Mechanism and improvement strategy of CoSe capacity change during lithiation/delithiation

Article abstract of DOI:10.1016/j.jallcom.2020.158099

Studying the reaction mechanism of materials in the process of lithiation/delithiation is important to understand and optimize the electrode materials. In this work, the petal-like CoSe is prepared by a simple hydrothermal method, and its lithium storage mechanism is studied. During the charge/discharge cycles, the capacity of the CoSe decreases first and then increases. In the initial stage, the volume of CoSe expands due to the intercalation of lithium ions, which results in the amorphousness of CoSe and reduces the specific capacity. The subsequent increase in capacity is due to the recrystallization of the material and the formation of a conductive SEI film. The petal-like CoSe displays a specific capacity of 450 mA h g^?1 at the current density of 100 mA g^?1 after 300 cycles. To improve the lithium storage performance, a CoSe/rGO composite is prepared. The addition of GO during the preparation of CoSe changes the morphology of CoSe from a larger petal shape to a smaller rod shape, which weakens the effect of volume change during lithium ion intercalation and shortens the lithium ion diffusion distance, so improves the reduction of specific capacity. At a current density of 100 mA g^?1, the specific capacity of CoSe/rGO composite can be as high as 730 mA h g^?1 after 200 cycles. Even under a large current density of 1000 mA g^?1, the specific capacity of the CoSe/rGO composite can still reach 570 mA h g^?1 after 1000 cycles.

Full text of DOI:10.1016/j.jallcom.2020.158099

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Mechanism and improvement strategy of CoSe
capacity change during lithiation/delithiation
Qingtao Wang, Minmin Hou, Dongxu Liu,
Xiaozhong Zhou, Ziqiang Lei
PII:
S0925-8388(20)34462-5
DOI:
https://doi.org/10.1016/j.jallcom.2020.158099
JALCOM158099
Reference:
To appear in:
Journal of Alloys and Compounds
Received date: 26 September 2020
Revised date: 16 November 2020
Accepted date: 23 November 2020
Please cite this article as: Qingtao Wang, Minmin Hou, Dongxu Liu, Xiaozhong
Zhou and Ziqiang Lei, Mechanism and improvement strategy of CoSe capacity
change during lithiation/delithiation, Journal of Alloys and Compounds, (2020)
doi:https://doi.org/10.1016/j.jallcom.2020.158099
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Mechanism and improvement strategy of CoSe capacity change
during lithiation/delithiation
Qingtao Wang *a, Minmin Hou a, Dongxu Liu a, Xiaozhong Zhou a, Ziqiang Lei a
a Key Laboratory of Eco-functional Polymer Materials of the Ministry of Education,
Key Laboratory of Eco-environmental Polymer Materials of Gansu Province, College
of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou
730070, China
E-mail: wangqt@nwnu.edu.cn
Abstract: Studying the reaction mechanism of materials in the process of
lithiation/delithiation is important to understand and optimize the electrode materials.
In this work, the petal-like CoSe is prepared by a simple hydrothermal method, and its
lithium storage mechanism is studied. During the charge/discharge cycles, the
capacity of the CoSe decreases first and then increases. In the initial stage, the volume
of CoSe expands due to the intercalation of lithium ions, which results in the
amorphousness of CoSe and reduces the specific capacity. The subsequent increase in
capacity is due to the recrystallization of the material and the formation of a
conductive SEI film. The petal-like CoSe displays a specific capacity of 450 mA h g-1
at the current density of 100 mA g-1 after 300 cycles. To improve the lithium storage
performance, a CoSe/rGO composite is prepared. The addition of GO during the
preparation of CoSe changes the morphology of CoSe from a larger petal shape to a

smaller rod shape, which weakens the effect of volume change during lithium ion
intercalation and shortens the lithium ion diffusion distance, so improves the
reduction of specific capacity. At a current density of 100 mA g-1, the specific capacity
of CoSe/rGO composite can be as high as 730 mA h g-1 after 200 cycles. Even under a
large current density of 1000 mA g-1, the specific capacity of the CoSe/rGO composite
can still reach 570 mA h g-1 after 1000 cycles.
Keywords: CoSe/rGO; lithium storage mechanism; crystal structure; lithium ion
batteries
1. Introduction
With the development of society, the secondary lithium-ion batteries have become
the main source of power for portable electronic products [1]. Compared with other
energy storage materials, lithium-ion batteries have attracted much attention due to
their large energy density [2-3]. At present, commercial graphite electrodes have
lower theoretical specific capacity (372 mA h g-1). So, looking for alternative
high-capacity materials is extremely urgent [4-6].
Transition metal chalcogenides (TMCs) have become an alternative anode material
for lithium-ion batteries due to their large theoretical capacity [7-9]. Compared with
sulfides, metal selenides have higher conductivity and larger interlayer spacing.
Thereby, the charge transfer resistance and electron transfer resistance can be reduced
[10-12], which makes them have better lithium storage properties. Besides, because it
undergoes a conversion reaction during the lithium ion intercalation process, the

formed Se can increase the specific capacity [13-15]. Therefore, it is expected to
become a more promising anode material for lithium ion batteries. However, due to
the large volume expansion of the selenide during the charge and discharge process,
the material is easily pulverized, which reduces its lithium storage capacity [16-17].
Faced with these shortcomings of metal selenide, the researchers mainly reduce the
influence of volume changes through nanometerization and composite with carbon
materials. For example, MoSe2/CNT [18], Co0.85Se/GO [19], SnSe/carbon [20], and
MoSe2@CoSe/NC [21] et al.
The fabrication of the electrode is also an important issue [22-23]. In this work, the
petal-like Co(OH)2 was prepared by a simple hydrothermal method. The CoSe
material was further obtained by hydrothermal selenization, which retains its
precursor’s morphology. The obtained CoSe was used as an electrode material for
lithium ion batteries. The cycle performance of CoSe showed a “U” shaped curve,
which was first reduced and then increased. Through an in-depth study of CoSe
materials with different charge/discharge cycles, the mechanism of charge and
discharge reaction of CoSe was revealed. On this basis, the CoSe/rGO composite was
prepared by adding GO during the preparation of CoSe. The specific capacity was up
to 730 mA h g-1 after 200 cycles at the current density of 100 mA g-1. Even under a
large current density of 1000 mA g-1, the specific capacity of the CoSe/rGO composite
can still reach 570 mA h g-1 after 1000 cycles.

2. Experimental section
2.1. Material preparation
2.1 1. Materials
All chemical reagents in our experiments are of analytical grade, including cobalt
nitrate (Co(NO3)2, Maclean), sodium borohydride (NaBH4, Sinopharm Group), and
selenium powder (Se, Maclean).
2.1.2. Preparation of Co(OH)2:
Typically, 5 mmol of Co(NO3)2•6H2O and 10 mmol of hexamethylenetetramine
(HMT) were dissolved in 35 mL of deionized water. The solution was transferred to a
50 mL of Teflon-lined stainless-steel autoclave and heated for 12 h at 100 °C. After
natural cooling to room temperature, the green product was centrifuged and washed
thoroughly with ethanol several times. Subsequently, the product was dried at room
temperature for 24 h.
2.1.3. Preparation of CoSe:
2.64 mmol of NaBH4 and 1 mmol of Se powder were dissolved in 15 mL of
deionized water via vigorous stirring for 10 min to obtain a NaHSe solution. Then, 1
mmol of prepared Co(OH)2 were added to this solution. The resultant suspension was
transferred into a 50 mL of Teflon-lined stainless-steel autoclave. After heating for 10
h at 140 °C, the autoclave was cooled to room temperature, and the solid was

centrifuged and washed thoroughly with ethanol several times. Finally, the product
was dried at 60 °C overnight in vacuum.
2.1.4. Preparation of CoSe/rGO:
The graphene oxide (GO) was prepared via a modified Hummers’ method in our
previous work[24]. The Co(OH)2/rGO composites were prepared via the same
reaction conditions mentioned above with the addition of 35 mL 8.68 g L-1 GO
aqueous solution. Then, Co(OH)2/rGO was converted into CoSe/rGO by the same
method as described above.
2.2. Materials Physical characterizations
The morphology and elemental mapping of the samples were observed by field
emission scanning electron microscope (FESEM, Germany, Carl Zeiss Ultra Plus),
transmission electron microscopy (TEM, United States, FEI TECNAI TF20), and
energy dispersive spectrometer (EDS, UK, X-Max 80). The structures of the samples
were measured on the power X-ray diffractometer (XRD, Germany, Bruker D8
advance) with Cu Kα radiation (λ = 0.15416 nm). The surface elements of the samples
were analyzed with X-ray photoelectron spectroscopy (XPS, US, PHI-5702). The
degree of redox and defects of GO were analyzed by Raman spectroscopy (Raman,
Germany, RFS 100/S). The content of the CoSe/rGO composites was determined by
thermogravimetric analysis (TGA, Germany, Netzsch STA449F3). The
Brunauer-Emmett-Teller (BET) specific surface areas were obtained by evaluating N2

adsorption-desorption isotherms obtained on an Autosorb-iQ2-MP adsorption
instrument.
2.3. Electrochemical measurements
The prepared sample, conductive carbon black, and polyvinylidene fluoride
(PVDF) binder were ground in a mortar at a mass ratio of 8:1:1. The mixture was then
dispersed in N-methyl-2-pyrrolidone (NMP) and stirred 6-8 h to form a uniform
slurry. The obtained slurry was coated on Cu foil with a typical mass loading of ∼1
mg cm−2 and dried in a vacuum oven at 60 °C for 12 h. Li foil was used as the counter
electrode and Celgard 2400 polypropylene film as the separator. 1.0 M LiPF6 solution
(EC:DEC = 1:1 vol.%) and 5.0 wt.% of fluoroethylene carbonate (FEC) were chosen
as electrolyte. The electrode materials were assembled into a 2025-type half-cell in an
argon-filled glove box.
The cycle performance carried out on a battery testing system (LANHE CT2001A,
Wuhan LAND electronic Co. Ltd., China) with a voltage range of 0.01-3.0 V (vs.
Li+/Li). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy
(EIS) tests were performed on an Autolab PGSTAT128N electrochemical workstation
(Metrohm, Switzerland). The CV test was tested at a scan rate of 0.1 mV s-1 within
0.01-3.0 V (vs. Li+/Li). The EIS measurements were measured with an alternating
voltage of 10 mV over the frequency range from 105 to 10-2 Hz. After cycling, the
batteries were disassembled in an argon-filled glove box. The obtained electrodes

were rinsed with dimethyl carbonate and dried in the glove box overnight for TEM,
XRD, and XPS testing.
3.
Results and discussion
The X-ray diffraction (XRD) patterns of the CoSe and CoSe/rGO are presented in
the supplementary material (Fig. S1). It can be easily seen that the prepared CoSe and
CoSe/rGO are consistent with hexagonal CoSe (ICDD PDF#70-2870). The diffraction
peaks at 33.2 °, 44.8 °, and 50.4 ° correspond to the (101), (102), and (110) crystal
planes of CoSe, respectively. The broadening of the diffraction peak of CoSe is
obvious, indicating that the size of the prepared CoSe is small. The intensity of the
diffraction peak of CoSe/rGO is lower, which indicates that the crystallinity is lower
or the grain size is smaller.
Fig. 1. SEM images of (a) Co(OH)2, (b) CoSe, (c) Co(OH)2/rGO composite, (d)
CoSe/rGO composite. The insets correspond to the low magnification SEM images of
each samples. EDS elemental mappings of (e-i) CoSe/rGO, Se, Co, C, and O.

As shown in Fig. 1a and the inset, the precursor Co(OH)2 is a petal-like structure
with a smooth surface. Fig. 1b and inset show that the as-prepared CoSe also
maintains the petal-like morphology, but the surface changed rougher. The SEM
image of the precursor Co(OH)2/rGO is shown in Fig. 1c. The nano-scale rod-like
Co(OH)2 is distributed on the surface of the rGO sheet. It can be seen from Fig. 1d
that the structure of CoSe/rGO was substantially unchanged after selenization, and
some rod-like Co(OH)2 of the surface were pulverized into nanoparticles. The EDS
elemental mappings of CoSe/rGO in Fig. 1e-i show that Se, Co, C, and O are
uniformly distributed. The presence of O may be due to partial oxidation of the
surface of CoSe/rGO.
Transmission electron microscopy (TEM) images of the as-prepared CoSe are
shown in Fig. S2a. It can be seen that the CoSe material is a sheet-like morphology.
Fig. S2b shows an HRTEM image of CoSe, in which the clear lattice spacing of
0.149, 0.313, and 0.122 nm correspond to the (112), (100), and (104) plane of CoSe.
Fig. S2c corresponds to the TEM image of CoSe/rGO. It can be seen that the CoSe
nanorods grown on the rGO sheets. In Fig. S2d, the lattice fringes of 0.181 nm, 0.156
nm, and 0.264 nm correspond to the (110), (200), and (002) crystal planes of CoSe,
respectively. The lattice fringes of 0.34 nm correspond to the interlayer distance of
rGO. This result indicates that the CoSe/rGO composite was successfully prepared.
The pore size distribution and specific surface area of the material can be
quantitatively analyzed by evaluating nitrogen adsorption-desorption isotherms (Fig.

S3). The Brunauer–Emmett–Teller (BET) specific surface areas of the CoSe and
CoSe/rGO was 33.51 m2 g-1 and 40.21 m2 g-1, respectively. CoSe/rGO has a larger
specific surface area, which facilitates the exposure of more catalytic sites. It can be
seen from the pore size distribution map that the mesopores of the material are mainly
distributed below 15 nm.
The CoSe/rGO composites were tested for carbon content by TG analysis, as
shown in Fig. S4a. The slight weight lost below 150 °C is the evaporation of water.
The mass lost between 200 and 600 °C is the oxidation of rGO and CoSe material.
Based on the chemical reaction of 3CoSe (s) + 5O2 (g) = Co3O4 (s) + 3SeO2 (g)
[25-28], the contents of the CoSe and rGO were calculated to be 58.3 wt.% and 41.7
wt.%, respectively. The Raman spectrum also confirmed the existence of rGO in the
CoSe/rGO composite. As presented in Fig. S4b, there are two peaks at the 1344 cm-1
and the 1596 cm-1, corresponding to the D-band and G-band of rGO. ID/IG ratio value
is 1.05, implying a higher degree of defects, which is favorable to ion
intercalation/deintercalation and improve the electrochemical properties [29].
Fig. 2. (a) the cyclic performance of CoSe at the current density of 100 mA g-1 and (b)
corresponding discharge/charge curves.

The electrochemical performance of CoSe is shown in Fig. 2. Fig. 2a is the cycle
performance of CoSe at the current density of 0.1 A g-1. It can be seen that the initial
discharging specific capacity is 606.4 mA h g-1 and coulomb efficiency is 82.2%.
Before the 30th cycle, the specific capacity is gradually reduced. Between the 30th
and 120th cycles, the specific capacity is maintained at about 150 mA h g-1. After
120th cycles, the specific capacity gradually rose. After 500th cycles, it was about 400
mA h g-1. The galvanostatic charge/discharge curves are shown in Fig. 2b. It can be
seen that the reaction platforms for intercalation/deintercalation of lithium ion are
clear in the initial three cycles. However, the curve of the 45th cycle shows almost no
obvious platform. Interestingly, in the curves of the 200th and 300th cycles, similar
reaction platforms can be seen clearly. The CoSe exhibits strange cycle performance
and charge/discharge curves. To understand the intercalation/de-intercalation
mechanism of lithium ion, we selected samples of the 45th and 200th cycles for
further analysis.
Fig. 3. TEM and HRTEM images of CoSe after (a), (b) 45th cycles and (c), (d) 200th
cycles.

The TEM image of CoSe after the 45th cycles in the full lithiation state is shown in
Fig. 3a. It can be seen that the CoSe material is a sheet-like morphology. Fig. 3b
displays a high-resolution TEM image of CoSe after the 45th cycles, and there are no
clear crystal regions. Fig. 3c and d show the structure of CoSe after the 200th cycles
in the full lithiation state. In Fig. 3c, it can be found that small nanoparticles
distributed on the sheet-like materials. Fig. 3d exhibit the lattice fringe of 0.372 nm,
corresponding to the (100) plane of Se, which matches with the XRD results (Fig. 4b).
The magnified images of Fig. 3b and d are shown in Fig. S5.
Fig. 4. XRD patterns of CoSe after (a) 45th and (b) 200th cycles.
Fig. 4a shows the XRD patterns of the CoSe material fully discharged and fully
charged after 45th cycles. It can be seen that the CoSe, whether fully charged or
discharged, has no diffraction peaks and is amorphous. Fig. 4b is the XRD patterns of
CoSe material after 200th cycles, whether fully charged or discharged, the spectra
have the same diffraction peaks, corresponding to CoSe and two different structures
of Se. When fully discharged, the CoSe and Se were not completely involved in the
intercalation reaction of lithium ion. Moreover, the diffraction peaks of Li2Se and Co

were not detected after the 45th and 120th cycles, indicating that the product formed
is amorphous during the intercalation of lithium ion.
Fig. 5. XPS spectra of the CoSe samples after 45th cycles. (a) XPS survey spectrum
of fully discharged state, (b) XPS survey spectrum of fully charged state, (c)
high-resolution Co 2p XPS spectrum of fully discharged state, (d) high-resolution Co
2p XPS spectrum of fully charged, (e) high-resolution Se 3d XPS spectrum of fully
discharged state, and (f) high-resolution Se 3d XPS spectrum of fully charged.

Fig. 5a and b show the XPS survey spectra of the CoSe material in the fully
discharged and fully charged state after 45th cycles, respectively. It can be seen that
the CoSe material surface contains Se, Co, and O elements in the fully discharged and
fully charged state, where O may be derived from adsorption of oxygen or oxidation
of the surface of the material. The sharp and strong peaks at about 690 eV can be
ascribed to the F element due to the residual electrolyte in the electrode. The
high-resolution Co 2p XPS spectrum of the CoSe samples in the fully discharged state
after 45th cycles is shown in Fig. 5c. The two peaks located at 779.0 and 796.3 eV can
be assigned to Co0 2p3/2 and Co0 2p1/2, respectively. It can be seen that the content of
metallic Co is very small. Two peaks appeared at 781.1 and 797.4 eV, corresponding
to Co2+ 2p3/2 and Co2+ 2p1/2 of CoSe, respectively. In addition, the peaks at 782.7 and
800 eV can be ascribed to Co2+ 2p3/2 and Co2+ 2p1/2 of CoO [10,30,31]. Meanwhile,
the other peaks are satellite peaks [32]. Fig. 5d shows the high-resolution Co 2p
spectrum of the CoSe samples in the fully charged state after 45th cycles. The binding
energies of Co 2p3/2 and Co 2p1/2 of CoSe were identified at 781.2 and 796.7 eV,
respectively. The two peaks at 782.7 and 798.4 eV can be ascribed to Co2+ 2p3/2 and
Co2+ 2p1/2 of CoO. The other peaks are satellite peaks. Compared to the fully
discharged state, there are no binding energies of Co0 2p3/2 and Co0 2p1/2. Fig. 5e and f
show high-resolution XPS spectra of Se 3d in the fully discharged state and fully
charged state after 45th cycles. It can be seen that except for the peaks of 3d5/2 and
3d3/2 of Se2- [31,33], the binding energy located at 59.2 eV can be ascribed to the

partial surface oxidation of Se. The peaks that appeared at 60.8 eV can be assigned to
the binding energy of Co 3p [20, 32].
Fig. S6a and b show the XPS survey spectra of the CoSe material in the fully
discharged and fully charged state after 200th cycles, respectively. It can be seen that
after 200th cycles, only Se was detected on the surface of the material, and Co was
not detected. Similarly, Co was hardly detected in the high-resolution Co 2p spectra in
Fig. S6c and d. Interestingly, Se can be detected in the high-resolution Se 3d XPS
spectra in Fig. S6e and f. This phenomenon may be due to after 200th cycles, the
formed Se dissolved and became a component of the solid electrolyte interface (SEI)
film [34-35]. As the number of charge/discharge cycles increases, a thicker SEI film
was formed. XPS can only detect elements on the surface of the material. So after
200th cycles, only Se was detected, and Co was not detected.
Fig. 6. CV curves of CoSe at the scan rate of 0.1 mV s-1. (a) initial three cycles,
(b) after 45th cycles, and (c) after 200th cycles.
Fig. 6 is the CV curves of CoSe at different charge/discharge cycles. According to
reports, the lithium storage mechanism of CoSe is a multi-step reaction process
[36-37]. Fig. 6a shows the CV curves of the first three cycles of the CoSe at the scan

rate of 0.1 mV s-1 and voltage range from 0.01 to 3 V vs. Li+/Li. During the first
cathodic scan, the peak at 2.15 V can be ascribed to the lithium insertion, and the peak
at 1.26 V corresponds to the sequential lithium insertion of CoSe to LixCoSe and the
formation of SEI film, and the peak at 0.75 V can be attributed to the formation of
metallic Co and Li2Se [36,38,13,28]. In the anodic scan, the peak at 2.15 V
corresponds to the delithiation process [34,37]. In the subsequent 2nd and 3rd cycle’s
curves, the two cathodic peaks at 1.53 and 1.38 V correspond to the formation of
LixCoSe. Meanwhile, the cathodic peak at 0.75 V also ascribes to the formation of
Li2Se and Co [39]. The anodic peaks from 2.15 V to 2.18 V could be attributed to the
delithiation processes, similar to the previously reported results [28,37]. Compared
with the CV curves of the first three cycles, the CV curves after 45th cycles in Fig. 6b
can be observed only one cathodic peak, which moved to 0.33 V, indicating that
lithium intercalation behavior became more difficult. In combination with the XRD
analysis in Fig. 4a, it was found that CoSe formed an amorphous structure during the
45th charge/discharge cycles.
Fig. 6c is the CV curves of CoSe after 200th cycles, significantly different from the
CV curves of the first three cycles and after the 45th cycles. According to the XRD
analysis of Fig. 4b, the electrode material not only contained CoSe, but also generated
Se after 200th cycles. The cathodic peak of 2.07 V corresponds to the insertion of Li+
into Se, and the two cathodic peaks at 1.74 V and 1.41 V correspond to the insertion
of Li+ into CoSe to form LixCoSe [36-37]. The cathodic peak at 0.59 V can be
attributed to the insertion of Li+ into LixCoSe, which is converted into Co and Li2Se
[28,36]. In the anodic scan, the peak at 1.31 V and 2.27 V are attributed to the gradual
delithiation processes and reversibly forming CoSe and Se [36-37,39-40].

Based on the above analysis and relevant literature, the reaction mechanism of the
CoSe during the entire charge/discharge cycles can be described as follows:
CoSe + xLi+ + xe-
LixCoSe + (2-x) Li+ + (2-x) e-
Se +2Li+ + 2e-
LixCoSe (1)
Co + Li2Se (2)
Li2Se (3)
As shown in Fig. 2a, the cycle performance of CoSe shows a “U” shaped curve.
During the first 45th cycles, the structure of the material gradually changed and
produced an amorphous material. The cathodic peak in the CV curve shifted to a
lower potential, indicating that lithium intercalation behavior became more difficult,
resulting in capacity degradation. However, after 200th cycles, the material gradually
changed from amorphous to crystalline structure and the capacity gradually increased.
On the other hand, Se is partially dissolved and became a component of the SEI film.
The formed conductive SEI film can facilitate charge transfer [34-35]. Also, the
reversible growth of the SEI film during charge and discharge can increase the
capacity [41-42].
Fig. S7 shows the Nyquist plots of CoSe electrodes after 45th and 200th
charge/discharge cycles. It can be found that after the 45th cycles, the diameter of the
semicircle in the high frequency range is larger than that of after the 200th cycles. The
result indicates that after 200th cycles, the electrode material has better electron
conductivity and lower charge transfer resistance. It further illustrates that
nanocrystals and conductive SEI films' formation make lithium's insertion/extraction
easier.

Fig. 7. (a) CV curves of CoSe/rGO at the scan rate of 0.2 mV s-1, (b) initial three
discharge/charge curves of CoSe/rGO at the current density of 100 mA g-1, (c) the
cycle performance of CoSe/rGO at the current density of 100 mA g-1, (d)
rate-performance of CoSe/rGO at the current densities ranging from 100 mA g-1 to
1000 mA g-1, and (e) long-term cycle performance of CoSe/rGO at the current density
of 1000 mA g-1.

In Fig. 7a, it is easy to find that the CV curves of CoSe/rGO are similar to that of
the pure CoSe material. Fig. 7b is the charge/discharge curves of the first three cycles
of CoSe/rGO. It can be seen that due to the formation of the SEI film in the first
charge/discharge process, the coulombic efficiency is low. The second and third
charge/discharge curves are basically coincident, and the coulombic efficiency is
higher. Compared with pure CoSe, the addition of rGO can effectively improve the
material's coulombic efficiency and capacity. Fig. 7c shows the cycle performance of
the CoSe/rGO composite at a current density of 100 mA g-1. For the first cycle, the
coulombic efficiency was 75%. After that, the coulombic efficiency increased rapidly.
After the 200th cycles, the specific capacity of CoSe/rGO was as high as 730 mA h
g-1, showing good cycling performance. Compared with CoSe, CoSe/rGO has a higher
specific capacity, and the specific capacity of CoSe/rGO decreases and increases to a
lesser extent during the cycles. It is maybe because the addition of rGO caused CoSe
to change from large flakes to small rods, which weakens the effect of volume change
during lithium ion intercalation and shortens the lithium ion diffusion distance, so
improves the reduction of specific capacity. Besides, the good conductivity of rGO
can also promote the cycling performance of the material.
The rate performance of CoSe/rGO at current densities from 100 mA g-1 to 1000
mA g-1 is shown in Fig. 7d. When the current density gradually increased (100, 200,
300, 500, and 1000 mA g-1), the delivered charge specific capacities were 703, 658,
631, 553, and 459 mA h g-1, respectively. More importantly, the specific capacity
reached 695 mA h g-1 when the current density was reduced back to 100 mA g-1. This

result demonstrates that CoSe/rGO composite has good rate performance. The
long-term cycling performance of CoSe/rGO was tested at a current density of 1000
mA g-1 (Fig. 7e). After 1000 cycles, the specific capacity was up to 570 mA h g-1. It
indicates that the CoSe/rGO composite has good cycle stability under large current
density.
Fig. 8. (a) CV curves of CoSe/rGO at different scan rates from 0.2 to 1.0 mV s-1, (b)
the linear relation of peak currents and the square root of scan rate, (c) the linear
relation of log (i, peak current) and log (v, scan rate), (d) plots of ν1/2 vs. i/ν1/2 at
different redox states for obtaining constants of k1 and k2, (e) capacitive contribution
(red) and diffusion controlled contribution (white) at the scan rate of 0.8 mV s-1, (f)
contribution ratios of capacitive (red) and diffusion controlled (white) contribution at
different sweep rates.

To further study this excellent electrochemical performance of the CoSe/rGO
composite, a series of CV tests were carried out at various scan rates. Fig. 8a shows
the typical CV curves for CoSe/rGO electrode at the scan rates from 0.2 to 1.0 mV s-1.
It can be seen that CoSe/rGO composite has a slight polarization with the increase of
scan rates. Fig. 8b shows the relation of the peak currents (Ip, mA) of CV curves and
the scan rates' square root (v1/2, mV1/2 s-1/2). According to previous reports, the
relationship between the peak currents and scan rates is as follows [43-45]:
i = avb (4)
logi = blogv + loga (5)
Where a and b are both variable parameters. The value of b determined by the slope of
logi vs. logv in Eq. (5). If the value of b is near 0.5, the diffusion-controlled behavior,
that is, cation intercalation, is prevailing during the charge/discharge process.
Whereas the value of b close to 1 indicates that the capacitive behavior is dominant
[46-47]. In this work, the relationship between logi and logv is presented in Fig. 8c. It
is found that the b values of the four anodic peaks (Peaks 1, 2, 3, and 4) and the
cathodic peaks (Peaks 5, 6, and 7) are 0.79, 0.83, 0.82, 0.85, 0.79, 0.70, and 0.87,
respectively. It implies that the redox kinetic process of CoSe/rGO composite contains
diffusion-controlled and capacitive behaviors.
In addition, the respective contribution of the capacitive and the
diffusion-controlled behaviors at a specific scan rate can be quantitatively
distinguished at a specific voltage by Eq. (6):

i(V) = k1v+ k2v1/2 (6)
Here k1 can be obtained by the slope of i(V)/v1/2 vs. v1/2 plots. In Eq. (6), k1v and k2v1/2
represent the capacitive contribution and diffusion-controlled contribution. Based on
the above discussion, as shown in Fig. 8d, the fitted line slope is k1, and the intercept
is k2. Fig. 8e displays the capacitive contribution (red) and diffusion controlled
contribution (white) at the scan rate of 0.8 mV s-1. The diffusion-controlled
contribution is mainly generated at around the redox peak in both lithiation and
delithiation processes. Likewise, the contribution ratios of the capacitive and diffusion
controlled behaviors at different scan rates were also quantified (Fig. 8f). The results
show that the capacitive contribution increases with the increase of the scan rates.
4. Conclusions
In this work, the CoSe and CoSe/rGO composite were prepared by a simple
hydrothermal method, and CoSe/rGO exhibited excellent electrochemical
performance. In particular, by studying the lithium storage mechanism of the CoSe
material, it is found that the material underwent phase transformation during the
charge and discharge cycles. After the 45th charge/discharge cycles, CoSe formed an
amorphous material. The cathodic peak in the CV curve shifted to a lower potential,
indicating that lithium intercalation behavior becomes more difficult, so the specific
capacity is reduced. In the subsequent electrochemical cycles, the amorphous
materials evolved into CoSe and Se nanocrystals, which increases the specific
capacity. Besides, Se is partially dissolved and became a component of the SEI film.

The conductive SEI film can promote the charge transfer, and the reversible growth of
SEI film during charge and discharge can increase the capacity. To increase the
stability of lithium storage, we prepared CoSe/rGO composite. The addition of rGO
caused CoSe to change from larger nanosheets to smaller nanorods, which weakens
the effect of volume change during lithium ion intercalation and shortens the lithium
ion diffusion distance, so improves the reduction of specific capacity. The good
electrical conductivity of rGO also promoted the lithium storage performance of
CoSe/rGO composite. After 200th cycles at the current density of 100 mA g-1, the
CoSe/rGO composite's specific capacity is 730 mA h g-1. Even under a large current
density of 1000 mA g-1, after 1000 cycles, the specific capacity of the CoSe/rGO
composite can still reach 570 mA h g-1.
Declaration of Competing interest
The authors declare no competing interests.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(21865032), the Innovation Team Basic Scientific Research Project of Gansu
Province (1606RJIA324), the Science and Technology program of Gansu Province
(19JR2RA020).

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