21st Interna tional Symposium on Plasma Chemistry (ISPC 21)
Sunda y 4 Augus t – Frida y 9 August 2013
Cai rns Convention Centre, Queensland, Aus tralia
Synthesis of Si-Ni composites by plasma spray PVD for negative electrode of
lithium-ion batteries
Narengerile and Kambara Makoto
Department of Materials Engineering, The University of Tokyo, Bunkyo -ku, Tokyo 113-8656, Japan
Abstrac t: The present work describes synthesis of the nanostructured Si-Ni co mposites for LiB
anodes by plasma spray physical vapor deposition method. The X-ray diffraction analysis and X-ray
photoelectron spectroscopy measurement of the resultant composites revealed that the formation of
high electrical conductivity of NiSi2 . Moreover, anodes prepared fro m the Si-Ni co mposites e xhib ited
a discharge capacity as high as 1286 mAh/g with ~96% colu mbic efficiency after 50 cycles.
Keywords : Plasma spray, nanostructured Si-Ni co mposites, anode material, lithiu m ion battery.
1. Introduction
Si is one of the most promising candidate materials for
lithiu m ion battery (LiB) anodes owing to the high theoretical capacity of 4200 mAh/g, more than 10 times higher than that of graphite wh ich is now co mmonly used as
an anode material in co mmercial LiB. However, presently,
practical applicat ions of Si as an anode material have two
major limitations [1]. First, Si has poor cyclability because of the large volume change (~400%) that occurs
during lith iation and delithiation. Th is large volu me
change can cause mechanical fracture and loss of electr ical contact with the current collector, leading to a rapid
capacity fade of the cell. Second, the low electrical conductivity of Si–based materials impedes fast lithiation [2].
To address the above issues, substantial effort has been
focused on synthesizing nanostructured alloys and dispersing the active material in an inactive matrix to form
composites. In this case, the inactive component plays a
structural buffering role to minimize the mechanical stress
induced by huge volume change of active silicon [3]. For
instance, the nanostructured alloys such as Li-Si, Co-Si,
Cu-Si, and Ti-Si, and composites such as Si/C, Si/TiN,
Si-Ni, and Si/TiB2 were widely synthesized and characterized [2-6]. However, many studies on the electrochemical behavior of alloys and composites have reported just
a slightly imp rovement in cycling performance [2-5].
The present work describes the use of plasma spray
physical vapour deposition (PVD) to prepare nanostructured Si-Ni co mposites for LiB anodes . Herein, n ickel
silicides in the co mposites are expected to acco mmodate
the volume changes and to insure good electronic and
ionic conductivities of the electrode material. The mo rphology of the resultant composites was characterized
using X–ray powder diffraction, X-ray photoelectron
spectroscopy, scanning electron microscopy, and transmission electron microscopy. Moreover, we tested the
electrochemical performance of anodes prepared from
Si-Ni co mposites by plasma spraying.
2. Experi mental
2.1. Preparation of Si-Ni composites
A schematic diagram of a direct current-rad io frequency
(DC-RF) hybrid plasma torch and a photograph of the
apparatus are shown in Fig. 1. The apparatus main ly consists of an RF generator, DC power supply, reactor chamber, reactants and gas supply system, and part icle collector. The DC plas ma torch is set at the upper centre, and
the arc jet generated fro m the DC torch which may play
an important role as an easy and stable igniter. The RF
plasma is generated by a three-turn copper coil at a frequency of 2.33 MHz. Detail of this system can be found
elsewhere [7,8]. A mixtu re of argon and hydrogen was
introduced as plasma-supporting gas. Argon was used
both as a carrier gas as well as sheath gas, which was injected in both the radial and tangential d irections to protect the inner surface of the quartz tube and stabilize the
plasma d ischarge. The operating conditions are summarized in Table 1.
An inexpensive Si (part icle size ~45 μm, purity 99.5%)
and Ni (part icle size ~20 μm, purity 99.9%) were used as
Fig. 1 Schematic diagram of plasma spray torch and photograph
of the apparatus.
21st Interna tional Symposium on Plasma Chemistry (ISPC 21)
Sunda y 4 Augus t – Frida y 9 August 2013
Cai rns Convention Centre, Queensland, Aus tralia
the raw materials after being ho mogeneously mixed using
a pot mill rotator (Asone, PM-001). The molar content of
nickel in the feed powders has been adjusted to 5 at.%.
The precursors were introduced at a feed rate of 1 g/ min
with argon as the carrier gas. The high enthalpy of the
plasma enables the precursors to instantaneously evaporate. The vapours of the injected Si and Ni powders are
transported with the plasma flow to the reaction chamber
and become supersaturated because of the rapid temperature decrease in the tail flame which leads to homogen eous nucleation. Moreover, the supersaturated vapour easily condenses on the nuclei by heterogeneous condens ation and/or co-condensation [8-10]. Thus, this technique
provides an efficient and one-step synthesis of
nanocomposite anodic materials for LiB.
The morphology of the composites was observed using
transmission
electron
microscope
microscopy
(H-9000NAR Hitachi Japan) operated at 300 kV and
scanning electron microscopy (SEM-4200 Hitachi Japan).
The phase compositions of the composites were determined by X-ray Photoelectron Spectroscopy (ESCA-850
Shimad zu Japan) and powder X-ray diffraction (Bru ker
D2 phaser) using monochromatic Cu Kα rad iation.
2.2. Electrochemical measurement
The prepared composites were d irectly used as active
materials after passing them through a 45-μm sieve. For
electrochemical studies, electrodes were prepared fro m a
mixtu re of 65 wt .% active material, 15 wt.% conductive
carbon, and 25 wt.% binder in N-methyl-2-pyrrolidone
(NMP). The slurry was applied by doctor blade onto an
etched copper foil current collector, and oven-dried at
110 °C for 60 min.
The electrochemical performance of the co mposite anode was investigated with a 2016 cell assembled in an
argon-filled g love bo x. Lithiu m metal foil was used as the
cathode, and the electrolyte was a 1-M solution of LiPF6
in ethylene carbonate/diethyl carbonate (EC/ DEC, 1:1
v/v), with a polypropylene-based membrane as the separator. The charge-discharge cycles were performed at 0.1
C-rate for 1–3 cycles and 0.5 C-rate for 4–50 cycles at
room temperature using an ACD01 charge-discharge battery test system.
Table 1 Experimental conditions
Parameters
Values
DC power
9 kW
RF power
Radial Ar
100 kW
140 sclm
Tangential Ar
30 sclm
Radial H 2
50 sclm
Deposition time
10 min
Chamber pressure
500 Torr
Fig. 2 XRD patterns of the prepared composites.
Fig. 3 Normalized XPS spectra of Ni 2p 3/2 peaks (a)
standard Ni and (b) the prepared composites.
3. Results and discussion
3.1. Characterization of the composites
The XRD patterns of composites prepared with 5 at.%
Ni in the feed powders is shown in Fig. 2. For co mparison,
the standard XRD patterns of NiSi2 , NiSi, and Ni2 Si are
also shown in Fig. 2, wh ich were derived fro m Ref [11].
The XRD patterns of the prepared composites indicate
that all the diffraction peaks can be ascribed to Si and/or
NiSi2 . However, the diffraction peaks of Ni powder and
nickel silicide of Ni2 Si and NiSi were not observed from
Fig. 4 Normalized XPS spectra of Si 2p peaks of
the prepared composites.
21st Interna tional Symposium on Plasma Chemistry (ISPC 21)
Sunda y 4 Augus t – Frida y 9 August 2013
Cai rns Convention Centre, Queensland, Aus tralia
the prepared composites. As shown in Fig. 2, the diffraction peaks of Si and NiSi2 are very close together, and are
difficult to distinguish fro m each other at low NiSi2 content. Therefore, we further analy zed the existence of NiSi2
in the prepared composites using XPS, as shown in Figs.
3 and 4. The high-resolution Ni 2p3/2 spectrum obtained
fro m both of the pure Ni (Fig. 3a) and the prepared composites (Fig. 3b). The position of the Ni 2p3/2 peak of
elemental Ni was set at 851.7 eV. Co mpared with the peak
of the pure Ni in Fig. 3(a), the Ni 2p3/ 2 peak is moved to
higher binding energy position of 853.2 eV, as shown in
Fig. 3(b), which is correspond to the NiSi2 [12]. It is kwon
fro m equilibriu m phase diagram that NiSi2 is the incongruent phase that forms through peritectic reaction [13].
This is contrast to the common observation that congruent
phases are formed preferentially by rapid quenching of
high-temperature metal vapor mixture during plasma
spraying PVD [7-10]. Th is fact induct suggests that NiSi2
phase form not via homogenous nucleation but via heterogeneous nucleation direct ly on the Si nanoparticles that
form in advance. This could make these composites powders unique as evidenced later in electrochemical test. In
Fig. 4 the Si 2p spectra present the peaks associated with
bulk silicon, silicide, and silicon dio xide, indicat ing the
interface Si was infinitesimally o xidized in the air.
Figures 5(a) and (b) display SEM images of the processors and the composites prepared with 5 at.% Ni content in the feed powders, respectively. In co mparison with
the precursors, shown in Fig. 5(a), the composites in Fig.
5(b) exh ibit a higher fract ion of large, irregular, and
spherical particles. Th is indicates that co mplete evaporation of raw powder in plas mas and aggregation occurred
during plasma treat ment. The structures of the prepared
composites were observed further by TEM, as shown in
Figs. 5(c) and (d). Fig. 5(d) is a magnificat ion TEM image of the large part icles. The part icles exh ibit a dense
internal structure, containing many smaller grains in Fig.
5(c). Such structures have a large grain boundary surface
area and a short diffusion path, useful for improving their
performance in LiB [2,5]. The magnificat ion TEM image
in Fig. 5(d) shows that the composite products were a
many nanoparticle aggregation in d iameter of ~50 n m.
3.2. Electrochemical investigation
Cycle perfo rmance of anode prepared fro m the Si-Ni
composites is shown in Fig. 6. For co mparison, Si aggregates anode prepared using plasma spray PVD with only
silicon (0 at.% Ni) as the raw materia l was also tested. As
can be seen, the cyclic performance of co mposite anodes
prepared with 5 at.% Ni concentration provides direct
evidence of the excellent lithiu m storage capabilities of
these materials; the rechargeable capacity 1286 mAh/g
after 50 cycles, which is approximately 400 mAh/g higher
than that of Si aggregates anode. Moreover, the obtained
results in this study were more than 4 times higher than
Fig. 5 SEM images of (a) the precursors and (b) the
composites prepared with 5at.% Ni and TEM images of
(c) the composites and (d) magnification TEM image.
Fig. 6 Cycle performance of anodes prepared
from the Si-Ni composites.
Fig. 7 Columbic efficiency of anodes prepared
from the Si-Ni composites.
21st Interna tional Symposium on Plasma Chemistry (ISPC 21)
Sunda y 4 Augus t – Frida y 9 August 2013
Cai rns Convention Centre, Queensland, Aus tralia
that of conventional graphite anode. The improvement in
electrical performance of the Si-Ni co mposite anodes is
attributed not only to the nanoscale structure of the co mposites but also to their high electrical conductivity.
The co lu mbic efficiency of the Si-Ni co mposites anode
is shown in Fig. 7. In the case of 1-5 cycles, the co lu mbic
efficiency was increased significantly. The lo w co lu mbic
efficiencies for the few first cycles can be attributed to
irreversible trapping of inserted Li ions by host materials,
formation of the solid electrode interface (SEI) layer, or
the loss of electrical contact between the electrode material and the current collector [4,5]. Fo r the subsequent
cycles (6-50), the composite anode exhibits very stable
capacity retention behavior and high colu mb ic efficiency
of >96%.
Figure 8 displays voltage profile and corresponding
differential capacity plot (DCP) of the prepared electrode
when discharged and charged for first, third, fifth and
tenth cycles. Only one broad peak at 0.07 V for Si-Ni
composites was observed during the first discharge (lithiu m alloy), wh ich was due to the phase transition of
amorphous Lix Si to crystalline Li15 Si4 [5,12]. At the end
of the first lithiation, all of the material has been tran sformed into amorphous Li xSi. In contrast to the first discharge, three peaks were observed at 0.25, 0.08, and 0.01
V in the third cycle and subsequent cycles. The peaks at
0.25 and 0.08 V may be due to the phase transitions between amorphous Lix Si. Here in, the intensities of the
peaks of the Si-Ni-silicide composites remained almost
the same, showing the great enhancement of the cycling
stability.
4. Conclusions
We prepared nanostructured Si-Ni co mposites using
plasma spray PVD. Moreover, anodes prepared from these composites are able to deliver as much as 1286 mAh/g
with >96% colu mb ic efficiency after 50 cycles. This observation indicates the potential of the development of
nano-sized alloy co mposite electrode materials with h igh
capacity at low cost using plasma spraying.
5. References
Fig. 8 DCPthof the Si-Ni composite
electrodes after (a)
the 1 cycle and (b) the 3th, 5th, and 10th cycles.
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