R. China
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta07848c
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23
nanomembranes with a graphene thickness of 5 nm. Previous of 20 sccm. A typical ALD sequence included the following: DEZ
investigation demonstrates that the fracture of the carbon layer is pulse (30 ms), waiting time (2 s), N purge (20 ms), DIW pulse
2
closely related to the coating thickness and the thicker one can (20 ms), waiting time (2 s), and N purge (20 ms). The thickness
2
22
sustain larger stress. Thus, in order to prevent the fracture of the of the ZnO nanomembrane was controlled by altering the
surface coating layer, a thicker coating for constraining the number of ALD cycles (e.g., 50, 100, 200, 400 and 800 cycles in
2
4,25
deformation of inner oxides is desired.
Nevertheless, a too the present work). Aer the ALD process, the samples were
+
ꢂ
ꢀ1
thick carbon coating would act as a barrier for Li transporting pyrolyzed at 700 C for 1 h in a N
2
ow of 600 mL min to
into inner oxides. Moreover, the isolated 2D nanosheets are carbonize the organic template. A freestanding interconnected
inclined to restack into a close packed structure. This process structure was formed, and aer being crashed, a large amount
would sharply decrease the active surface and block ion transport of CZC nanosheets were obtained. The nanosheets obtained
26,27
kinetics.
In this regard, porous 2D nanostructures, ensuring were washed with ethanol and DIW to remove the residual
efficient penetration of the electrolyte and fast charge transfer, are impurities.
11,13,28
highly desired for improving the power density of batteries.
Preparation of CTC nanosheets. The CTC nanosheets were
In the present work, we therefore designed porous carbon also fabricated by the same strategy. First, TiO
2
was deposited
ꢂ
conned oxide nanosheets, in which an oxide layer with onto the template at 150 C. Tetrakis(dimethylamido)titanium
controllable thickness was sandwiched between two porous (TDMAT) and DIW were used as precursors. A typical ALD
carbon layers with relatively larger thickness (>30 nm). In the sequence includes the following: TDMAT pulse (200 ms), wait-
methodology, template-assisted atomic layer deposition (ALD) ing time (2 s), N purge (30 s), H O pulse (50 ms), waiting time (2
2
2
was used to deposit the oxide layer on a self-made organic s), and N purge (30 s). Aer 200 deposition cycles, the sample
2
ꢂ
ꢀ1
template, followed by a simple pyrolysis process. The architecture was pyrolyzed at 700 C in N ow (600 mL min ) for 1 h. The
2
is designed with several superiorities. First, the thick carbon nanosheets obtained were washed with ethanol and DIW to
coating on the surface of oxides is thought to constrain the remove the residual impurities.
volume expansion of oxides by its mechanical robustness, thus
Preparation of ZnO nanosheets. Aer deposition of ZnO
Second, the porous feature of the thick onto the template, the sample was treated in an O atmosphere
carbon coating alleviates the restacking issues of the nanosheets (600 mL min ) at 700 C for 3 h to remove the organic
11,29
providing a stable SEI.
2
ꢀ1
ꢂ
+
26
and facilitates Li diffusion into the inner oxides. Third, the template. The ZnO nanosheets obtained were washed with
carbon outside also provides an electric contact network around ethanol and DIW to remove the residual impurities.
ꢀ1
oxides. Due to its high theoretical capacity of 978 mA h g and
natural abundance, ZnO is chosen as the active anode material in
the present study. The composite nanosheets (porous carbon/
ZnO/porous carbon nanosheets, hereaer CZC nanosheets) in
this study maintain high rate capacity and stability. A capacity of
Microstructural characterization
The morphologies of the samples were investigated using
a scanning electron microscope (SEM, Phenom Prox) and
a transmission electron microscope (TEM, Nova NanoSem 450).
The energy dispersive spectrum (EDS, Oxford X-Max 80T) was
utilized to analyze the composition of the sample. Dual beam
focused ion beam (FIB, Helios NanoLab 600) was used to cut the
nanosheet for cross-sectional imaging. The crystallinity of the
samples was characterized by X-ray diffraction (XRD) on
ꢀ
1
ꢀ1
330 mA h g was obtained at 5000 mA g and no obvious
capacity decay was observed aer more than 1650 cycles. An
optimized thickness ratio between ZnO and the carbon coating
(
ꢁ1 : 1) was also revealed. In addition, carbon conned TiO
2
2
composite nanosheets (porous carbon/TiO /porous carbon
nanosheets, hereaer CTC nanosheets) were fabricated to prove
the universality of the current approach. The results obtained
here may have important applications in the eld of high energy
and stable lithium storage.
a Bruker D8 X-ray diffractometer with Cu Ka radiation (l ¼
˚
1
.5418 A). The X-ray photoemission spectroscopy (XPS)
measurements were conducted with a ULVAC PHI5000 VERSA
PROBE system. Thermogravimetric analysis (TGA) was per-
ꢂ
formed using an SDT Q600 in the range of 25–900 C (heating
ꢂ
ꢀ1
Experimental section
Preparation of samples
rate: 10 C min ) under air-ow. Nitrogen sorption isotherms
were measured on a Quadrasorb evo (America Quantachrome).
Before measurements, the samples were degassed in a vacuum
Preparation of a porous organic template. A degradable
polyurethane foam template was prepared by crosslinking
poly(propylene oxide) (PPO) and poly-(propylene oxide)-block-
ꢂ
at 250 C for 3 h. The Raman spectrum was collected by using
a
Jobin Yvon LabRAM HR800 spectrometer, excited by
2
a 632.8 nm He–Ne laser with a laser spot size of ꢁ1mm .
30
polylactide (PPO-b-PLA) copolymers using isocyanate. More
details can be found in ESI S1.†
Electrochemical measurements
Preparation of CZC nanosheets. The CZC nanosheets were
31,32
prepared by a template-assisted ALD approach
and a subse- The electrochemical tests were performed on a coin-type 2016
quent pyrolysis process as outlined in Fig. 1a. First, deposition half-cell with metallic lithium serving as both the counter and
of a ZnO nanomembrane onto a porous organic template was reference electrodes. The working electrode was composed of
ꢂ
conducted in a home-made reactor at 150 C. Diethylzinc (DEZ) 80 wt% active material, 10 wt% Super-P (conductive additive
and deionized water (DIW) were used as precursors. Nitrogen agent), and 10 wt% polyvinylidene diuoride (binder). The
(N ) gas served as both the carrier and purge gas with a ow rate electrolyte used for Li-ion half cells consisted of 1 M LiPF
2 6
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Fig. 1 (a) Schematic of the synthesis procedure of CZC nanosheets. (b) SEM image of the porous network of the sample after pyrolysis. The inset
shows a curled edge of a nanosheet. (c) SEM image of CZC nanosheets after being crashed. (d) TEM image and EDS mapping of an individual CZC
nanosheet. (e) Cross-sectional TEM image of a CZC nanosheet. The nanosheet is composed of a sandwiched structure. (f) High resolution TEM
image of the C/ZnO interface. (g) Nitrogen adsorption–desorption isotherms and pore size distribution (inset) of the CZC nanosheet sample.
solution in a 1 : 1 mixture of ethylene carbonate (EC)/diethyl surface of the porous organic template (see Fig. 1a and S2†) and
carbonate (DEC) (Shenzhen Kejing Star Technology CO., LTD). then the ZnO coated template is treated in an inert atmosphere
The two electrodes were assembled into a cell in an argon at high temperature. During the pyrolysis process, the template
33,34
circulating glovebox (H
2
O and O
2
<1 ppm). Galvanostatic is transferred to carbon.
Meanwhile, hydrocarbons (e.g.,
charge/discharge cycling was carried out on a battery testing methane, ethane, ethylene, and propylene) are generated with
35
system (LAND CT2001A) with a voltage window of 0.001–3 V vs. increasing temperature. These volatile products undergo
+
36
Li /Li at different current rates. Electrochemical impedance a secondary pyrolytic deposition on all exposed surfaces. Aer
spectroscopy (EIS) measurements were carried out on a Zen- the pyrolysis procedure, a monolith of porous network, with the
nium/IM6 electrochemical workstation in a frequency range framework similar to that of the original template, is obtained.
from 1 MHz to 100 mHz with an amplitude of 5 mV. CV tests Fig. 1b demonstrates the typical morphology of the sample with
ꢀ1
were also carried out at a scan rate of 0.2 mV s from 0 to 3 V on 200 ALD cycles. The structure consists of interconnected
a Zennium/IM6 electrochemical workstation.
nanosheets, and the inset of Fig. 1b shows a curled edge of
a nanosheet, indicating the exibility. Aer the porous structure
is crashed manually, large amounts of composite nanosheets
Results and discussion
(i.e., CZC nanosheets) are obtained (Fig. 1c). For a more detailed
The CZC nanosheets were prepared by a template-assisted ALD identication of the structural features and chemical compo-
31,32
method
and a subsequent pyrolysis process as outlined in sitions, the EDS of an individual CZC nanosheet was conducted,
Fig. 1. The ZnO nanomembrane is rst deposited onto the as shown in Fig. 1d. The corresponding mapping results
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indicate the homogeneous distribution of C, Zn, and O and (103) lattice planes of hexagonal ZnO (PDF #36-1451),
37
elements in the nanosheet. And the atomic concentrations of C, respectively. The Raman spectrum also indicates the amor-
Zn, and O are 47%, 15%, and 38%, respectively. Besides, the phous nature of the carbon layer as a very intense D band
ꢀ1
cross-sectional structure of the nanosheet was investigated with (1339 cm , associated with disordered carbon) is observed
30,32
the help of the FIB technique. From the cross-sectional TEM (Fig. S5†).
The chemical bonding states of CZC nanosheets
image (Fig. 1e), one can see that the nanosheet is composed of were also studied by XPS measurements (Fig. S6†). The presence
a three-layer structure with 32 nm-thick ZnO sandwiched of C, O, and Zn elements is conrmed using the characteristic
between two carbon layers with thicknesses of 30 and 38 nm, spectra of C 1s, O 1s, and Zn 2p (see the detailed discussion in
respectively. No voids and impurities are observed between the the ESI†).
adjacent layers, suggesting a good electrical contact (Fig. 1e and
Fig. 2a shows the rst three cyclic voltammetry (CV) curves of
f). The high resolution TEM image in Fig. 1f demonstrates the the anode made from CZC nanosheets with 200 ZnO ALD cycles
amorphous structure of the carbon layer and the nanocrystal- (CZ200C nanosheets, 68% ZnO; Fig. S7†). In the rst cathodic
line feature of ZnO. We notice that enlarged TEM images reveal scan, four peaks at 1.7, 0.9, and 0.6 and near 0.1 V can be dis-
that a large amount of voids/pores exist in the carbon layer cerned carefully. The peak at 1.7 V is ascribed to the decom-
(
Fig. S3†). On the basis of the Barrett–Joyner–Halenda model, position of the electrolyte to generate a SEI layer and gradually
38,39
well-dened average pore sizes of 2.2, 3.8, and 5.8 nm are decreases in the subsequent cycles.
The peak at 0.9 V
determined (inset of Fig. 1g). This porous carbon layer, together presumably originates from the conversion reaction between
+
38,40
with the 2D nanosheet geometry, leads to a remarkably ZnO and Li to form Zn and Li O.
The peak at 0.6 V is
2
2
ꢀ1
+
enhanced surface area (266 m g , see Fig. 1g). This unique ascribed to the multi-step alloying process between Zn and Li
structural property is of benet to fast diffusion of Li in the to generate Li Zn. The sharp peak near 0.1 V can be attributed to
+
x
+
41
composite CZC nanosheets and will be discussed later.
the insertion of Li into the amorphous carbon. In the
The crystallographic structure of the CZC nanosheets was subsequent cathodic scans, a broad peak gradually stabilizes at
analyzed by XRD. As shown in Fig. S4,† the XRD patterns prove 0.8 V, corresponding to the reduction of ZnO and the alloying
+
42
the coexistence of the amorphous carbon layer and multi-crystal process between Zn and Li . For the anodic curve, four peaks
ꢂ
ZnO layer aer being treated at 700 C in a N
2
atmosphere. Both at 0.2, 0.5, 1.7, and 2.2 V are observed. The oxidation peaks at
CZC nanosheets and ZnO nanosheets show diffraction peaks at 0.2 and 0.5 V indicate the multi-step dealloying process of the
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
2
q ¼ 31.769 , 34.421 , 36.252 , 47.538 , 56.602 , and 62.862 , Li Zn alloy to form Zn, and the peaks at 1.7 and 2.2 V indicate
x
40,41
which are associated with the (100), (002), (101), (102), (110), the oxidation of Zn to generate ZnO.
It is notable that
Fig. 2 (a) CV curves of the CZ200C nanosheet anode. (b) Galvanostatic discharge/charge profiles of the CZ200C nanosheet anode. (c) Gal-
vanostatic discharge/charge capacities of CZC nanosheets with 50, 100, 200, 400, and 800 ALD cycles at different current densities. (d)
Comparison of the capacity of the CZ200C nanosheet anode with those of ZnO-based anodes reported in recent publications.
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a plateau from 1.3 to 2.1 V is observed in the charge proles respectively. And only around 5% remains with the current
Fig. 2b and S8†) and is ascribed to the oxidation of ZnO, cor- density increasing by 64 times for CZ800C nanosheets. When
(
ꢀ
1
responding to the peaks at 1.7 and 2.2 V in the CV anodic scan. further cycled at 1280 mA g for 300 cycles, as shown in Fig. 3a,
The CV results are consistent with the charge/discharge proles. the CZ200C nanosheet anode retains a capacity of 598 mA h g
ꢀ
1
,
ꢀ
1
The rate performance of the CZC nanosheets was investi- which is higher than those of CZ100C (470 mA h g ), CZ400C
ꢀ
1
1
ꢀ1
gated (Fig. 2c). In order to get composite nanosheets with (405 mA h g ), CZ50C (300 mA h g ), and CZ800C
ꢀ
preferable energy density and mechanical stability, composite (180 mA h g ). The performance of the batteries was also
nanosheets with different ZnO thicknesses (i.e., different investigated at even higher current densities. For the CZ100C
numbers of ALD cycles: 50, 100, 200, 400, and 800 cycles in the nanosheet anode, the battery is cycled for 500 cycles at
present work (Fig. S9†)) were tested. For the composite with 50 1250 mA g , 500 cycles at 2560 mA g , and 1000 cycles at
ALD cycles (CZ50C nanosheets), a steady capacity of 486, 418, 5120 mA g , and no capacity decay occurs (Fig S10†). For the
ꢀ
1
ꢀ1
ꢀ
1
ꢀ
1
ꢀ1
3
28, 242, 235, and 176 mA h g (based on the total mass of the CZ200C nanosheet anode, the battery is cycled at 5000 mA g
composite) is observed at current densities of 80, 320, 640, 1280, for more than 1450 cycles aer cycling for 200 cycles at
ꢀ1
ꢀ1
2
560 and 5120 mA g , respectively (Fig. 2c). The capacity 4500 mA g (Fig. 3b); although a capacity decay of around 8% is
increases gradually with the number of ALD cycles in the case of observed at the early cycles, it gradually stabilizes at around
ꢀ
1
ꢀ1
a thin ZnO layer because of the higher content of ZnO (Fig. S7†). 330 mA h g at a current density of 5000 mA g , much higher
The CZ200C nanosheet anode demonstrates the highest than that of the CZ100C nanosheet anode. The excellent rate
ꢀ
1
ꢀ1
reversible capacity of 1081 (80 mA g ), 889 (320 mA g ), 780 performance of the CZC nanosheet anode with a thinner ZnO
ꢀ
1
ꢀ1
ꢀ1
(
(
640 mA g ), 668 (1280 mA g ), 520 (2560 mA g ), and 380 layer is ascribed to its unique features. First, in CZC nanosheets,
5120 mA g ) mA g in all 5 samples (as shown in Fig. 2b and thick carbon coating layers on both sides of the ZnO layer help
ꢀ
1
ꢀ1
c), and the values are also higher than those of previously re- to provide an electric contact network around ZnO and the
3
7–39,43–45
+
ported Zn-based anodes (Fig. 2d).
increase of ALD cycles, the CZ400C and CZ800C nanosheet layer.
anodes suffer from severe capacity decay (Fig. 2c). With the provides a shortened ion-diffusion length. Thus when applied
However, with further porous feature facilitates Li diffusion into the inner ZnO
11,13,27
Second, the thin ZnO nanomembrane inside
3,4
ꢀ
1
current increasing from 80 to 640 mA g , the CZ400C and for lithium storage, the composite nanosheets enable fast and
CZ800C nanosheets lose 60% and 70% of their initial capacity, high capacity. Besides, because of its exible properties, the 2D
ꢀ1
Fig. 3 (a) Cycling performance of the CZC nanosheet anode with 50, 100, 200, 400 and 800 ALD cycles at 1280 mA g . (b) Cycling perfor-
ꢀ1
ꢀ1
mance of the CZ200C nanosheet anode at 4500 mA g for 200 cycles and 5000 mA g for 1450 cycles.
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17,27
nanostructure allows strain accommodation upon lithiation aer cycling.
R
ct/CPEct from the second semicircle is
and the carbon coating on the outer layer also helps to restrict assigned to the charge transfer process. W is the Warburg
s
5
,22
the volume change of ZnO.
impedance, and Cint is the intercalation capacitance, corre-
47–49
To probe the effect of structural features on the remarkable sponding to the sloping straight line.
The fresh cells show
performance enhancement, we conducted EIS measurements low resistance, indicating a favorable conducting network and
for CZC nanosheet anodes. The Nyquist plots of all samples charge transfer at the electrode interface. Fig. 4b lists the values
were recorded and are shown in Fig. 4a and S11.† All the plots of RSEI aer different galvanostatic cycles in the full charge
for the fresh cell demonstrate a similar shape with one semi- state. For the CZ50C and CZ100C nanosheet anodes, the RSEI
circle at high-to-medium frequency (two semicircles for the remains similar to that of the initial cycle, suggesting a stable
electrode aer cycling, Fig. S11†), followed by the appearance of SEI. A slightly higher RSEI is observed in the CZ200C nanosheet
a straight line at low frequency. The Nyquist plots were carefully anode: the RSEI increases to around 25 U and becomes stable,
analyzed with an equivalent circuit, as depicted in the upper while CZ800C exhibits a considerably higher RSEI. The RSEI
20,31,46–49
panel of Fig. 4a.
e
Herein, R represents the electrolyte jumps to around 60 U aer 5 cycles and nally reaches around
resistance. RSEI/CPESEI from the rst semicircle in the high 80 U (Fig. 4b).
frequency region is attributed to the resistance and capacitance,
In the present work, we consider that the increase of RSEI is
respectively, due to the formation of a SEI layer on the electrode attributed to the growth of the SEI layer, which is caused by
cracking of the surface carbon layer. A maximum tensile stress
(s
max) will be developed in the carbon layer by the expansion of
inner ZnO, and if this stress reaches fracture strength (s ), the
f
21,22
carbon coating is prone to fracture.
Previous investigation
demonstrated that the tensile stress smax should increase with
22
the diameter of the oxide core. The stable RSEI of CZ50C and
CZ100C nanosheets indicates that tensile stress smax is less
f
than the fracture strength s , so no crack occurs in the carbon
coating. For CZ200C nanosheets, the slightly increased RSEI
suggests that smax is gradually approaching s with increasing
f
ZnO ALD cycles. This consequently leads to a mild crack of the
carbon layer and a moderate growth of the SEI layer. The situ-
ation is much severer with the further increase of ZnO thickness
to 800 ALD cycles because of the much larger tensile stress. In
the rst several cycles, the SEI layer together with the carbon
coating is not robust yet to constrain the lithiation-induced
expansion of ZnO. The destruction and regeneration of the
9
SEI layer continues and causes a consecutively increased RSEI.
+
The consumption of electrolyte/Li in the thick SEI and its
electrically insulating nature nally induce severe capacity
7
–10
degradation of the CZ800C nanosheet anode.
Higher ZnO
content/larger ZnO thickness (see Fig. S7†) certainly contributes
to higher energy density, but a too high ZnO content sacrices
the durability: a high ZnO concentration of 89% in the CZ800C
nanosheet anode demonstrates poor stability. Therefore, one
may deduce that a critical thickness ratio of the ZnO layer to the
carbon layer exists. In the present work, the thicknesses of the
carbon layers are constant (Fig. S7†) and the growth rate of ZnO
is about 0.16 nm per ALD cycle. Thus, the thickness ratios of
ZnO/C are about 1 : 4, 1 : 2, 1 : 1, 2 : 1, and 4 : 1 for CZ50C,
CZ100C, CZ200C, CZ400C, and CZ800C, respectively. The mild
crack of the carbon layer in CZ200C (Fig. 4b) may suggest that
the thickness ratio in CZ200C is close to the critical value. When
the thickness ratio of ZnO/C < 1 : 1, as in the cases of CZ50C and
CZ100C, a stable cycling performance can be ensured. When the
Fig. 4 (a) EIS spectra of CZC nanosheets with 50, 100, 200, and 800
ALD cycles before galvanostatic discharge/charge. The upper panel
shows the equivalent circuit used to fit the experimental results. Here thickness ratio is close to 1 : 1, a slight capacity decay in the
e
R , RSEI, and Rct represent the electrolyte resistance (e), SEI resistance,
and charge transfer resistance (ct), respectively. CPE is the respective
constant-phase element accounting for the depressed semicircle in
the experimental spectra. W is the Warburg impedance, and Cint is the
s
intercalation capacitance. (b) The variation of SEI resistance as
a function of the cycle number.
early cycles and subsequent stability are noticed. When the
thickness ratio > the critical value 1 : 1, as in the cases of
CZ400C and CZ800C, the carbon layers are prone to fracture.
And an even thicker ZnO layer should lead to severer capacity
decay.
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Fig. 5 SEM images demonstrating the change of the electrode structure before and after 200 discharge/charge cycles. (a–h) Cross-sectional
view and top view of the CZ50C, CZ100C, CZ200C, and CZ800C electrodes before (a, c, e, and g) and after (b, d, f, and h) 200 cycles. (i) and (j)
display the thickness change of all electrodes after 200 discharge/charge cycles.
In order to specically elucidate the constrain effect of the change of CZ100C nanosheets is 33% and increases to 47% for
carbon layer on the ZnO layer in composite CZC nanosheets, we CZ200C (see also Fig. 5i and j). No signicant morphology
also investigated the volume expansion and structural change change can be observed for the electrodes made from CZ50C,
of the electrode before and aer 200 discharge/charge cycles CZ100C, and CZ200C nanosheets before and aer all charge/
(Fig. 5). As presented in Fig. 5a and b, for instance, the thickness discharge cycles, and the nanosheet structure is still distin-
of the CZ50C nanosheet electrode increases from 60 to 80 mm, guishable (Fig. 5b, d, and f). This indicates that when the
with 33% thickness change aer 200 cycles. The thickness thickness ratio of ZnO/C # 1 : 1, the carbon coating outside
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effectively connes the expansion of the ZnO nanosheets and
maintains their structural continuity without fracture. However,
for the CZ800C nanosheet anode, the electrode exhibits
a subversive deformation of surface morphology (Fig. 5g and h).
The nanosheets suffer from pulverization due to cracking of the
carbon layer and no apparent nanosheets are observed (Fig. 5h).
The thickness change reaches 91% (Fig. 5i and j). The
comparison of the electrode thickness before and aer
discharge/charge cycles in Fig. 5i and j implies that, for
a thickness ratio of the ZnO layer to the carbon layer below the
critical value (ꢁ1 : 1), the carbon coating layer in composite
nanosheets can sufficiently suppress electrode volume expan-
sion and retain structural integrity aer long-term cycling,
thereby improving the battery performance compared to the
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with a stable SEI for fast and stable lithium storage. By this
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1
1
707433.
1
1
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the volume change because of its mechanical robustness and
+
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critical thickness ratio of the ZnO layer to the carbon layer is
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fracture. This nding provides a guideline for designing sand-
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1
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Conflicts of interest
There are no conicts to declare.
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