The electrochemical confrontation between CoP microflake and Co3O4 microsphere via a similar synthesis process as anodes for lithium ion batteries

Article abstract of DOI:10.1016/j.jallcom.2017.09.086

In this article, the synthesis of CoP microflake and Co₃O₄ microsphere via a simple two-step strategy is presented. Startlingly, electrochemical performance comparisons of as-prepared two compounds as anode materials for lithium ion batteries (LIBs) are investigated. CoP microflake depicts a prominent capacity retention and a long cycle life with a discharge capacity of 619.2 mAh/g at a high current density of 1000 mA/g after 800 cycles, while Co₃O₄ microsphere only shows a capacity of 93.1 mAh/g after 800 cycles under the current of 1000 mA/g. This work may offer a facile approach for the preparation of metal phosphides as promising anodes for LIBs.

Full text of DOI:10.1016/j.jallcom.2017.09.086

Journal of Alloys and Compounds 728 (2017) 910e916
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
The electrochemical confrontation between CoP microflake and Co3O4
microsphere via a similar synthesis process as anodes for lithium ion
batteries
,
b
,
,
b
,
c
,
,
b
,
,
b
,
, b, c
Qing Guo a c, Qiang Ru a
*, Bei Wang a c, Yudi Mo a c, Zhen Wang a
,
Peng Zhang a c, Xianhua Hou a c, Shejun Hu a
,
b
,
,
b,
, b, c
a Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South
China Normal University, Guangzhou 510006, PR China
b Guangdong Engineering Technology Research Center of Efficient Green Energy and Environmental Protection Materials, South China Normal University,
Guangzhou 510006, PR China
c Engineering Research Center of Materials and Technology for Electrochemical Energy Storage (Ministry of Education), Guangzhou 510006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, the synthesis of CoP microflake and Co3O4 microsphere via a simple two-step strategy is
presented. Startlingly, electrochemical performance comparisons of as-prepared two compounds as
anode materials for lithium ion batteries (LIBs) are investigated. CoP microflake depicts a prominent
capacity retention and a long cycle life with a discharge capacity of 619.2 mAh/g at a high current density
of 1000 mA/g after 800 cycles, while Co3O4 microsphere only shows a capacity of 93.1 mAh/g after 800
cycles under the current of 1000 mA/g. This work may offer a facile approach for the preparation of metal
phosphides as promising anodes for LIBs.
Received 20 July 2017
Received in revised form
7 September 2017
Accepted 8 September 2017
Available online 9 September 2017
Keywords:
© 2017 Published by Elsevier B.V.
CoP microflake
Long cyclic stability
Lithium ion batteries
1. Introduction
promising anode candidates for next generation LIBs becomes
imperative.
As the popularization of electronic vehicles and wearable
equipment, such as Tesla and Apple Watch, the ever-increasing
requirements for novel energy storage devices with high capacity,
long duration and eco-friendliness have been more and more ur-
gent [1e4]. A global group of scientists have paid great attention to
exploring various types of high-performance electrochemical en-
ergy storage systems, including supercapacitors, fuel cells and
rechargeable batteries [5e7]. Nowadays, lithium ion batteries (LIBs)
have dominated a great proportion of power supplies in our life
owing to the superiorities of high power density, fast recharge
ability and long cycle life [8e12]. However, commercially available
anodes for LIBs are still carbonaceous materials, low Li-storage
capacity (LiC6, 372 mAh/g) and safety issues may hamper further
applications to a large extent [13e15]. So, the exploitation of
Currently, cobalt-based materials have been a research hotspot
by virtue of higher theoretical capacities, such as cobalt sulfide
(CoS, 588 mAh/g), Sn-Co alloy (SnCo, 663 mAh/g) and cobalt oxide
(Co3O4, 889 mAh/g) [16e20]. Nevertheless, some defects of low
intrinsic conductivity and drastic volume change generated by re-
petitive Liþ intercalation and extraction limit their practical appli-
cations [21]. In order to overcome adverse impacts, constructing
nanostructured electrodes or designing special composites con-
taining carbonaceous materials are considered to be valid methods
[22]. For instance, Wang et al. fabricated CoS/CNTs nanocomposites
as the anode for LIBs through a solvothermal method, a reversible
capacity of 780 mAh/g was still obtained after 50 cycles at a current
density of 100 mA/g [23]. Simultaneously, Li et al. prepared
Sn0.31Co0.09C0.6 compound by the simple solution polymerization
and pyrolysis approach, which shows
a specific capacity of
706.7 mAh/g after 100 cycles at 50 mA/g [24]. Mo et al. reported
that dendritic crystal Co3O4 with a diameter of 20e50 nm exhibited
a reversible capacity of 951.7 mAh/g after 100 cycles at a current
density of 100 mA/g [25].
* Corresponding author. Guangdong Provincial Key Laboratory of Quantum En-
gineering and Quantum Materials, School of Physics and Telecommunication En-
gineering, South China Normal University, Guangzhou 510006, PR China.
E-mail address: rq7702@yeah.net (Q. Ru).
http://dx.doi.org/10.1016/j.jallcom.2017.09.086
0925-8388/© 2017 Published by Elsevier B.V.

Q. Guo et al. / Journal of Alloys and Compounds 728 (2017) 910e916
911
Beyond that, transition metal phosphides (TMPs) are also ex-
pected to be attractive anodes due to earthly abundance, good
thermal stability and unique conversion mechanism [26,27]. TMPs
still own lower Li-intercalation potential and higher long cycling
stability than cobalt oxides, which is of great benefit to the adapt-
ability in practical full cells. Yang et al. synthesized CoP hollow
nanoparticle with carbon coating layer by a facile thermal decom-
posing strategy [28], which shows a specific capability of 630 mAh/
g after 100 cycles at 178 mA/g (0.2 C). At the same time, it also
expressed an excellent rate capability of 256 mAh/g at 4450 mA/g
(5 C).
the resultant black hybrids were washed several times thoroughly
with dilute hydrochloric acid (~0.5 mol/L), deionized water and
dehydrated ethanol respectively to obtain the CoP.
To fabricate Co3O4, the identical precursor (Co(OH)2) was
annealed at 350 ꢀC for 4 h under a heating rate of 1 ꢀC/min which
was same as above procedure. However, it was critical to note that
this process was adopted the muffle furnace under an oxygen (air)
atmosphere instead of Ar.
2.3. Material characterization of techniques
Hence, we report a facile co-precipitation and low-temperature
thermal treatment to successfully synthesize CoP microflake and
Co3O4 microsphere by using same precursor (Co(OH)2). Surpris-
ingly, CoP microflake demonstrates a prominent reversible capacity
and a long cycle life with a discharge capacity of 619.2 mAh/g and a
coulombic efficiency of 99.4% at 1000 mA/g after 800 cycles,
implying a promising anode candidate for LIBs. As a contrast, Co3O4
microsphere only has a capacity of 93.1 mAh/g after 800 cycles
under the current of 1000 mA/g.
The crystal structure of as-prepared CoP and Co3O4 was per-
formed by using a powder X-ray diffraction (XRD; PANalytical
X'Pert PRO, Cu/Ka radiation,
with a 2
l
¼ 1.5406 Å) at a scan rate of 2ꢀ/min
q
range of 20ꢀe80ꢀ. To further explore the electronic state,
X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD-600W)
was also carried out. The morphology and microstructure of the
product were investigated by the field emission scanning electron
microscopy (FESEM; ZEISS ULTRA 55) and high-resolution trans-
mission electron microscopy (TEM; JEM-2100HR).
2. Experimental
2.1. The synthesis of precursor
2.4. Battery assembly and electrochemical measurements
All of the chemicals in this work were purchased from Aldrich.
As illustrated in Scheme 1, Co(OH)2 precursor was synthesized by a
co-precipitation reaction under the water bath. Typically, 5 mmol of
Co(NO3)2$6H2O dissolved in 100 mL deionized water (DI water)
was stirred for 10 min to yield amaranthine aqueous solution.
Subsequently, excess ammonium hydroxide was added dropwise to
above solution with intense stirring. Afterwards, the pink suspen-
sion was under strong magnetic stirring for more 5 h and the above
procedures were also adopted water bath with a constant tem-
perature of 80 ꢀC. After sufficient reaction, the resultant suspension
was separated by centrifuge and rinsed several times through DI
water and absolute ethyl alcohol. Finally, the precipitate was dried
under vacuum at room temperature overnight to obtain the pre-
cursor (Co(OH)2).
The electrochemical tests of as-prepared materials were evalu-
ated with coin-type cells (CR2430) under the ambient environ-
ment. The working electrode for coin cells was fabricated by
spreading a slurry of 70 wt % active materials (CoP or Co3O4), 20%
acetylene black and 10 wt % polyvinylidene difluoride (PVDF)
binder onto a copper foil substrate. The working electrode sheet
was then dried at 80 ꢀC under the vacuum over 12 h to remove the
solvent. The typical loading density of the active materials is almost
between ~0.8 mg/cm2. ACR2430 coin cell was assembled in an Ar-
filled glove box under condition where the contents of oxygen and
moisture were both below 0.5 ppm. The electrode was separated
from the lithium reference electrode by a separator (Celgard 2400
membrane). And 1.0 M LiPF6 dissolved into a mixture of EC, DEC
and EMC (1:1:1, in volume, provided by Chei Industries Inc., South
Korea) was worked as electrolyte.
2.2. The process of phosphidation and oxidation
The galvanostatic charge and discharge in the voltage window of
0.01e3.0 V was recoded with a NEWARE Battery Testing System at
constant temperature of 25 ꢀC. Coin cells are cycled The cyclic
voltammetry (CV) was performed on a Solartron 1470E electro-
chemical workstation under a scan rate 0.2 mV/s in the range of
0.01e3.0 V. And the result of electrochemical impedance spec-
troscopy (EIS) measurement was collected from the CHI604D
Electrochemistry System in the frequency ranging from 0.01 Hz to
100 Hz.
To synthesize CoP, enough sodium hypophosphite (NaH2PO2)
was put at the upstream of the porcelain crucible in the center of
the tube furnace. And as-prepared Co(OH)2 precursor was placed at
the downstream. After flushed with Ar atmosphere for 1 h to
remove foreign gas, the tube furnace was heated up to 350 ꢀC with a
slow rate of 1 ꢀC/min, and maintained for 4 h with argon. Then the
furnace was naturally cooled down to room temperature. Finally,
Scheme 1. Schematic illustration of the experimental process for the Co3O4 microsphere and CoP microflake.

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Q. Guo et al. / Journal of Alloys and Compounds 728 (2017) 910e916
3. Result and discussion
[32].
Fig. 2 depicts the morphologies of two samples through scan-
ning electron microscopy. Obviously, Co3O4 product is composed of
lots of microspheres with ~2 m from Fig. 2(aeb). As can be seen in
3.1. Phases and microstructures
m
In order to affirm the phase and crystal structure, the results of
powder X-ray diffractometer are depicted in Fig. 1(aeb). It is
Fig. 2(ced), plenty of particles stack up to form CoP microflake.
Although Co3O4 and CoP was synthesized based on the identical
precursor, different morphologies may be resulted from the process
of oxidation and phosphidation. It must also be mentioned that
there is a number of nanoparticles attaching to CoP microflake,
which is useful to provide more contact areas of the electrode-
electrolyte interface and offer more active sites for lithium storage.
To achieve a better understanding of this unique structure, TEM
results of CoP are presented in Fig. 3. It is apparent that the diam-
obvious that the precursor with four sharp peaks at 2
q
¼ 19.1ꢀ,
32.5ꢀ, 38.1ꢀ, 51.5ꢀ are consistent well with hexagonal Co(OH)2 from
Fig. 1 (a). During the calcination process, PH3 with strong reduc-
ibility released from NaH2PO2 reacted readily with Co(OH)2 to
generate cobalt phosphide. As shown in Fig. 1 (b), relevant XRD
pattern reveals that the peaks of final product are indexed to
orthorhombic CoP with main peaks at 2
q
¼ 31.6ꢀ, 36.3ꢀ, 46.2ꢀ, 48.1ꢀ,
52.2ꢀ, 56.7ꢀ, corresponding to the (011), (111), (112), (211), (103) and
(301) planes. On the other hand, the same precursor was heated by
the oxidation routine under the air to obtain Co3O4. From Fig. 1 (b),
the crystal phase of Co3O4 can match our previous report [25].
These two reactions can be accounted for [29]:
eter of microflake is about 1.5 mm, which is in line with the above
discussion. From Fig. 3(aeb), it can also be inferred that CoP
products consist of multi thin flakes and these nano-sized particles
distribute on the surface with more buffer space, which accom-
modates the stress relaxation and facilitates the transport of Liþ/eꢁ
to improve the cyclic stability. A HRTEM image is shown in Fig. 3 (c),
the annotated lattice fringe with a d-spacing of 0.19 nm coincides
with the (211) lattice plane of the CoP phase shown in the XRD
analysis. Fig. 3 (d) exhibits a series of discontinuous diffraction
rings, corresponding to crystallographic directions of (011), (111),
(112), (211) and (301) planes, respectively. It can be concluded that
this unique structure can buffer the huge volume expansion, pro-
vide more contact areas of the electrode-electrolyte interface and
offer more active sites for lithium ions.
We also take the morphologies of two electrodes after 800 cy-
cles at a current density of 1000 mA/g to explore the excellent
electrochemical property. Obviously, plenty of narrow cracks
(marked with red cycles) are presented on the surface of the Co3O4
electrode (Fig. 4 a), which can be attributed to the expansion of
volume by conversion reaction. In detail, the structure of Co3O4
microsphere displays severe agglomeration for the formation of SEI
film. It results in the reduction of effective contact areas between
active materials and the electrolyte. As a comparison, the CoP
electrode only has several slight cracks and maintains its integrality
(Fig. 4 c), owing to the multi-layer structure with more buffer space.
Co(OH)2 þ PH3 / CoP þ H2O
Co(OH)2 þ O2 / Co3O4 þ H2O.
It is worth pointing that, no evident peaks of the residue or the
contaminant can be detected in two as-synthesized materials,
revealing the high purity of the products.
To gain more insights, additional surface electronic state was
certified by XPS as shown in Fig. 1(ced), which confirmed the
presence of Co, P element and two distinct valence states. For CoP
sample, the high-resolution spectrum of Co 2p can be assigned to
the Co 2p3/2 at 778.8 eV and Co 2p1/2 at 793.9 eV, respectively [30].
While two other peaks of Co 2p at 781.8 and 798.1 eV can be
attributed to the oxidized Co species (Co 2p3/2 and Co 2p1/2). What's
more, there are also two apparent shake-up satellite peaks in Fig. 1
(c), usually described as the high-spin Co2þ ions [31]. In addition,
the P 2p signal reveals two main peaks at 129.7 and 130.4 eV, which
coincides with the P 2p3/2 and P 2p1/2. A broad peak at 133.3 eV is
assigned to oxidized phosphorus species due to the contact with air
Fig. 1. XRD pattern of (a) the precursor and (b) the final products. (ced) XPS spectra of as-prepared CoP.

Q. Guo et al. / Journal of Alloys and Compounds 728 (2017) 910e916
913
Fig. 2. SEM images of (aeb) the Co3O4 microsphere and (ced) the CoP microflake.
Fig. 3. (aeb) TEM, (c) HRTEM of the CoP microflake and (d) the corresponding electron diffraction (SAED) pattern.
In general, the architecture of as-prepared CoP microflake has the
preponderance for LIBs.
layer [35]. In the next cathodic sweep process, cathodic peaks
moved to about 1.0 V and 2.2 V, consisting with the reduction of
CoO to Co. And from the first anodic scan, all anodic peaks at ~2.1 V
express the coincident shape, attributed to the process that Co was
reversibly oxidized to Co3O4. For the Co3O4, the mechanism of
lithium-ion storage might follow the redox reaction as:
3.2. Electrochemical analysis
In order to trace the electrochemical behavior, the Li storage
property of Co3O4 and CoP is evaluated. A series typical cyclic vol-
tammetry curves of Co3O4 at a scanning rate of 0.2 mV/s between
0.01 and 3 V (vs Li/Liþ) are plotted [33]. In Fig. 5(a), there is a
distinct reduction peak vanishing in the next lithium intercalation
around 0.7 V during the first cycle discharge. It is in good accor-
dance with the initial reduction of Co3O4 to metallic cobalt and the
transformation into Li2O [34]. In addition, it may partially involve
the formation of irreversible solid electrolyte interface (SEI) film
Co3O4 þ 8Li þ þ8e ꢁ 43Co þ 4Li2O
Fig. 5 (b) shows the cyclic voltammetry result of the initial five
scans for CoP under the same condition with Co3O4. On the con-
trary, a broad cathodic peak ranging from 1.0 to 0.6 V, is due to the
multi-step Liþ insertion into CoP [36]. Similarly, it is also related to
the formation of SEI film layer owing to the decomposition of

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Q. Guo et al. / Journal of Alloys and Compounds 728 (2017) 910e916
Fig. 4. Typical SEM images of (aeb) Co3O4 and (ced) CoP electrodes after 800 cycles at a current of 1000 mA/g.
Fig. 5. Typical voltage versus specific capacity profiles of the initial 5 cycles at a scan rate of 0.2 mV/s of (a) the Co3O4 microsphere and (b) the CoP microflake. (ced) Galvanostatic
charge-discharge curve for the 1st, 2nd, 800th cycles at a current density of 1000 mA/g of the two electrodes respectively.
electrolyte. In subsequent cathodic sweep process, a sharp peak
around ~0.6 can be ascribed to the reaction:
profiles (1st, 2nd, and 800th cycles) under a same current density of
1000 mA/g in the voltage of 0.01e3.0 V. There is a distinct discharge
plateau located at ~0.8 V for the first cycle, and the lithiation
plateau of the second cycle increases to ~1.0 V for Co3O4. However,
different from the cobalt oxide, the cobalt phosphide shows a
consecutive multi-step lithiation reaction. Also, all these conse-
quences are consistent with the above CV results. Interestingly, the
first discharge capacities and initial coulombic efficiency of two
V
CoP þ 3Li þ þ3e ꢁ /Co þ Li3P [28]. In the first charge, a repre-
sentative peak at ~1.2 V is ascribed to the reversible lithium
extraction of Li3P [37]. During the subsequent cycles, the CV curves
are almost identical, indicating the stable electrochemical charac-
teristics of CoP and the enhanced lithium-ion insertion kinetics.
Furthermore, Fig. 5(ced) present the charge-discharge voltage

Q. Guo et al. / Journal of Alloys and Compounds 728 (2017) 910e916
915
2500 mA/g, respectively. More specially, when the current gradu-
ally rebounds to 2000,1500 and 1000 mA/g, CoP microflake still
exhibits higher capacities of 504.9, 545.3 and 591.2 mAh/g. This is in
good consistent with the discussed effect of activation and micro-
flake structure. In the other hand, this excellent electrochemical
behavior is attributed to the unique architecture with more contact
areas and active sites for lithium storage. Importantly, the CoP
electrode maintains an excellent specific capacity of 659.4 mAh/g
under 500 mA/g after 200 cycles, which implies a promising
candidate anode for LIBs.
4. Conclusion
In summary, we developed a facile two-step approach to syn-
thesize the Co3O4 microsphere and CoP microflake, which revealed
extremely different electrochemical performance. Due to the
gradual activation and unique microflake architecture, the CoP
microflake demonstrated a more stable discharge capacity of
619.2 mAh/g after 800 cycles even at a high current density of
1000 mA/g, than that of Co3O4 (93.1 mAh/g). These consequences
imply that the CoP microflake could be a hopeful candidate anode
for the LIBs to handle the existent energy crisis.
Acknowledgements
This work was financially supported by the Scientific and
Technological Plan of Guangdong Province (2017A030310166,
Fig. 6. (a) Cycling performance of the Co3O4 and the CoP at a constant current density
of 1000 mA/g. (b) Rate performance of the CoP microflake at various current densities
from 200 to 2500 mA/g.
2017A030310166,
2017A040405047,
2016A050503040,
2016B010114002), the union project of National Natural Science
Foundation of China and Guangdong Province (No. U1601214), the
samples are nearly similar (1026.5 mAh/g and 73.7% for Co3O4;
1034.5 mAh/g and 74.4% for CoP), which resulted from the
approximate theoretical specific capacity. Nevertheless, there is no
doubt that the 800th discharge capacity of 619.2 mAh/g for CoP is
higher than that of Co3O4 (93.1 mAh/g).
Natural
Science
Foundation
of
Guangdong
Province
(2014A030313436), the Scientific and Technological Plan of
Guangzhou City (201607010274, 201607010322) and the Extra-
curricular Scientific Research of South China Normal University
(17WDGB04).
The cyclic performance and corresponding coulombic efficiency
of the samples are displayed in Fig. 6. As can be seen, as-prepared
samples depict two contrasting electrochemical tendencies.
Although Co3O4 microsphere delivers a similar initial capacity with
CoP microflake at the current density of 1000 mA/g, the capacity
gradually increases from the second cycle. This is a normal phe-
nomenon for the transition meal oxides, which results from the
activation process during cycling. Nonetheless, the subsequent ca-
pacity of the Co3O4 electrode is apt to deteriorate drastically until
800th cycle, which only exhibits 93.1 mAh/g with 12.0% of the
second discharge capacity. Large capacity loss may be caused by the
severe volume expansion and pulverization of the structure. On the
contrary, it can be seen that the capacities of CoP microflake also
improve step by step from 30th to 400th cycle. This increment is
supported by the other studies of TMPs [38,39], which demon-
strates the reason that the gradual activation and special microflake
structure. Furthermore, the SEI film formed on the surface during
the charge-discharge process also contributed to the gradual in-
crease in charge storage [40e42]. Finally, even at a high current
density of 1000 mA/g, the CoP electrode still shows a stable ca-
pacity of 619.2 mAh/g with 99.4% coulombic efficiency.
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