A facile method for construction of a functionalized multi-layered separator to enhance cycle performance of lithium manganese oxide

Article abstract of DOI:10.1039/c3ra44917c

A facile approach has been proposed to improve the cycle performance of LiMn₂O₄-based cathodes by chemical modification of the separator. This strategy is based on the construction of a multilayer structure consisting of an interlayer with polar hydroxyl (OH) groups to promote the chemical reactivity between a hydrophobic separator and a surface layer with a HF scavenger to remove any HF remaining in the electrolyte. Systematic studies performed in this work indicate that separator functionalization is effective in increasing the structural stability of LiMn₂O₄ cathodes.

Full text of DOI:10.1039/c3ra44917c

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A facile method for construction of a functionalized
multi-layered separator to enhance cycle performance
of lithium manganese oxide†
Cite this: RSC Adv., 2013, 3, 25657
Received 6th September 2013
Accepted 24th October 2013
Taeeun Yim, Hyo-Jeong Ha, Min-Sik Park, Ki Jae Kim,* Ji-Sang Yu and Young-Jun Kim
DOI: 10.1039/c3ra44917c
www.rsc.org/advances
A facile approach has been proposed to improve the cycle perfor- researchers have attempted to improve the structural stability of
mance of LiMn -based cathodes by chemical modification of the the cathodes at elevated temperatures. The two most frequently
separator. This strategy is based on the construction of a multilayer used approaches that essentially focus on cathode modica-
2 4
O
1
3–17
structure consisting of an interlayer with polar hydroxyl (OH) groups tion
and utilization of additives in the electrolyte, which
18–20
to promote the chemical reactivity between a hydrophobic separator chemically scavenge the HF formed in the electrolyte.
and a surface layer with a HF scavenger to remove any HF remaining However, to the best of our knowledge, there are no reports so
in the electrolyte. Systematic studies performed in this work indicate far in the literature that deal with separator modication as a
that separator functionalization is effective in increasing the struc- possible approach for preventing the dissolution of Mn ions. To
tural stability of LiMn
O
2 4
cathodes.
this end, we propose a novel approach of separator modication
by a HF scavenging layer for improving the electrochemical
2 4
performance of LiMn O -based cathodes employed in LIBs. In
Recently, lithium manganese oxide (LiMn O ) has emerged as a
2
4
contrast to conventional cathode and electrolyte modication
techniques, the separator modication approach proposed in
this study is novel and has not been attempted before.
Among the candidates for HF scavenging, tetraethylorthosi-
licate (TEOS) has been shown to be effective in enhancing cycle
promising cathode material with the potential to replace
commercial lithium cobalt oxide (LiCoO ), owing to its low cost,
natural abundance, low toxicity, good safety characteristics, and
2
1–5
2 4
high power density. Despite these advantages, LiMn O still
lacks commercial implementation because of insufficient cycle
performance arising from structural instability, especially at
2 4
performance of LiMn O -based cathodes owing to their desir-
able chemical structure: TEOS provides signicantly more
elevated temperatures. The instability of LiMn O is mainly
2
4
reaction sites to remove HF (there are three reaction sites in one
caused by the accelerated dissolution of Mn ions in the pres-
ence of hydrouoric acid (HF) produced by the reaction between
À
21,22
mole of TEOS) through the reaction of F and Si.
However,
direct installation of a TEOS layer on a polyethylene (PE) sepa-
rator is unattainable owing to the lack of chemical functional-
ities of PE separators as they are composed of chemically stable
LiPF
6
and H
2
O in the electrolyte, resulting in irreversible
to MnO, MnO , and MnF by
In addition, Mn ions dissolved in the
decomposition of LiMn
reacting with HF.
2
O
4
2
2
6–8
2 2
olen (–CH CH –) units. Therefore, an interlayer with specic
electrolyte readily form reduced Mn particles on the anode
surface, which hinder the formation of stable solid electrolyte
interfaces (SEIs) and it signicantly affects the cycle perfor-
mance of the cathode in lithium ion batteries (LIBs).
Therefore, there is a critical need for strategies to prevent the
dissolution of Mn ions in the electrolyte in order to enhance the
functionality (which enables its components to react with TEOS)
is required to install the TEOS layer on the separator surface. In
this regard, alcohol functionality is highly desirable for the
interlayer component: the polar hydroxyl (OH) group readily
9–12
reacts with TEOS via an alcohol–ester condensation reac-
23–25
tion.
Among the many candidates for the interlayer, SiO is
2
cycle performance of LiMn
the dissolution of Mn ions from LiMn
2
O
4
-based LIBs. Thus far, to prevent
-based cathodes,
one of the more attractive components: it has a sufficient
number of polar hydroxyl (OH) groups during the synthesis
process to advance the reaction rate of the interlayer and TEOS.
2 4
O
Advanced Batteries Research Center, Korea Electronics Technology Institute, 68 Additionally, from the viewpoint of chemical reactivity, the
Yatap-dong, Bundang-gu, Seongnam-si, Gyeonggi-do, 463-816, Republic of Korea.
condensation reaction between the OH group and orthosilicate
has many advantages such as low reaction temperature, high
processability, and eco-friendliness. With this synthetic
strategy, a novel multi-layered functional separator has been
E-mail: kijaekim@keti.re.kr; Fax: +82 31 789 7499; Tel: +82 31 789 7492
†
Electronic supplementary information (ESI) available: Experimental details,
FT-IR analysis for cathode and separator with carbonates-based electrolyte. See
DOI: 10.1039/c3ra44917c
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developed to prevent dissolution of Mn ions and improve the The components of the modied layer were characterized by
cycle performance of LiMn O -based cathodes. energy dispersive spectroscopy (EDS) analysis: they contained
2
4
Fig. 1 illustrates the synthetic scheme of the modied 13.7 wt% of Si derived from TEOS as a result of the chemical
separator proposed in this study. For the facile chemical reaction with OH in an interlayer. The elemental mapping
modication of the hydrophobic separator, an alcohol-func- results also conrmed the presence of a nely dispersed TEOS
tionalized interlayer was introduced by dip coating the PE layer: the main elements of the outer layer (Si and O) completely
separator with the SiO
enging layer was successfully introduced by chemical treatment Fig. 2c. This is in good agreement with results of the wettability
via a simple alcohol–ester condensation reaction (detailed tests using polar electrolytes (Fig. S2†): the PE–SiO –TEOS
2
. Furthermore, an additional HF scav- covered the surface of the modied separator, as shown in
2
experimental procedure and mechanism for alcohol–ester separator was more compatible with polar electrolytes because
condensation are presented in ESI†). The SEM analysis (Fig. 2a the more hydrophilic TEOS layer was formed on the hydro-
and b) revealed that the morphology of the separator was quite phobic PE surface. The experiments for electrolyte uptake also
different: contrary to the bare PE, a uniformly covered layer was indicated higher compatibility of the PE–SiO
2
–TEOS separator:
clearly observed on the surface of the PE–SiO
2
–TEOS separator. a 165.6% increase in weight was observed for the absorbed
Fig. 1 Synthetic scheme for TEOS coated separator.
2 2
Fig. 2 SEM images for (a) bare (b) PE–SiO –TEOS, (c) EDS and elements mapping results for PE–SiO –TEOS.
2
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electrolyte in the separator, although the PE and PE–SiO
2
three kinds of separators, namely, bare PE separator (black), PE
separators only exhibited 74.3% and 138.8% increase in weight, coated with SiO (red), and PE coated with SiO -TEOS (blue).
2
2
respectively. Because cycle performance of LIBs is considerably The cycle performance of the cell with bare PE showed drastic
affected by their wettability with electrolyte, improved physical capacity fading, with only 45.8% retention aer 50 cycles. On
properties of the separator is expected to provide a positive the other hand, the cell assembled with a SiO
inuence on the LIB cycle performance. separator showed improved cycle performance with higher
Further FT-IR analysis provided informative spectroscopic retention (65.3%) aer 50 cycles. More interestingly, the cell
evidences for the formation of a new layer, as shown in Fig. 3. with a PE–SiO –TEOS separator showed much more improved
2
-coated PE
2
The FT-IR spectrum of the PE–SiO
2
–TEOS separator exhibits cycle performance, exhibiting a high retention of 80.7% aer 50
À1
peaks corresponding to Si–O–CH (880 cm ) and Si–O–Si (850– cycles. The observed outstanding electrochemical performance
3
À1
À1
7
90 cm , 478 cm ), which were formed as a result of free could be explained by considering the favorable interaction
alcohol capping on the interlayer. These results conrm the between HF and Si coated on the separator. The Si component
formation of a HF scavenging multilayer with TEOS groups on effectively scavenges HF and increases the structural stability of
the interlayer surface.
Prior to tests of the electrochemical performance of the performance. The superior electrochemical performance of the
modied separator, ionic conductivity and air permeability PE–SiO –TEOS separator when compared to that of the PE–SiO
2 4
the LiMn O cathode, thereby resulting in enhanced cycle
2
2
tests were performed to verify whether the TEOS layer formed separator could have resulted from the difference in the
on the surface had a signicant effect on the resistance of the chemical functionality developed on its surface. As shown in
separator (Fig. S3 and S4†). Because an additional layer on the Fig. S5a in ESI,† the SiO component in the interlayer could
2
separator can act as an extra resistance factor as a result of remove HF in the electrolyte as a result of the strong chemical
disturbances in ion migration during the electrochemical bonds formed in Si–F by nucleophilic substitution reaction of
À
process, an evaluation of these properties is necessary to esti- F . However, the presence of an increased number of alcohol
mate the electrochemical performance. According to the functional groups on PE–SiO
2
promoted the generation of HF by
(Fig. S5b†). This
means that these two kinds of chemical reactions competed
results, the ionic conductivity was slightly decreased in PE– a nucleophilic substitution reaction with LiPF
6
À1
À1
À1
SiO
2
–TEOS (0.242 mS cm ) compared to PE (0.291 mS cm
)
and PE–SiO (0.269 mS cm ). These results agree well with the during the electrochemical process (HF scavenging reaction vs.
2
results for air permeability tests: PE–SiO –TEOS took longer for HF generation reaction), leading to unremarkable cycle
2
3
1
00 cm of air permeation (311.2 s) than PE (239.1 s) and PE– performance. On the contrary, the free alcohol groups of the
SiO
2
(285.6 s). These results indicate the resistance of PE–SiO
2
–
2
interlayer on the PE–SiO –TEOS separator already participated
TEOS was slightly increased with formation of the TEOS layer, in the formation of new local bonds (O–Si–C, outer layer).
although it was conrmed that such increase would not lead to Therefore, there was less HF remaining in the electrolyte than in
serious changes in utilization of LIBs.
2
the case of the PE–SiO separator owing to remarkable cycle
To demonstrate the effect of the TEOS layer on the PE performance.
separator, electrochemical performance testing was conducted
These observations were further substantiated by ICP-MS
ꢀ
at an elevated temperature (60 C), with LiMn O4 as the analysis. The anode cycled with the PE–SiO –TEOS separator
2
2
cathode, graphite as the anode, and water-content-controlled showed lesser Mn-ion dissolution (anode: 321.5 ppm, separator:
electrolytes (1000 ppm) to accelerate HF generation in electro- 342.4 ppm) than with the bare PE separator (anode: 691.8 ppm,
lytes (Fig. 4). The electrochemical cells were assembled with
ꢀ
Fig. 4 Cycle performance results of graphite/LiMn
2
O
4
full cell at 60 C (a) bare
PE (black) (b) PE–SiO
2
(red) (c) PE–SiO
2
–TEOS (galvanostatically charged to 4.2 V
+
Fig. 3 Characterization of separator by FT-IR (black: bare PE, red: PE–SiO
This journal is ª The Royal Society of Chemistry 2013
2
–TEOS).
and discharged to 3.0 V vs. Li/Li with 0.5 C).
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separator: 636.9 ppm) and the PE–SiO
2
separator (anode: 401.5 functionalized TEOS group is consumed by the chemical reac-
ppm, separator: 537.3 ppm). Mn-ion dissolution is mainly tion with HF in the electrolyte, resulting in the formation of Si–F
caused by cathode deterioration due to the reaction with HF in bonding. The relatively low intensity of signal corresponding to
the electrolyte. Since the PE–SiO –TEOS separator is effective in Si–F in the PE–SiO2 separator indicates that the chemical
2
scavenging the existing HF in the electrolyte, it leads to less reactivity of the TEOS group is more desirable for trapping HF
dissolution of Mn ions from the cathode, thereby resulting in than SiO
2
in the interlayer. Based on the spectroscopic evidence,
cathodes at a detailed illustration of the plausible reaction mechanism
pertaining to the PE–SiO –TEOS separator is suggested in Fig. 6.
enhanced electrochemical performance of LiMn
higher temperatures.
2 4
O
2
The specic role of a functionalized separator was demon- Once the separator is exposed to the electrolyte consisting of
strated by comparing the FT-IR spectra of the separator before HF, protonation of oxygen takes place spontaneously by a
and aer cycling (Fig. 5). The main FT-IR peak corresponding to Bronsted acid–Lewis base reaction. Since the resulting oxonium
À1
the existence of a TEOS layer (Si–O–C, 880 cm , blue dot) intermediate is quite unstable, the remaining uoride anions
almost disappeared aer cycling. However, a new transmittance readily attack silicon to escape from the unstable oxonium state
À1 26–28
peak corresponding to Si–F bonding originated at 845 cm
.
by recovering two electrons from silicon (bond breakage of
This is an important spectroscopic evidence that explains the Si–O). Repetition of this procedure effectively scavenges HF in
action of a TEOS layer on the separator, along with the disap- the electrolyte, thereby preventing the deterioration of the
pearance of Si–O–C signals. According to the FT-IR results, the LiMn O cathode. Consequently, the cell with the TEOS-func-
2 4
tionalized separator showed improved cycle performance.
Further studies on this mechanism are underway.
In summary, we developed a novel synthetic methodology for
the facile chemical modication of a PE separator and evaluated
2 4
the cycle performance of LiMn O -cathode-based LIBs at high
temperatures. For effective scavenging of HF in the electrolyte,
the PE separator was modied by forming an interlayer con-
sisting of OH groups that was subsequently converted to a
siloxane chemical moiety (TEOS) via a simple alcohol–ester
condensation reaction. The cell composed of the PE–TEOS
separator showed excellent retention (80.7%) aer 50 cycles.
Systematic spectroscopic studies indicate favorable interaction
between the scavenging layer and HF. ICP-MS results conrm
lesser dissolution of Mn ions (anode and separator), and FT-IR
analysis clearly indicates the formation of Si–F bonding as a
result of a chemical reaction between the TEOS group and HF.
Accordingly, we have also suggested a plausible mechanism
underlying the chemical interaction between the PE–TEOS
separator and HF. We believe that the novel approach proposed
in this study will open up new opportunities to improve the
electrochemical performance of Mn-based cathodes.
ꢀ
Fig. 5 FT-IR analysis of separator after cycling at 60 C (black line: bare PE, red
line: PE–SiO , blue line: PE–SiO –TEOS, blue dot: PE–SiO –TEOS (before cycling)).
2
2
2
Fig. 6 Mechanistic consideration of HF scavenging by modified separator.
2
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This work was supported by the R&D program (Development 13 D. Capsoni, M. Bini, G. Chiodelli, V. Massarotti, C. B. Azzoni,
of KW Vanadium Redox Flow Battery Technology for Green IDC,
program no. 10041942) of Ministry of Trade, Industry and
Energy, Republic of Korea.
M. C. Mozzati and A. Comin, Phys. Chem. Chem. Phys., 2001,
3, 2162.
14 D. Capsoni, M. Bini, G. Chiodelli, P. Mustarelli,
V. Massarotti, C. B. Azzoni, M. C. Mozzati and L. Linati,
J. Phys. Chem. B, 2002, 106, 7432.
Notes and references
1
5 P. Aitchison, B. Ammundsen, D. J. Jones, G. Burns and
1
E. Hosono, T. Kudo, I. Honma and H. Matsuda, Nano Lett.,
009, 9, 1045.
D. K. Kim, P. Muralidharan, H.-W. Lee, R. Ruffo, Y. Yang, 17 A. Blyr, C. Sigala, G. G. Amatucci, D. Guyomard, Y. Chabre
J. Rozi `e re, Mater. Chem., 1999, 9, 3125.
16 J. C. Hunter, J. Solid State Chem., 1981, 39, 142.
2
2
C. K. Chan, H. Peng, R. A. Huggins and Y. Cui, Nano Lett.,
008, 8, 3948.
X. Li, F. Cheng, B. Guo and J. Chen, J. Phys. Chem. B, 2005,
09, 14017.
H.-W. Lee, P. Muralidharan, R. Ruffo, C. M. Mari, Y. Cui and
D. K. Kim, Nano Lett., 2010, 10, 3852.
and J.-M. Tarascon, J. Electrochem. Soc., 1998, 145, 194.
18 M. Y. Saidi, F. Gao, J. Barker and C. Scordilis-Kelley, US Pat.,
US5,846,673, 1998.
19 K. Takechi, A. Koiwai and T. Shiga, US Pat., US6,077,628,
2000.
2
3
4
5
6
7
8
9
1
20 K. Takechi and T. Shiga, US Pat., US6,235,431, 2001.
I. Belharouak, Y.-K. Sun, W. Lu and K. Amine, J. Electrochem. 21 T. N. M. Bernards, M. J. V. Bommel and J. A. J. Jansen, J. Sol-
Soc., 2007, 154, A1083.
Gel Sci. Technol., 1998, 13, 749.
O. K. Park, Y. Cho, S. Lee, H.-C. Yoo, H.-K. Song and J. Cho, 22 C. S. Lee, J. T. Baek, H. J. Yoo and S. I. Woo, J. Electrochem.
Energy Environ. Sci., 2011, 4, 1621.
Soc., 1996, 143, 1099.
T.-J. Kim, D. Son, J. Cho and B. Park, J. Power Sources, 2006, 23 V. Y. Davydov, A. V. Kiselev and L. T. Zhuravlev, Trans.
54, 268.
Faraday Soc., 1964, 60, 2254.
S. B. Park, S. M. Lee, H. C. Shin, W. I. Cho and H. Jang, J. 24 M. L. Hair, J. Non-Cryst. Solids, 1975, 19, 299.
1
Power Sources, 2007, 166, 219.
S. Komaba, T. Ohtsuka, B. Kaplan, T. Itabashi, N. Kumagai
and H. Groult, Chem. Lett., 2002, 31, 1236.
0 S. Komaba, T. Itabashi, T. Ohtsuka, H. Groult, N. Kumagai,
B. Kaplan and H. Yashiro, J. Electrochem. Soc., 2005, 152, A937.
25 A. Anedda, C. M. Carbonaro, F. Clemente, R. Corpino and
P. C. Ricci, J. Phys. Chem. B, 2003, 107, 13661.
26 E. Pretsch, P. Buhlmann and C. Affolter, Structure
determination of organic compounds, Springer, Germany, 3rd
edn, 2000.
1
1
1
1 S. Komaba, B. Kaplan, T. Ohtsuka, Y. Kataoka, N. Kumagai 27 A. N. Alcaraz, J. Codnia and M. L. Azc ´a rate, J. Photochem.
and H. Groult, J. Power Sources, 2003, 119–121, 378.
Photobiol., A, 2004, 165, 209.
2 M.-H. Ryou, G.-B. Han, Y. M. Lee, J.-N. Lee, D. J. Lee, 28 T. Kanashima, Y. Kurioka, T. Imai, H. Yamamoto and
Y. O. Yoon and J.-K. Park, Electrochim. Acta, 2010, 55, 2073.
M. Okuyama, Jpn. J. Appl. Phys., 1997, 36, 2460.
This journal is ª The Royal Society of Chemistry 2013
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