Journal of The Electrochemical Society, 153 共12兲 A2262-A2268 共2006兲
A2262
0013-4651/2006/153共12兲/A2262/7/$20.00 © The Electrochemical Society
Fabrication and Electrochemical Characterization of Copper
Selenide Thin Films by Pulsed Laser Deposition
Ming-Zhe Xue,a Yong-Ning Zhou,b Bin Zhang,a Le Yu,a Hua Zhang,b and
Zheng-Wen Fua,*,z
a
Department of Chemistry and Laser Chemistry Institute, Shanghai Key Laboratory of Molecular Catalysts
and Innovative Materials, and bDepartment of Material Science, Fudan University, Shanghai, 200433
China
Copper selenide, Cu2Se, CuSe, and CuSe2, thin films have been successfully fabricated using pulsed laser ablation of mixed targets
of Cu and Se. The substrate temperature was a key factor in preparing high-quality thin films of the copper selenides, with
different crystalline structures and stoichiometries. Although these three kinds of compounds 共Cu2Se, CuSe, and CuSe2兲 all form
thin films with the same anion and cation in their chemical formula, their electrochemical properties clearly differ from each other,
evidently associated with their various compositions and structures. The galvanostatic cycling measurement indicates that a
Cu2Se/Li cell has the best cycling performance when compared to the other two possible cells, CuSe2 /Li and CuSe/Li. The
conversion reaction mechanism between Cu2+ 共CuSe and CuSe2兲 and Cu+ 共Cu2Se兲 are proposed for CuSe2 /Li and CuSe/Li cell,
while the Cu2Se/Li cell may undergo a “displacement” reaction mechanism according to X-ray diffraction, transmission electron
microscopy, and selected area electron diffraction data. These complex and unique mechanisms make copper selenides interesting
materials for rechargeable lithium batteries.
© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2358854兴 All rights reserved.
Manuscript submitted May 24, 2006; revised manuscript received August 7, 2006. Available electronically October 17, 2006.
The electrochemical properties of various 3d metal compounds
have been widely investigated as anode materials for Li ion
batteries.1-6 The commonly used 3d metals are Fe, Co, Ni, Cu, and
Zn. Among them, copper, a coinage element, is one of the most
important metals. It cannot alloy with lithium like zinc and is
monovalent in many compounds, which is different from iron, cobalt, and nickel. Much attention has been paid to copper-based
transition-metal compounds such as Cu3N, Cu2O, CuO, Li2CuP,
Cu3P, and CuP2.4,7-17 Grugeon et al.7 reported on lithium electrochemical activity with Cu2O and found that Cu nanograins dispersed
into a lithia 共Li2O兲 matrix formed during the initial discharge, which
could enhance their electrochemical activity toward the formation/
decomposition of Li2O. Pereira et al.4 examined the electrochemistry of Cu3N with lithium and showed a similar reversible lithium/
copper nitride conversion process driven by Cu nanoparticles. In
addition, they found that oxidation of Cu metal into Cu2+ formed
copper oxide, and they believed it to be associated with electrolyte
degradation. Crosnier et al.14 studied the lithium electrochemical
process of Cu3P, and their results were obviously different from
those for metal oxides and nitrides. In their results, the electrochemical reaction of Cu3P with lithium occurs via a multistep process that
leads to the formation of metallic copper. An almost fully reversible
topotatic “displacement” reaction occurs between the three intermediate phase, Cu3P, Li2CuP, and Li3P. Bichat et al.’s work16 confirmed this mechanism and they further showed that not only
Li2CuP but also LiCu2P is an intermediate product in the successive
biphasing processes. Wang et al.17 reported that there are no detectable ternary lithium copper phosphides in the discharge process of
CuP2 /Li cell. Apparently, the lithium electrochemical reaction with
transition compounds based on metallic copper is still complicated,
and more work must be done to understand the nature of electrochemical reactions of transition compounds based on copper with
lithium.
Copper selenide is an interesting metal chalcogenide semiconductor material. It exists in a wide range of stoichiometric compositions such as copper 共I兲 selenide 共Cu2Se兲, copper 共II兲 selenide
共CuSe兲, or copper diselenide 共CuSe2兲 with various crystallographic
forms. It has a number of applications in solar cells, super ionic
conductors, and photo-detectors.18-20 However, there is no report on
the electrochemical properties of copper selenide.
* Electrochemical Society Active Member.
z
E-mail: [email protected]
There are many techniques for the fabrication of copper selenide
thin films currently in use such as vacuum evaporation,19 chemical
bath deposition,21 flash evaporation,22 seleniation,23 and solid state
reaction.24 Here we report the first demonstration of the fabrication
of copper selenide films, with different crystal structures and stoichiometries, by pulsed laser deposition 共PLD兲, which is a simple and
effective method for depositing thin-film electrodes and has the ability to control chemical composition in the deposition of thin films.
The electrochemical behavior, structure, composition, and morphology of these thin films were characterized by galvanostatic cycling,
X-ray diffraction 共XRD兲, scanning electron microscopy 共SEM兲,
transmission electron microscopy 共TEM兲, and selected area electron
diffraction 共SAED兲. The motivation of this work is to elucidate the
relationship of lithium electrochemical reaction mechanisms of copper selenide thin films with their compositions and structures.
Experimental
The apparatus used for pulsed laser deposition has been described elsewhere.25 Experimental conditions for depositing thin
films are described briefly as follows. A 355 nm laser beam, provided by the third harmonic frequency of a Q-switched Nd:yttrium
aluminum garnet 共YAG兲 laser 共Quanta-Ray GCR-150兲, was focused
onto the surface of the target. The incident angle between the laser
beam and the target surface normal was 45°. The laser energy intensity was about 2 J/cm2. The repetition rate and pulse width of the
laser were 10 Hz and 10 ns, respectively. The targets were made
from Cu and Se powders 共both pure 99.9%兲, they were mixed and
ground in certain elemental molar ratios of Cu:Se = 1:1.2 or 1:5.0,
then were pressed to form a 1.3 cm diam pellet as the ablated target.
An excess of Se in a mixture target can compensate for Se loss due
to its vacuum sublimation during laser ablation. The base pressure of
the chamber was 10−2 Pa, and the ambient Ar gas pressure during
deposition was kept at 5 Pa by a needle valve. The thin films were
deposited on stainless steel substrates, which were kept at 200° or
400°C. The distance between target and substrate was 4 cm.
XRD patterns and the thin film electrodes morphologies were
recorded by a Bruker D8 advance diffractormeter equipped with
Cu-K␣ radiation 共␭ = 1.5406 Å兲 and a SEM 共Philips XL30 microscope兲, respectively. TEM and SAED measurements were carried
out in a 200 kV side entry JEOL 2010 TEM with an energy dispersive X-ray 共EDX兲 analyzers. The weight of the thin film was examined with an electrobalance 共BP 211D, Sartorius兲.
For the electrochemical measurements, the cells were constructed using the as-deposited thin films as a working electrode and
a lithium sheet as a counter electrode. The electrolyte consisted of
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Journal of The Electrochemical Society, 153 共12兲 A2262-A2268 共2006兲
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Figure 1. XRD patterns of the target 共a兲 and as-deposited thin films at
different substrate temperature of 共b兲 200°C with target ratio Cu:Se
= 1:5.0, 共c兲 200°C with target ratio Cu:Se = 1:1.2, 共d兲 400°C with target
ratio Cu:Se = 1:1.2. The peaks marked with asterisk corresponding to stainless steel substrate.
1 M LiPF6 in a nonaqueous solution of ethylene carbonate 共EC兲 and
dimethyl carbonate 共DMC兲 with a volume ratio of 1:1 共Merck兲. The
cells were assembled in an Ar filled glove box. Galvanostatic
charge-discharge measurements were carried out at room temperature with a Land CT 2001A battery test system. The cells were
cycled between 1.0 and 2.5 V vs Li+ /Li at a current density of
5 ␮A/cm2.
To gain insight into the reaction mechanism of copper selenide
with lithium, ex situ XRD, TEM, and SEAD measurements were
collected on the copper selenide thin film electrodes at selected voltage points during the initial discharge and charge process. The
model cells were dismantled in an Ar-filled glove box and the electrodes were rinsed in anhydrous, dimethyl carbonate 共DMC兲 to
eliminate residual salts. For TEM and SAED measurements, the
active materials were scratched from the stainless steel substrate.
The loosened powders were then mixed with ethanol to prepare a
slurry, out of which one drop was taken, and deposited on a copper
grid. To avoid exposure to oxygen or water, the thin films or copper
grids were rapidly transferred into the chambers for cleanliness.
Results
Figure 1 shows the typical XRD patterns of the as-deposited thin
films on stainless steel substrate by pulsed laser deposition. The
XRD pattern of the target is used for comparison 共Fig. 1a兲, in which
the diffraction peaks at 2␪ = 43.3° and 50.4° can be assigned to the
共111兲 and 共200兲 reflection of Cu 共JCPDS card no. 04-0836兲 while the
peaks at 2␪ = 23.4°, 29.7°, and 43.5° can be assigned to the 共100兲,
共101兲, and 共012兲 reflection of Se 共JCPDS card no. 86-2246兲. When
the substrate temperature was 200°C, cubic CuSe2 thin films were
formed using the target element ratio of Cu:Se = 1:5.0. XRD of
those deposits are represented by the pattern in Fig. 1b, in which the
only peak, at 2␪ = 29.1°, can be assigned to the 共200兲 reflection of
cubic structure of CuSe2 共JCPDS card no. 26-1115兲, indicating an
epitaxial CuSe2 thin film. When using a target ratio of Cu:Se
= 1:1.2 instead of 1:5.0, CuSe with a hexagonal structure was prepared with a substrate temperature of 200°C. Two peaks, at 2␪
= 28.0° and 31.1°, were assigned to the 共102兲 and 共006兲 reflection of
CuSe 共JCPDS card no. 34-0171兲 共Fig. 1c兲. When the substrate temperature was elevated to 400°C, another copper selenide, Cu2Se,
with a cubic structure, was observed. Two peaks at 2␪ = 26.7° and
44.4° were assigned to the 共111兲 and 共220兲 reflection of the textured
structure of Cu2Se 共JCPDS card no. 65-2982兲, indicating a well
crystallized Cu2Se thin film 共Fig. 1d兲. Thus, three copper selenide
Figure 2. SEMs of the as-deposited thin films 共a兲 CuSe2, 共b兲 CuSe, and 共c兲
Cu2Se.
thin films 共Cu2Se, CuSe, and CuSe2兲 were controllably prepared by
optimizing the substrate temperature and target element ratios using
pulsed laser deposition.
Figure 2 shows typical SEMs of the as-deposited CuSe2, CuSe,
and Cu2Se thin films, prepared under the conditions mentioned
above. The surface of CuSe2 thin film was composed of large agglomerated particles with a petal-like appearance 共Fig. 2a兲 while
club like particles of CuSe thin film were observed 共Fig. 2b兲. When
the substrate temperature was elevated to 400°C the morphology of
the thin film changed 共Fig. 2c兲. Some spherical particles were dis-
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Journal of The Electrochemical Society, 153 共12兲 A2262-A2268 共2006兲
Figure 4. Cycle performances of CuSe2 /Li, CuSe/Li, and Cu2Se/Li cells
cycled between 1.0 and 2.5 V.
Figure 3. Galvanostatic curves of the as-deposited thin films 共a兲 CuSe2, 共b兲
CuSe, 共c兲 Cu2Se, and 共d兲 the discharge capacities of the as-deposited thin
films as a function of cycle number. All cells are cycled between 1.0–2.5 V.
tributed randomly on the surface, with an average size of 150 nm.
Apparently, the as-deposited CuSe2, CuSe, and Cu2Se thin films
exhibit utterly different surface morphologies.
The galvanostatic curves of the CuSe2 /Li, CuSe/Li, and
Cu2Se/Li cells cycled between 1.0 V–2.5 V are shown in Fig. 3a-c,
respectively. For CuSe2 /Li cell, the open-circuit voltage 共OCV兲 lies
close to 2.4 V. The first lithium insertion into CuSe2 is characterized
by two flat voltage plateaus at 2.0 and 1.55 V and provides an initial
capacity of 471.2 mAh/g. The second Li-insertion process delivers a
reversible capacity of 201.7 mAh/g, indicating a large irreversible
capacity loss. Similarly, CuSe/Li cell has the OCV of 2.6 V, there
are also two flat voltage plateaus at 2.0 and 1.55 V in the initial
discharge process. The only difference of the first discharge process
between CuSe2 /Li and CuSe/Li cells are in the length of these two
plateaus. In CuSe2 /Li cell, the first 共and the higher兲 plateau is longer
than the second 共and the lower兲 while in the CuSe/Li cell, they are
almost equal. The initial and the second capacities of CuSe/Li cell
are 367.8 and 258.4 mAh/g, respectively. Interestingly, the electrochemical behavior of Cu2Se/Li cell is utterly different from those of
CuSe2 /Li and CuSe/Li cells. Its OCV is 2.4 V vs Li+ /Li. There are
two sloping voltage plateaus from 1.7 to 1.6 V and 1.5 to 1.4 V in
the initial discharge process. The capacity of the first discharge is
found to be 243.8 mAh/g. The second discharge process yields a
reversible capacity of 210.2 mAh/g. The specific capacities of
CuSe2, CuSe, and Cu2Se thin films obtained as a function of the
cycle number are presented in Fig. 4. It shows that the discharge
capacity of CuSe2 /Li and CuSe/Li cells fade fast in the initial several cycles, only 45% and 30% of the initial capacity are reserved
after 10 cycles. On the contrary, Cu2Se/Li cell shows a good cycle
performance. Although it has an initial capacity loss of about 14%,
the subsequent cycles show stability and 73% of the initial discharge
capacity 共177.7 mAh/g兲 is reserved after 100 cycles.
Note that the first discharge curves for both the CuSe2 /Li and the
CuSe/Li cells have two flat plateaus and their cycle performances
are poor. To avoid the second plateau 共the lower one兲, we tried to
cycle both cells in a limited voltage ranges of 1.8–2.5 V. Figure 5a
and b show the galvanostatic curves of the CuSe2 /Li and CuSe/Li
cells cycled between 1.8 and 2.5 V, respectively. It is remarkable to
see that although the CuSe2 /Li cell has a larger capacity loss between the first two cycles, it cycles well in the subsequent cycles.
After 100 cycles, the reserved discharge capacity is 196.6 mAh/g,
even higher than the specific capacity of CuSe2 /Li cell cycled between 1.0–2.5 V 共133.0 mAh/g兲. For CuSe/Li cell, the initial capacity loss was about 12%. After 100 cycles, the reserved discharge
capacity was 118.9 mAh/g, 67% of the first cycle.
To reveal the electrochemical reaction features of copper selenide thin films with lithium, ex situ XRD, TEM, and SAED measurements were performed on CuSe2, CuSe, and Cu2Se thin-film
electrodes arrested at different stages during the first cycle. For comparison, XRD pattern of the as-deposited thin films are also included.
Figure 6 shows the ex situ XRD patterns of CuSe2 thin film
electrodes: the as-deposited, that after discharging to 1.8 V and that
after charging to 2.5 V, respectively. When the thin film was discharged to 1.8 V, the original CuSe2 peak disappeared 共Fig. 6b兲. Six
peaks at 25.0°, 27.7°, 28.7°, 31.3°, 44.5°, and 45.2° were assigned to
the 共101兲, 共200兲, 共111兲, 共210兲, 共310兲, and 共221兲 reflection of Cu3Se2
共JCPDS card no. 47-1745兲 while the strongest peak at 26.7° was
assigned to the 共111兲 reflection of Cu2Se 共JCPDS card no. 65-2982兲.
This means that decomposition of CuSe2 results in the formation of
other two other kinds of copper-rich selenides: Cu3Se2 and Cu2Se.
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Journal of The Electrochemical Society, 153 共12兲 A2262-A2268 共2006兲
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Figure 7. Ex situ XRD pattern of CuSe thin film at various states during the
first cycle of CuSe/Li cell with a limited voltage range of 1.8–2.5 V. 共a兲
As-deposited, 共b兲 discharging to 1.8 V, and 共c兲 charging to 2.5 V.
Figure 5. Galvanostatic curves of the as-deposited thin films 共a兲 CuSe2, 共b兲
CuSe, and 共c兲 the discharge capacities of the as-deposited thin films as a
function of cycle number. All cells are cycled between 1.8–2.5 V.
After charging to 2.5 V, the CuSe2 共200兲 diffraction peak appeared
again, indicating reconstruction of crystalline CuSe2 under these
conditions 共Fig. 6c兲.
Figure 7 shows XRD patterns for CuSe thin film electrodes in the
as-deposited state, when discharging to 1.8 V and after charging to
2.5 V, respectively. Although CuSe2 and CuSe have different compositions and structures, both have similar discharge plateaus at
2.0 V and the same discharge products 共Fig. 7b兲. Five peaks at
25.0°, 27.8°, 28.6°, 44.5°, and 45.2° were assigned to the 共101兲,
Figure 6. Ex situ XRD pattern of CuSe2 thin film at various states during the
first cycle of CuSe2 /Li cell with a limited voltage range of 1.8–2.5 V. 共a兲
As-deposited, 共b兲 discharging to 1.8 V, and 共c兲 charging to 2.5 V.
共200兲, 共111兲, 共310兲, and 共221兲 reflection of Cu3Se2 共JCPDS card no.
47-1745兲 while one peak at 27.0° was assigned to the 共111兲 reflection of Cu2Se 共JCPDS card no. 88-2043兲. During the charging process, diffraction peaks for Cu3Se2 and Cu2Se disappeared while four
peaks at 26.4°, 27.9°, 30.8°, and 45.8° were assigned to the 共101兲,
共102兲, 共006兲, and 共110兲 diffraction of CuSe 共JCPDS card no. 340171兲 共Fig. 7c兲. However, the diffraction peak shapes of reconstructed CuSe thin film become wider than the as-deposited, indicating smaller crystallite size.
Figure 8 shows the ex situ XRD patterns of Cu2Se thin film
electrodes for the as-deposited, discharged to 1.0 V, and charged to
2.5 V. XRD pattern 共Fig. 8b兲 shows that the diffraction peaks of
Cu2Se disappear after discharging to 1.0 V. There are no detectable
diffraction peaks from Cu or Li2Se. This may be due to the sizes of
the Cu and Li2Se nanoparticles formed, less than the X-ray coherence length 共6 nm兲, which could not be identified by XRD. This
indicates the complete structural collapse of the copper selenide and
the formation of nanosized Cu metal in the Li–Se matrix which was
confirmed by SAED data 共Fig. 9兲. Upon the charging process,
lithium was gradually extracted from the Li2Se matrix, with the
simultaneous selenidation of copper. After charging to 2.5 V,
lithium was fully displaced by copper to yield the original Cu2Se
Figure 8. Ex situ XRD pattern of Cu2Se thin film at various states during the
first cycle of Cu2Se/Li cell with a voltage range of 1.0–2.5 V. 共a兲 Asdeposited, 共b兲 discharging to 1.6 V, 共c兲 discharging to 1.0 V, 共d兲 charging to
2.0 V, and 共e兲 charging to 2.5 V.
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Journal of The Electrochemical Society, 153 共12兲 A2262-A2268 共2006兲
Table I. d-spacings (Å) derived from SAED analysis of the discharging to 1.0 V of Cu2Se thin-film electrode. JCPDS standards
for Li2Se and Cu are given for reference purposes.
Discharge to 1.0 V
JCPDS Standard
Li2Se-Fm 3m
3.01
1.07
a = 6.04
3.01 共200兲
1.06 共440兲
a = 6.02
JCPDS Standard
Cu-Fm 3m
1.83
1.29
a = 3.65
1.81 共200兲
1.28 共220兲
a = 3.62
of copper selenides,26 there are several kinds of copper selenides in
the binary phase diagram of Cu–Se, that are air-stable. When the
deposition temperature was elevated to 400°C, only Cu2Se was
formed because both CuSe and CuSe2 were unstable above 377°C.
When the deposition temperature was 200°C, it was prone to form
CuSe2, with a large selenium excess, using the laser ablated target
with a Cu/Se ratio of 1/5. Otherwise, it was prone to form CuSe
with the target Cu/Se ratio of 1/1.2, with volatilization of residual
selenium, due to its high vapor-pressure 共9.8 Pa兲 at this temperature.
Although CuSe2, CuSe, and Cu2Se all can react with lithium
electrochemically, their electrochemical behaviors differ. From XRD
and SAED data, we could deduce the electrochemical reaction
mechanisms of CuSe2, CuSe, and Cu2Se with lithium 共CuSe2 /Li and
CuSe/Li cells were cycled over a limited voltage range, 1.8–2.5 V,
while the Cu2Se/Li cell was cycled between 1.0–2.5 V兲
Figure 9. 共a兲 TEM and 共b兲 corresponding SAED pattern of the Cu2Se thin
film discharged to 1.0 V.
phase. This behavior was confirmed by the XRD pattern, wherein
the peak at 2␪ = 26.5° was characteristic of a Cu2Se phase 共Fig. 8c兲.
The crystallinity of Cu2Se was poor. The slight 2␪ shift at small
angles in peak positions between the as-deposited Cu2Se thin film
and the electrochemically formed Cu2Se may be due to the formation of nonstoichiometric copper selenide particles, just as with the
results from Cu2O, reported by Grugeon et al.7
TEM bright-field images and SAED patterns collected for the
fully reduced 共discharging to 1.0 V兲 Cu2Se electrode are shown in
Fig. 9. Several bright rings made up of discrete spots are related to
the polymicrocrystalline nature of the materials. These concentric
rings are unambiguously indexed to Li2Se 共cubic structure with a
= 6.017 Å兲 and Cu 共cubic structure with a = 3.615 Å兲 共Table I and
Fig. 9b兲. The broad, weak rings of Li2Se indicate its partially amorphous state while two clear rings are made up of some discrete spots
that were indexed to nanocrystalline Cu. These results show that the
fully reduced Cu2Se electrode consists of Li2Se and Cu.
Discussion
It is interesting that three kind of copper selenide 共Cu2Se, CuSe,
and CuSe2兲 thin films were fabricated by controlling substrate temperature and target element ratios. According to the phase diagram
5CuSe2 + 14Li+ + 14e− Cu2Se + Cu3Se2 + 7Li2Se
关1兴
5CuSe + 4Li+ + 4e− Cu2Se + Cu3Se2 + 2Li2Se
关2兴
Cu2Se + 2Li+ + 2e− Li2Se + 2Cu
关3兴
The electrochemical cycling of the CuSe2 /Li and CuSe/Li cells
result in the same discharge products: Cu2Se and Cu3Se2. To our
knowledge, CuSe2 obviously has a different stoichiometric ratio
共Cu:Se兲 and crystal structure from CuSe. CuSe2 crystallizes in the
cubic structure of the pyrite type, space group Pa3̄共Z = 4兲 共Fig.
10a兲. Cu atoms are on the Wyckoff position 4共a兲 with coordinates
共0, 0, 0兲 and Se atoms are on the 8共c兲 Wyckoff positions with coordinates 共x, x, x兲.27 Each cation is in the center of an anion octahedron and each anion has a tetrahedral coordination consisting of one
anion atom and three cations. The experimental lattice parameter is
a = 6.116 Å and the only internal degree of freedom is Sex
= 0.3891共5兲. On the contrary, CuSe 共klockmannite兲 is isostructural
with covellite, CuS. This phase has a hexagonal cell based on
P63 /mmc space group 共Z = 6兲 共Fig. 10b兲, with two Cu1 atoms on
2共d兲 Wyckoff sites 共1/3, 2/3, 3/4兲, four Cu2 atoms on 4共 f兲 sites
共1/3, 2/3, Cuz兲, two Se1 atoms on 2共c兲 sites 共1/3, 2/3, 1/4兲, and
four Se2 atoms on 4共e兲 sites 共0, 0, Sez兲.28 The atomic arrangement
consists of alternative packing of hexagonal close-packed layers of
关CuSe兴 and 关Cu2Se2兴 in the c direction. Cu atoms in the 关CuSe兴
layers are triangularly coordinated by Se atoms, and Cu atoms in the
关Cu2Se2兴 layers are surrounded by four tetrahedrally arranged Se
atoms. The experimental lattice parameter is a = 3.939 Å,
c = 17.25 Å and the internal degrees of freedom are Cuz = 0.107,
Sez = 0.068. When the CuSe2 /Li cell and CuSe/Li cells are electrochemically reduced to 1.8 V, some of the selenium atoms are pulled
out from the CuSe2 and CuSe lattice to form Li2Se while others are
combined with copper atoms to form Cu3Se2 and Cu2Se. These
conversion reactions experience large structure and volume changes,
resulting in the large irreversible capacity loss. However, CuSe has a
smaller irreversible capacity loss than CuSe2. In our opinion, it may
be due to the lower volume change during the cycling, which is
derived from the structure conversion, and the unchangeable valence
of Se in CuSe.
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Journal of The Electrochemical Society, 153 共12兲 A2262-A2268 共2006兲
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XRD and SAED data, we could deduce that the electrochemical
cycling of a Cu2Se/Li cell results from a reversible structural evolution of Cu2Se and Cu during discharge and charge. We suppose
that upon lithiation 共delithiation兲, lithium is gradually inserting into
共extruding from兲 the face-centered cubic Se matrix to yield Li2Se
共Cu2Se兲. Such “displacement” reactions between Cu2Se and Li2Se
could be conceivable with our system because the pristine 共Cu2Se兲
and lithiated 共Li2Se兲 products have the same cubic-type structure
with a = 5.76 and 6.03 Å 共calculated by XRD and SAED data兲,
respectively 共Fig. 10c and d. The lithium atoms in Li2Se occupy the
same positions in the face-centered cubic structure as the copper
atoms in Cu2Se. So that one could imagine a selenium framework in
which Li 共Cu兲 will enter, pushing the copper 共lithium兲 out. Because
the lattice parameter of Cu2Se and Li2Se is similar, causing inconsiderable changes in volume and structure during the cycles. But
CuSe2 and CuSe have obviously different structures compared to
Li2Se. The difference of the composition and structure could be used
to explain the well cycling performance Cu2Se/Li cell than
CuSe2 /Li and CuSe/Li cells.
Conclusion
In this paper, three kinds of copper selenide thin films, CuSe2,
CuSe, and Cu2Se have been successfully prepared by the reactive
PLD method in argon. The selenium ratio in the target and substrate
temperature is the decisive factor for the composition and structure
of the as-deposited thin film. The thin film deposited at 200°C consists of CuSe2 with a target ratio of 1:5.0 共Cu:Se兲 or CuSe with a
target ratio of 1:1.2 共Cu:Se兲, respectively. When the substrate temperature was elevated to 400°C, the as-deposited thin film was composed of crystallized Cu2Se. The reversible capacity of CuSe2 /Li,
CuSe/Li, and Cu2Se/Li cells are 201.7, 258.4, and 210.2 mAh/g,
respectively. The cycle performance in the range of 1.0–2.5 V of
CuSe2 /Li and CuSe/Li cells are very poor while the Cu2Se/Li cell
could retain 73% of the initial discharge capacity even after
100 cycles. When cycled in a limited voltage range of 1.8–2.5 V,
the CuSe2 /Li and CuSe/Li cells have elevated cycle performance.
64% and 73% of the initial discharge capacity was held after
100 cycles, respectively. By using ex situ XRD and TEM, the conversion reaction mechanisms between Cu2+ 共CuSe and CuSe2兲 and
Cu+ 共Cu2Se兲 were proposed for the CuSe2 /Li and CuSe/Li cells. On
the contrary, the Cu2Se/Li cell underwent a “displacement” reaction.
This displacement reaction between Cu2Se and Li2Se largely reduced the volume and structure change during the electrochemical
cycles and enhanced the cycle ability remarkably. Our results demonstrate that the lithium electrochemistry of copper selenides was
complex and may enlighten and expand understanding of lithium
electrochemical reaction mechanisms for copper-based compounds
and re-open opportunities in this fascinating field of copper-based
storage materials for Li-ions batteries.
Acknowledgment
This work was supported by the National Nature Science Foundation of China 共project no. 20203006兲.
Fudan University assisted in meeting the publication costs of this article.
References
Figure 10. Crystal structure of 共a兲 CuSe2, 共b兲 Cu2Se, 共c兲 CuSe, and 共d兲
Li2Se.
Entirely different from the electrochemical behaviors of
CuSe2 /Li and CuSe/Li cells, the galvanostatic curves of the
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retention even after 100 cycles between 1.0–2.5 V. From ex situ
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