Synthesis and characterization of alkylsilane ethers with oligo(ethylene oxide) substituents for safe electrolytes in lithium-ion batteries

Article abstract of DOI:10.1039/c0jm01596b

Alkylsilane ethers, containing one or three carbon spacer groups between the silicon atom and oligo(ethylene oxide) moiety, were designed and synthesized. These compounds are non-hydrolyzable and less flammable than their alkoxysilane counterparts. A full cell test using them as electrolyte solvents showed good cycling performance in lithium-ion batteries.

Full text of DOI:10.1039/c0jm01596b

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www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterization of alkylsilane ethers with oligo(ethylene oxide)
substituents for safe electrolytes in lithium-ion batteries†
ab
c
c
a
c
Lingzhi Zhang, Leslie Lyons, Jocelyn Newhouse, Zhengcheng Zhang, Megan Straughan, Zonghai Chen,
d
d
a
a
Khalil Amine, Robert J. Hamers and Robert West*
Received 25th May 2010, Accepted 2nd August 2010
DOI: 10.1039/c0jm01596b
Alkylsilane ethers, containing one or three carbon spacer groups
between the silicon atom and oligo(ethylene oxide) moiety, were
designed and synthesized. These compounds are non-hydrolyzable
and less flammable than their alkoxysilane counterparts. A full cell
test using them as electrolyte solvents showed good cycling perfor-
mance in lithium-ion batteries.
1S1Mn (n ¼ 1–3, compounds denoted as in Scheme 1A),
containing one methylene spacer, were synthesized through a nucle-
ophilic reaction of lithiated hydroxymethyltrimethylsilane with the
corresponding tosylate of ethylene glycol monomethyl ether in
satisfactory yield (Scheme 1A). The intermediates, acetatomethyl-
trimethylsilane and hydroxymethyltrimethylsilane, were prepared as
4
described in the literature. 1S3Mn (n ¼ 1–3, compounds denoted as
in Scheme 1A), containing a spacer of 3 methylene groups, were
synthesized by reacting trimethylsilylmethyl-magnesium chloride
with the tosylate of ethylene glycol monomethyl ether in the presence
There are increasing concerns about the safety of the conventional
electrolytes used in consumer electronic products because of the
recent accidents involving lithium-ion batteries. In order to improve
the safety of energy storage devices, much effort has been devoted to
developing safer electrolytes such as solid electrolytes and ionic
5
of copper(I) iodide as a catalyst (Scheme 1A). Alternatively, 1S1Mn
and 1S3Mn can be synthesized easily by reacting the corresponding
sodium salt of ethylene glycol monomethyl ether with 3-iodome-
1
liquids. We have been developing organosilicon based electrolytes
2
for energy storage applications during the past several years. Orga-
thyltrimethylsilane and iodopropyltrimethylsilane, respectively
6
Scheme 1B). However, it was observed that moderate cleavage of
(
1
Si–C bonds (about 10%, estimated from H-NMR) took place in
nosilicon compounds have lower vapor pressure, higher flash point,
and withstand higher operating voltages than the commercial alkyl
carbonates. All these properties point to their increased safety when
these reactions. Similar cleavage of Si–C bond has been reported in
8
the literature. To investigate the chemical structure effect on the
3
2a
9
used as electrolytes in devices.
flammability, 1NM3 and 2SM3, corresponding to a trimethylsilyl
triethylene glycol monomethyl ether without carbon spacer and
a disiloxane compound with a 3 carbon spacer, respectively, were
synthesized for comparison.
Recently, we reported the synthesis of trimethylsilyl oligo(ethylene
glycol) ethers though a trimethylsilylation reaction by directly
refluxing oligo(ethylene glycols) or oligo(ethylene glycol) mono-
2a
methyl ethers with trimethylchlorosilane or hexamethyldisilazane.
The synthesized alkylsilane ethers (1S1Mn/1S3Mn, n ¼ 1–3) have
very low glass transition temperatures (Table 1). 1S1Mn has a lower
These materials have high ionic conductivity, great electrochemical
stability, and good cycling performance when used as electrolyte
solvents in lithium-ion batteries. However, these trimethylsilyl ethers
may be hydrolyzed by moisture. In this communication we report the
synthesis of compounds in which a trimethylsilyl group is attached to
g g
T than that of 1S3Mn with the same EO chain length. T increases
with increasing EO chain length for both 1S1Mn and 1S3Mn families
as expected. However, all 1S3Mn compounds exhibit a crystallization
peak followed by a melting peak, probably due to the ease of packing
of the long methylene spacer for crystallization during the DSC
heating process. 1S1M1 and 1S1M2 show only a Tg peak
(Fig. S1, ESI†). But for 1S1M3, a broad crystallization peak was
observed before melting, similar to 1S3Mn. In this case, the longer
an oligo(ethylene glycol) through a one-carbon (CH
2
) or three-
carbon (CH CH CH ) spacer, in order to minimize the possibility of
2
2
2
hydrolysis. The conductivity, thermal properties, hydrolysis experi-
ments, and the cycling performance of lithium cells using these
compounds as electrolytes are reported. The flammability of these
organosilicon electrolytes is characterized by measuring the flame
propagation rate and compared with a commercial carbonate
electrolyte.
a
Organosilicon Research Center, Department of Chemistry, University of
Wisconsin, Madison, Wisconsin, 53706, USA. E-mail: west@chem.wisc.
edu; Fax: +1 608 262 6143; Tel: +1 608 262 1873
b
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences,
No.2 Nengyuan Rd., Guangzhou, 510640 Guangdong, China
c
Department of Chemistry, Grinnell College Grinnell, IA, 50112, USA
d
Chemical Engineering Division, Argonne National laboratory, Argonne,
IL, 60439, USA
†
Electronic supplementary information (ESI) available: Experimental
details, synthesis, chemical characterization, and DSC curves of the
alkylsilane ether compounds. See DOI: 10.1039/c0jm01596b
Scheme 1 Synthesis of alkylsilane compounds.
8
224 | J. Mater. Chem., 2010, 20, 8224–8226
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Table 1 Physical data, conductivity and VTF parameter for trimethylsilyl compounds
+
ꢀ1
a
ꢁ
ꢁ
ꢀ1
ꢀ1
Compound
EO/Li
s/mS cm
h/cP
3
T
g
/ C
T
0
/ C
E
a
/kJ mol
0
s /S cm
1
1
1
1
1
1
1
2
S
S
S
S
S
S
1
1
1
3
3
3
M1
M2
M3
M1
M2
M3
7 : 1
10 : 1
10 : 1
10 : 1
15 : 1
10 : 1
10.1 : 1
10 : 1
1.4
1.5
0.86
1.3
0.90
0.94
1.9
0.50
1.60
2.75
1.70
2.10
3.75
1.4
3.87
4.40
5.42
3.82
4.44
5.48
5.13
4.28
ꢀ130
ꢀ118
ꢀ110
ꢀ115
ꢀ112
ꢀ109
ꢀ148
—
ꢀ108
ꢀ113
ꢀ69.2
ꢀ72.2
ꢀ95.2
ꢀ95.2
ꢀ67.2
—
3.12
4.20
2.77
0.95
3.67
4.40
2.1
16.5
75
29
0.32
43.5
79
NM3
SM3
27
—
0.37
3.85
—
a
ꢁ
2
SM3 shows a crystallization peak and a melting peak in DSC curve at ꢀ72 and ꢀ33 C.
EO chain seems cooperatively involved in the crystallization process.
The viscosity and dielectric constant increase with increasing length of
the EO unit and carbon spacer.
To compare the flammability of our organosilicon electrolytes with
the conventional electrolytes, a flame test was conducted based on
14
a modified condition of UL Flame Rating—UL94.
The ionic conductivity of the synthesized compounds is listed at
their optimum doping levels, determined from a working curve of
conductivity vs. doping level (Table 1). Generally, the viscosity
dominates the conductivity for these alkylsilane ethers. For one
exception, 1S1M2 has slightly higher conductivity than that of
The propagation rates of a commercial carbonate electrolyte
(EC/DMC with 1M LiPF6) and different organosilicon electrolytes
are plotted in Fig. 2. The commercial electrolyte has a highest
ꢀ1
propagation rate of 450 mm min . The organosilicon electrolytes are
much less flammable, the propagation rates being 280, 233 and
ꢀ1
1S1M1 because 1S1M2 provides one more active oxygen atom for
116 mm min for 1NM3, 1S1M3 and 2SM3. It is not surprising that
2SM3 has a smallest propagation rate because of the inorganic nature
of the Si–O–Si bond. However, it was unexpected that the compound
with one carbon spacer, 1S1M3, is somewhat less flammable than the
non-spacer compound, 1NM3.
Li-ion coordination. It was found in our previous studies that the
trimethylsilyl group reduces the complexing ability of the neighboring
2a,10
oxygen atom.
These alkylsilane ethers have somewhat lower
conductivities than the conventional electrolytes (e.g. propylene
1c,11
carbonate has a conductivity value of 5.1 mS cm with LiTFSI).
ꢀ1
The performance of lithium cells was investigated using these
Arrhenius plots of the ionic conductivity of these compounds with
alkylsilane compounds as electrolyte solvents, LiNi0.8Co0.15Al0.05O
2
ꢀ1
LiTFSI were slightly curved (Fig. 1a). The ln (sT) versus (T ꢀ T
0
)
as the cathode, and MCMB graphite as the anode. LiBOB was
chosen as the lithium salt because it provides an effective passivation
15
plots fit well linearly (Fig. 1b), where T and other parameters were
0
12
derived from the VTF equation and listed in Table 1. This suggests
film on the carbon anode. The charge/discharge cycling perfor-
that the motion of EO chain is involved in transporting the ions of the
mance of a lithium cell [MCMB anode/0.8 M LiBOB in 1S1M2/
13
electrolytes.
The hydrolysis experiments were conducted by stirring 1S1M3,
2
LiNi0.8Co0.15Al0.05O cathode] is plotted in Fig. 3, and is comparable
1
6
with that of conventional electrolytes. The cell was charged and
discharged at a C/5 rate and cycled between 3.0 and 4.2 V at room
temperature. It exhibits excellent cycling performance with only 9%
capacity loss over 200 cycles.
1
S3M3 and 1NM3 with 20% v/v water. 1NM3 was rapidly hydro-
lyzed, by about 20% in the initial 50 min, and then the hydrolysis
proceeds more slowly with time (Fig. 1c). The compounds with one
or three carbon spacer, 1S1M3 and 1S3M3, did not show
any hydrolysis after 24 h, even when stirred in the presence of LiTFSI
salt.
In summary, we synthesized oligo(ethylene glycol) functionalized
alkylsilane ethers with different carbon spacers (one or three) between
the silicon atom and the EO unit. The carbon spacer prevents the
Fig. 1 (a) Variable temperature ionic conductivity of the electrolytes with LiTFSI salt at their optimum doping levels; (b) VTF curves for trimethylsilyl
electrolytes. Linear fits are shown as solid lines; and (c) hydrolysis versus time for 1NM3, 1S1M3 and 1S3M3 in water.
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funded by the Ministry of Education, Science and Technology (R33-
10082) for support. LZ thanks the Hundred Talents Program of
Chinese Academy of Sciences and NSFC (0901021001) for support.
The NMR instruments in this research were funded by NIH (Grant
NIH 1 S10 RRO 8389-01 and CHE-96296887). Instrumentation at
Grinnell was funded by the NSF-MRI program (Grant no. 0116159).
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6
a
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3
7
3 2
BF $OEt . .
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Acknowledgements
1
This work is based in part on research sponsored by the National
Science Foundation Grant 0724469. RW, RH, and LZ have
a financial interest in the results of this work. RW thanks the WCU
program through the National Research Foundation of Korea
8
226 | J. Mater. Chem., 2010, 20, 8224–8226
This journal is ª The Royal Society of Chemistry 2010