One-pot galvanic formation of ultrathin-shell Sn/CoOx nanohollows as high performance anode materials in lithium ion batteries

Article abstract of DOI:10.1039/c3cc42109k

The thermolysis of a heterobimetallic Sn-Co organometallic precursor resulted in the formation of hollow Sn/CoO_x nanomaterials through galvanic reaction between metal species. The Sn/CoO_x nanohollows with 6.1 nm diameter and ～1.5 nm shell thickness showed excellent lithium storage capacity of up to 857 mA h g^-1 after 30 cycles.

Full text of DOI:10.1039/c3cc42109k

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One-pot galvanic formation of ultrathin-shell Sn/CoOx
nanohollows as high performance anode materials in
lithium ion batteries†
Juan Xu,a Jaewon Jin,a Kyeongyeol Kim,a Young Jun Shin,a Hae Jin Kimb and
Seung Uk Son*a
Cite this: Chem. Commun., 2013,
49, 5981
Received 22nd March 2013,
Accepted 14th May 2013
DOI: 10.1039/c3cc42109k
www.rsc.org/chemcomm
The thermolysis of a heterobimetallic Sn–Co organometallic precursor
A single-precursor approach for the preparation of nanomaterials
resulted in the formation of hollow Sn/CoOx nanomaterials through is attractive because the synthetic steps can be simplified.9 For the
galvanic reaction between metal species. The Sn/CoOx nanohollows synthesis of multi-component materials, precursors having the
with 6.1 nm diameter and B1.5 nm shell thickness showed excellent corresponding multi-elements have been prepared.9 Usually, single
lithium storage capacity of up to 857 mA h gÀ1 after 30 cycles.
precursors with a tailored stoichiometry of elements have been
used for the selective control of target stoichiometric phases.
The chemical activities of nanomaterials are closely related to In comparison, as far as we are aware, the use of multi-
their size and surface area.1 In addition, the shape effect of component single precursors for the shape-controlled synthesis
nanomaterials can be critical in their chemical functions.2 For of nanomaterials has not been reported. Our research group
example, various inorganic nanomaterials have been applied as has studied the shape-controlled synthesis of functional
energy storage materials in lithium ion batteries.3 For evaluating nanomaterials.10 In this work, we report the synthesis of tin
the electrochemical performance of electrode materials, the containing hollow nanomaterials with ultrathin shells using a
energy storage capacity and stability are important parameters. multi-component single precursor and their performance as an
To enhance the energy storage capacity, the size of electrode anode material in lithium ion batteries.
materials has been controlled to sub-10 nm. The shape of
As shown in Fig. 1, a heterobimetallic precursor with tin and
electrode materials is related to their electrochemical stability. cobalt was prepared.11 Originally, this precursor was designed for
The existence of inner empty space in materials enhances the the preparation of Sn–Co hetero-component anode materials
stability of their electrochemical functions because it enables the because both Sn and cobalt oxide are active as anode materials.3b
electrode materials to maintain their functions against volume First, to confer nucleophilicity on cobalt, Co2(CO)8 was reduced
change after lithium insertion.3b In this regard, various hollow to Co(À1) species using sodium naphthalenide. The resultant
nanomaterials have been prepared to achieve high capacity and NaCo(CO)4 was reacted with a tin electrophile (Ph3SnCl) to form
stability as electrode materials in lithium ion batteries.4
the target precursor, Ph3SnCo(CO)4. Its elemental analysis (theore-
Group IVA elements are extensively used as anode materials in tical values for C: 50.72 and H: 2.90, observed values for C: 50.78
lithium ion batteries.5 Their electrochemical performances are and H: 2.96) confirmed the high purity of the obtained precursor.
based on the alloying and de-alloying processes triggered by For the synthesis of ultrathin Sn/CoOx hollow nanospheres (US-HS),
lithium insertion and desertion. Among the group IVA elements,
tin-containing nanomaterials have been most widely prepared.4,6–8
Zerovalent tin can be used as an anode material in lithium ion
batteries.7,8 However, the size of zerovalent tin materials reported
thus far has been relatively large (>10 nm), due to the limited
synthetic routes, especially for sub 3 nm-sized materials.7,8
Moreover, hollow zerovalent tin materials are quite rare.8
a Department of Chemistry and Department of Energy Science,
Sungkyunkwan University, Suwon 440-746, Korea. E-mail: sson@skku.edu;
Fax: +82-031-290-4572
b Korea Basic Science Institute, Daejeon 350-333, Korea
† Electronic supplementary information (ESI) available: Experimental details,
HR-TEM image and cyclovoltammogram of an electrochemical cell. See DOI:
10.1039/c3cc42109k
Fig. 1 Synthesis of
a Sn–Co heterobimetallic precursor and ultrathin-shell
Sn/CoOx hollow nanospheres (US-HS).
c
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the Sn–Co heterobimetallic precursor was dissolved in well-dried
oleylamine and injected into a hot oleylamine solution at 260 1C.
The reaction mixture was heated for additional 12 hours. After
cooling to room temperature, excess methanol was added to
form precipitates which were retrieved by centrifugation (refer to
the ESI† for details).
Transmission electron microscopy (TEM) analysis of the
obtained materials showed nanospheres with empty inner
space (Fig. 2). The average size of materials was 6.1 Æ 0.5 nm
with a shell thickness of B1.5 Æ 0.3 nm (Fig. 1 and Fig. S1 in
the ESI†).12 To elucidate the formation mechanism of the
hollow nanospheres, the materials formed at different reaction
times were analyzed using energy dispersive X-ray spectroscopy
(EDS), powder X-ray diffraction studies (PXRD), TEM and X-ray
photoelectron microscopy (XPS) (Fig. 3).
The materials formed after 30 min showed nearly a 1 : 1
stoichiometric ratio of tin to cobalt in EDS analysis. As time
passed, the cobalt content gradually decreased to 0.2 eq. to tin
after 12 hours (Fig. 3a). According to PXRD studies, the powders
obtained after 30 min and 1 hour showed amorphous character
(Fig. 3b). Interestingly, PXRD peaks corresponding to zerovalent
tin (JCPDS# 86-2265) appeared after 12 hours. In the XPS analysis
of the materials obtained after 30 min and 12 hours, the 2p3/2
orbital peak of cobalt shifted from 778.3 eV to a higher energy
level of 780.0 eV (Fig. 3c). The peaks at 778.3 eV and 780.0 eV were
close to those of zerovalent Co and Co(+2), respectively.13 In
contrast, the 3d5/2 orbital peak of tin shifted from 486.4 eV to a
lower energy level of 485.4 eV. The peaks at 486.4 eV and 485.4 eV
were close to Sn(+2) and zerovalent Sn, respectively.13 Thus, as the
Fig. 3 (a) EDS (* = Cu from grid) and (b) PXRD studies of the materials obtained
after heating Ph3SnCo(CO)4 in oleylamine at 260 1C for 30 min, 1 h and 12 hours.
(c) XPS and TEM analysis (also, refer to Fig. S4 in the ESI†) of the materials
obtained after 30 min (d) and 12 hours (e).
reaction proceeded after 30 min, cobalt was oxidized to cobalt(+2) High-resolution TEM studies on the hollows showed poor crystal-
ions and at the same time, tin(+2) was reduced to zerovalent tin. linity due to the size effect (Fig. S3, ESI†). After 18 hours, the hollow
Considering the standard reduction potentials of cobalt(+2) and structures collapsed. Thus, it can be speculated that cobalt was
tin(+2) ions, a redox reaction between cobalt and tin species is gradually removed from the materials via oxidation and zerovalent
thermodynamically reasonable.14 TEM analysis of the materials tin gradually formed in the hollows with inner empty space.
obtained after 30 min and 12 hours showed B7 nm-sized spheres
Next, we investigated the electrochemical properties of US-HS
(Fig. 3d) and hollow spheres with B6 nm diameter (Fig. 3e). as an anode material in lithium ion batteries. Using US-HS,
working electrodes were fabricated (see the ESI† for detailed
procedures). Then, coin-type electrochemical cells were assembled
using lithium metal as a counter electrode. The cells were dis-
charged from open voltage and cycled between 1 mV and 2 V with
a current density of 50 mA gÀ1. It has been reported that metallic
tin acts as an anode material through reversible lithiation and
de-lithiation to form a Li4.4Sn alloy.7,8 In addition, CoO is active as
an anode material.3b US-HS showed the typical cyclovoltammo-
gram of tin-based anode materials (Fig. S4, ESI†). The redox
reaction at +0.8 V (vs. Li/Li+) by CoO can be overlapped (Fig. S5
in the ESI†). Fig. 4a shows the cycling performance of the cell of
US-HS. The theoretical maximum storage capacity of metallic tin is
known to be 992 mA h gÀ1 7,8
The first discharge capacity of the
.
cell of US-HS was observed to be 2530 mA h gÀ1, which originated
from both irreversible and reversible electrochemical processes.
It has been suggested in the literature that this irreversible process
in the early stages of tin anode materials is related to the formation
of a solid electrolyte interface (SEI).7,8 The relatively large irrever-
sible capacity in the first cycle originated from the size effect of
materials. The discharge capacities were gradually stabilized in
successive runs. After 30 cycles, the electrochemical cells
Fig. 2 TEM image of ultrathin-shell Sn/CoOx hollow nanospheres (US-HS). Also
refer to the magnified image in the ESI† (Fig. S2).
c
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In conclusion, in this work we have shown that ultrathin-
shell hollow nanomaterials can be prepared in a one-pot
process through an organometallic single-precursor approach.
We believe that more diverse nano-structured materials can be
prepared in one-pot by the delicate design of single precursors
with multi-elements with different electrochemical properties.
This work was supported by grants NRF-2012-004029 (Basic
Science Research Program) through the National Research
Foundation of Korea funded by the MEST. JX thanks for grants
NRF-2012-1040282 (Priority Research Centers Program) and
R31-2008-10029 (WCU program).
Notes and references
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Fig. 4 (a) Cycling performance, (b) coulombic efficiencies, (c) charge–discharge
curves with a 50 mA gÀ1 current density and (d) current density-dependent
discharge capacities of the electrochemical cells prepared using US-HS.
maintained a discharge capacity of 857 mA h gÀ1 (based on the
weight of US-HS), which corresponds to 86% and 91% of the
theoretical maximum capacity of zerovalent tin and Sn0.8/(CoO)0.2
anode materials respectively.3b,15 Notably, in the last five cycles,
the change in discharge capacity was only within 3 mA h gÀ1. In
the literature, the sizes of zerovalent tin-based anode materials
were in the range of 10–100 nm.7,8 The lithium storage capacities
were observed to be in the range of 500–800 mA h gÀ1 7,8
.
Considering these values, it can be speculated that the high
capacity of electrochemical cells of US-HS and the stability
are attributed to the sub 10 nm size and hollow structure.16
Especially, it is noteworthy that the Li diffusion length corre-
sponds to the shell thickness of US-HS, which is B1.5 nm.
As shown in Fig. 4b, the coulombic efficiencies sharply
increased from 46% to 97% in the first 10 cycles and maintained
the higher values in successive runs. The charge–discharge curves
showed typical patterns of electrochemical cells based on the tin
alloying–de-alloying process (Fig. 4c).7,8 The shoulder peak in the
B1–1.5 V range in the first discharge curve is typically assigned to
the irreversible decomposition of the electrolyte or the formation
of SEI.7,8 The voltages below 1 V in Fig. 4c correspond to the
reversible delithiation–lithiation, and match well with the conven-
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12 The diameters and shell thicknesses of 120 and 40 particles were
cells maintained good performance at higher current densities
measured respectively.
(Fig. 4d). At current densities of 100 mA gÀ1 and 200 mA gÀ1
,
13 J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben, Handbook
of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., 1992.
14 The standard reduction potentials of Co2+ and Sn2+ are À0.28 V and
À0.14 V respectively.
the cells maintained discharge capacities of 802 mA h gÀ1 and
770 mA h gÀ1, corresponding to 81% and 78% of the theoretical
maximum capacity, respectively. Even with current densities of
0.5 A gÀ1 and 1 A gÀ1, the cells showed discharge capacities of
663 mA h gÀ1 and 578 mA h gÀ1, which are 67% and 58% of the
theoretical maximum capacity of tin, respectively, and much higher
than that of commercialized graphite (372 mA h gÀ1).
15 According to EDS and XPS analysis, US-HS contains 20% CoO
(theoretical capacity: 716 mA h gÀ1, ref. 3b). The theoretical maximum
capacity of US-HS can be estimated to be 937 mA h gÀ1
.
16 In comparison, the SnCo(1 : 1)Ox materials obtained after 30 min
and US-HS maintained 745 mA h gÀ1 and 947 mA h gÀ1 discharge
capacities after 10 cycles, respectively.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5981--5983 5983