Nanocomposite Polymer Electrolyte Doped with Nanosized
Li
0.1Ca
0.9TiO
3for Lithium Polymer Batteries
Lishi Wang, Wensheng Yang,
*
,zXingwang Li, and David G. EvansState Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
A poly共ethylene oxide兲 共PEO兲-based nanocomposite polymer electrolyte 共NCPE兲 doped with nanosized Li0.1Ca0.9TiO3, a lithium fast ionic conductor, has been developed. The ionic conductivity and lithium ion transference number of PEO12–LiClO4–Li0.1Ca0.9TiO3 NCPE are both enhanced by the addition of nanosized Li0.1Ca0.9TiO3, with a maximum ionic conductivity of 1.02⫻ 10−5 S cm−1at room temperature and a maximum lithium ion transference number of 0.533 at 70°C when the Li0.1Ca0.9TiO3content is 15 wt %. A broad electrochemical stability window suggests that the NCPE is a viable candidate for the electrolyte material in lithium polymer batteries.
© 2009 The Electrochemical Society. 关DOI: 10.1149/1.3257593兴 All rights reserved.
Manuscript submitted June 12, 2009; revised manuscript received October 7, 2009. Published November 4, 2009.
A solid-state lithium polymer battery has attracted the attention of many researchers because of its features, such as flexibility in the shape of the cell design, leak-proof electrolyte, and high safety. The key component in a solid-state lithium polymer battery is the poly-mer electrolyte. For a fully dried polypoly-mer electrolyte, poly共ethylene oxide兲 共PEO兲 has been considered one of the most favorable lithium conducting matrices. The main drawback of PEO-based polymer electrolytes is the relatively high crystallinity of PEO at room
temperature.1 This reduces the ionic conductivity of PEO-based
polymer electrolytes to a level that is too low to satisfy the general requirements of batteries or other practical electrochemical devices. To improve the ionic conductivity at ambient temperature, most re-search efforts have been dedicated to obtaining solid polymer elec-trolyte films containing a variety of stable amorphous phases; these afford us good flexibility of the polymer chains, which favors ion transport. To date, most of the successful work has been carried out using inorganic materials, such as ceramic and nano-oxides,2-7 lay-ered clays,8-10 organic–inorganic hybrid materials,11,12 and mi-croporous molecular sieves13-15as fillers to enhance the ionic con-ductivity of the polymer electrolytes. However, none of these inorganic fillers is an ionic conductor that introduces lithium ions into the polymer electrolyte. In an effort to further enhance the ionic conductivity, Wang et al.16introduced a lithium fast ionic conductor Li3−2x共Al1−xTix兲2共PO4兲3共x = 0.55–1.0兲, produced by a conventional solid-state reaction, into a PEO-based polymer electrolyte. The ionic conductivity of a PEO–LiClO4–Li1.3Al0.3Ti1.7共PO4兲3film with
eth-ylene oxide 共EO兲/Li = 8 reached a maximum of 7.985
⫻ 10−6 S cm−1at room temperature when the Li
1.3Al0.3Ti1.7共PO4兲3 content was 15 wt %. However, the modal grain size of the Li1.3Al0.3Ti1.7共PO4兲3powders is larger than 0.5 m, which may ad-versely affect the electrochemical properties of the resulting solid composite polymer electrolytes. Nanosized fillers have been chosen as additives in PEO-based composite polymer electrolytes and are effective in enhancing the ionic conductivity, lithium ion
transfer-ence number, and electrochemical stability.1,4 Nanosized
Li0.1Ca0.9TiO3 is a lithium fast ionic conductor just like
Li1.3Al0.3Ti1.7共PO4兲3. It has a higher conductivity 共4.53 ⫻ 10−4 S cm−1兲 17
as a solid electrolyte at room temperature than that of Li1.3Al0.3Ti1.7共PO4兲3共10−5–10−6 S cm−1兲.16Thus, the use of nanosized Li0.1Ca0.9TiO3as a filler has the potential to further im-prove the electrochemical properties of PEO-based solid composite polymer electrolytes.
In this work, the preparation of a PEO-based solid nanocompos-ite polymer electrolyte共NCPE兲 doped with nanosized Li0.1Ca0.9TiO3 is described. The effects of adding the nanosized Li0.1Ca0.9TiO3 filler on the crystallinity of the PEO phase as well as on the ionic
conductivity, lithium ion transference number, and electrochemical stability of the PEO-based polymer electrolyte are investigated.
Experimental
Nanosized Li0.1Ca0.9TiO3 powders were prepared by a sol–gel method according to Ref.17as follows: A stoichiometric amount of Ca共NO3兲2·4H2O and LiNO3 were dissolved in deionized water. Then, Ti共OC4H9兲4, absolute alcohol, and acetylacetone关the volume ratio employed was Ti共OC4H9兲4:absolute alcohol:acetylacetone = 4:4:1兴 were consecutively added dropwise into the mixture under constant stirring. After a reaction at 40°C for 4 h, a Li0.1Ca0.9TiO3 sol was obtained. The sol was kept in an oven at 80°C for 7 days to afford the Li0.1Ca0.9TiO3dried gel. The final powdered product was obtained by sintering the dried gel in air at 700°C for 2 h.
PEO with a molecular weight of 100,000 and LiClO4supplied by Alfa Aesar were dried under vacuum at 50 and 100°C, respectively, for at least 48 h before use. Nanosized Li0.1Ca0.9TiO3powders were heated under vacuum at 150°C for 48 h to remove water before use. Films were prepared by the conventional solution cast technique as follows: Various amounts of nanosized Li0.1Ca0.9TiO3 and PEO were dispersed in acetonitrile with the aid of ultrasonic dispersion, followed by the addition of PEO and LiClO4with a fixed关EO兴/关Li兴 molar ratio of 12.12,14The solution was stirred at room temperature for 24 h until a complete homogenization of the mixture had oc-curred. The slurry was then cast onto a self-designed Teflon plate, and the solvent was allowed to evaporate slowly under N2protection at room temperature for 24 h. Finally, the samples were dried under vacuum at 50°C for 48 h and kept in an argon-filled Unilab glove
box at room temperature. The films obtained were 150–200 m
thick. The NCPEs that contain Li0.1Ca0.9TiO3 were designated as PEO12–LiClO4− x wt % Li0.1Ca0.9TiO3, where 12 indicates the fixed关EO兴/关Li兴 molar ratio and x denotes the amount of the nano-sized Li0.1Ca0.9TiO3in the PEO with x = 0, 5, 10, 15, or 20.
X-ray diffraction共XRD兲 patterns of the samples were obtained
by using a Shimadzu XRD-6000 diffractometer with Cu K␣
radia-tion共40 kV and 30 mA兲 at a scanning rate of 5° min−1.
Differential scanning calorimetry共DSC兲 was employed to deter-mine the melting point共Tm兲 and glass transition temperature 共Tg兲 of the polymer electrolyte using a Netzsch differential scanning calo-rimeter 204 F1 instrument. The measurements were carried out at a
heating rate of 10°C min−1 from ⫺80 to 100°C. A flow of dry
nitrogen gas was maintained over the perforated pan to avoid any contact with atmospheric moisture. Sample weights were in the range of 3–5 mg, and an empty aluminum pan was used as the reference.
All IR absorption spectra were recorded on a Bruker Fourier
transform IR spectrometer 共model Vector 22兲 over the range of
4000–400 cm−1. For the measurements, the mixed slurry was cast
on a KBr wafer and dried via the same steps used in the preparation of the nanocomposite electrolyte films.
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z
E-mail: [email protected]
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Sample morphologies were investigated using a Hitachi S4700
field-emission-scanning electron microscope共FESEM兲.
The ionic conductivity of the samples was measured using ac impedance techniques after sandwiching the samples between two
stainless steel 共SS兲 blocking electrodes, which formed an SS/
NCPE/SS cell. The measurements were performed using an
electro-chemical workstation共IM6e, Germany兲 between 100 kHz and 10 Hz
at various temperatures ranging from 30 to 80°C. A thermostatic
bath共Julabo Labortechnik GmbH, Germany兲 was utilized to control
the temperature to within⫾0.1°C of the target value. The samples were thermally equilibrated at each temperature for at least 2 h before the measurements. The bulk resistance共Rb兲 was obtained by reading the intercept of the impedance spectrum, and the ionic con-ductivity was calculated from the expression
= L/共RbA兲 关1兴
where L is the thickness of the electrolyte film and A represents the electrode area.
The lithium ion transference number, TLi+, was evaluated using the method of ac impedance combined with the steady-state current technique, proposed by Vincent and co-workers.18,19The NCPE was sandwiched between two lithium-unblocking electrodes to form a symmetrical Li/NCPE/Li cell.
The electrochemical stability window of the NCPE was deter-mined by running a linear sweep voltammogram in three-electrode cells using SS as the working electrode and lithium as both the counter and the reference electrode. An IM6e electrochemical work-station was used with a scanning rate of 1 mV s−1.
All the above-mentioned cells were assembled and sealed in an
argon-filled Unilab glove box 共M. Braun Ltd., Germany兲 共O2
⬍ 1 ppm, H2O⬍ 1 ppm兲.
Results and Discussion
The XRD pattern of Li0.1Ca0.9TiO3powder sintered at 700°C for 2 h is shown in Fig.1. The appearance of the typical 111, 311, 400, and 222 reflection peaks indicates that Li0.1Ca0.9TiO3powders have crystallized well and formed a cubic perovskite structure, as
sug-gested in the literature.17 Figure 2 shows the FESEM image of
Li0.1Ca0.9TiO3 powder sintered at 700°C for 2 h. The powder is monodisperse, and the mean grain size is about 80 nm. These results confirm that the nanosized Li0.1Ca0.9TiO3 powders have been pre-pared.
Figure3displays the XRD patterns of pure PEO, polymer
elec-trolyte PEO12–LiClO4, and the PEO12–LiClO4
− 15 wt % Li0.1Ca0.9TiO3 NCPE. The characteristic diffraction
peaks of crystalline PEO are apparent between 2 = 15 and 30°
共Fig.3a兲.7These diffraction peaks become broader and less
promi-nent after the addition of LiClO4共Fig.3b兲 compared with those of pure PEO, indicating a decrease in the crystallinity of PEO. The characteristic peak intensities decrease further when nanosized Li0.1Ca0.9TiO3is added to form the NCPEs 共Fig.3c兲, which con-firms that nanosized Li0.1Ca0.9TiO3can effectively decrease the PEO crystallinity.
The glass transition temperature共Tg兲, melting temperature 共Tm兲, recrystallization enthalpy 共⌬Hm兲, and crystallinity 共Xc兲 values of
pure PEO and the polymer electrolytes PEO12–LiClO4
− x wt % Li0.1Ca0.9TiO3with x = 0, 5, 10, 15, and 20 are shown in TableI. TableIshows that the values of Tmand Xcboth decrease when LiClO4is introduced into the PEO matrix. The values of Tm and Xcdecrease further when the third component Li0.1Ca0.9TiO3is
added to the PEO12–LiClO4complex to form the NCPEs. The
val-ues of Tg, Tm, and Xc of PEO12–LiClO4− x wt % Li0.1Ca0.9TiO3 NCPEs all initially decreased markedly and subsequently increased slightly as the content of Li0.1Ca0.9TiO3was increased from 0 to 20 wt %. The observed decrease in Tm indicates that the addition of nanosized Li0.1Ca0.9TiO3 can inhibit the reorganization of PEO chains effectively and hence decrease the crystallization of PEO.7,16
The decrease in Tg indicates that the addition of nanosized
Li0.1Ca0.9TiO3can increase the flexibility of the PEO chains.20This may be because the presence of LixCa1−xTiO3 perturbs the PEO chain conformation and therefore introduces additional free space Figure 1. XRD pattern of the Li0.1Ca0.9TiO3powder.
Figure 2. FESEM image of the Li0.1Ca0.9TiO3powder.
Figure 3. XRD patterns of 共a兲 pure PEO, 共b兲 PEO12–LiClO4, and 共c兲 PEO12–LiClO4− 15 wt % Li0.1Ca0.9TiO3.
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between the polymer segments,4thus increasing PEO flexibility and lowering its Tg; this could be responsible for the enhanced ionic conductivity at a low temperature.
Figure4displays the IR spectra recorded for the pure PEO and PEO12–LiClO4− x wt % Li0.1Ca0.9TiO3NCPEs with x = 0, 5, and 15. The presence of a crystalline PEO phase is confirmed by the triplet peaks of the C–O–C stretching vibrations with maxima at about 1146, 1107, and 1062 cm−1.21In Fig.4, we can observe that these triplet peaks become broader when Li+salt is added into the PEO matrix, and the intensities of the peaks become weaker after the third component Li0.1Ca0.9TiO3is added. This is indicative of a reduced amount of crystalline PEO, consistent with the DSC results.
In the pure PEO sample, the CH2wagging mode peak共observed at
about 1350 cm−1 in amorphous PEO兲 is split into two peaks at
⬃1360 and ⬃1343 cm−1; this is further evidence for the presence of a crystalline PEO phase.21-23After the addition of the Li+salt, the
two peaks at⬃1360 and ⬃1343 cm−1become broader. When the
third component Li0.1Ca0.9TiO3is added, these two peaks disappear and only the peak at 1350 cm−1can be observed. This confirms that
the addition of nanosized Li0.1Ca0.9TiO3can effectively reduce the crystallization of PEO in PEO12–LiClO4− x wt % Li0.1Ca0.9TiO3. The perchlorate stretching vibration peaks共ClO4−兲 in the region 650–600 cm−1are frequently used to analyze ion–ion interactions in
PEO–LiClO4-based composite electrolytes. Salomon et al.
sug-gested that the共ClO4−兲 band centered between 630 and 635 cm−1is associated with the presence of contact-ion pairs, whereas the band centered at about 623 cm−1can be attributed to free ClO4−anions.21 In Fig.4b-d, only the peak at 623 cm−1characteristic of free ClO
4 − is apparent for all the PEO-based polymer electrolytes, indicating
that an abundance of free Li+ ions is present in each of these
electrolytes.21 The broader peaks characteristic of H2O at
⬃3500 cm−1are also observed for all the samples because exposure to moisture cannot be avoided during the preparation of the samples and the recording of the spectra.
The ionic conductivities of PEO12–LiClO4
− x wt % Li0.1Ca0.9TiO3, with x = 0, 5, 10, 15, and 20 at the tem-perature range of 30–80°C, are shown in Fig.5. The introduction of Li0.1Ca0.9TiO3powders has a marked effect on the ionic conductiv-ity of the NCPEs. The enhancement of ionic conductivconductiv-ity is most
pronounced at a low temperature 共⬍Tm兲. The ionic conductivity
first increases and then decreases with increasing Li0.1Ca0.9TiO3 content. The ionic conductivity of the NCPEs reaches a maximum
value of 1.02⫻ 10−5 S cm−1 at 30°C when the content of
Li0.1Ca0.9TiO3is 15 wt %. This value is about 100 times higher than that of PEO12–LiClO4at the same temperature. The enhancement of ionic conductivity of NCPEs at low temperature is mainly because the presence of nanosized Li0.1Ca0.9TiO3as a filler results in a larger reduction in crystallinity and more flexible local chains of PEO in the NCPE, as indicated by the reduced values of Tmand Tg. When Table I. Glass transition temperature„Tg…, melting temperature „Tm…, recrystallization enthalpy „⌬Hm…, and crystallinity „Xc… values of the polymer electrolytes PEO12–LiClO4− x wt % Li0.1Ca0.9TiO3with x = 0, 5, 10, 15, and 20.
Sample Tg 共°C兲 共°C兲Tm 共J g⌬Hm−1兲 Xca 共%兲 Pure PEO — 67.5 ⫺107.1 50.12 PEO12–LiClO4 ⫺19.4 64.1 ⫺79.02 36.97
PEO12–LiClO4− 5 wt % Li0.1Ca0.9TiO3 ⫺39.6 54.0 ⫺31.71 14.84
PEO12–LiClO4− 10 wt % Li0.1Ca0.9TiO3 ⫺45.5 50.7 ⫺26.39 12.35
PEO12–LiClO4− 15 wt % Li0.1Ca0.9TiO3 ⫺44.5 50.1 ⫺21.95 10.27
PEO12–LiClO4− 20 wt % Li0.1Ca0.9TiO3 ⫺42.0 51.7 ⫺33.82 15.82
aX
c=共⌬Hm/⌬Hmⴱ兲 ⫻ 100, where ⌬Hmⴱ = 213.7 J g−1.
20
Figure 4. IR spectra of 共a兲 pure PEO and PEO12–LiClO4
− x wt % Li0.1Ca0.9TiO3with共b兲 x = 0, 共c兲 x = 5, and 共d兲 x = 15.
Figure 5. Dependence of the ionic conductivities of PEO12–LiClO4 − x wt % Li0.1Ca0.9TiO3 NCPEs on Li0.1Ca0.9TiO3 loading x at different temperatures:共a兲 30, 共b兲 40, 共c兲 50, 共d兲 60, 共e兲 70, and 共f兲 80°C.
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the content of Li0.1Ca0.9TiO3in the NCPEs is further increased to 20 wt %, the ionic conductivity of the NCPEs decreases. This decrease in ionic conductivity can also be attributed to the change in the crystallinity of PEO in the NCPE. Because of the aggregation of Li0.1Ca0.9TiO3particles at relatively high loadings, the reduction in the crystallinity of PEO in the NCPE with 20 wt % Li0.1Ca0.9TiO3is smaller than that with 15 wt % Li0.1Ca0.9TiO3, as indicated by the DSC results. Furthermore, the ionic conductivities of NCPEs are also higher than PEO12–LiClO4at a high temperature共⬎Tm兲. It is possible that the presence of Li0.1Ca0.9TiO3 itself is an important factor in enhancing the ionic conductivity of NCPE; namely, the conduction occurs not only through amorphous PEO but also within the solid state Li0.1Ca0.9TiO3. This is suggested by the fact that at a high temperature, the ionic conductivities of the NCPEs increase with the increasing content of Li0.1Ca0.9TiO3.
The lithium ion transference numbers共TLi+兲 of PEO12–LiClO4
− x wt % Li0.1Ca0.9TiO3NCPEs measured at 70°C are shown in
Table II. It can be seen in Table II that the TLi+ value of
PEO12–LiClO4 is relatively low. A possible reason is that Li+ can coordinate not only to the ether oxygen atoms in PEO but also to the oxygen atoms in ClO4−, which restricts its transport ability.24After the addition of Li0.1Ca0.9TiO3, the TLi+ value of PEO12–LiClO4 − x wt % Li0.1Ca0.9TiO3NCPE first increases and then decreases with increasing Li0.1Ca0.9TiO3content from 0 to 20 wt %. The in-crease in TLi+with the increasing content of Li0.1Ca0.9TiO3up to 15 wt % can be attributed to two factors: Li0.1Ca0.9TiO3not only com-plexes strongly with PEO and results in the release of more free Li+ ions but also provides abundant free Li+ions in its own right. There-fore, a high concentration of mobile charge carriers in the amor-phous phase results.16The decrease in TLi+observed with the further increase in Li0.1Ca0.9TiO3content from 15 to 20 wt % may be due to the aggregation of Li0.1Ca0.9TiO3 particles at relatively high load-ings.
Figure6shows the results of linear sweep voltammetry measure-ments at 70°C for cells prepared with the NCPEs. In Fig.6, we can see that PEO12–LiClO4–Li0.1Ca0.9TiO3NCPEs exhibit a good
elec-trochemical stability关above 5.5 V 共vs Li+/Li兲兴. The good chemical stability suggests that these NCPEs are candidate electro-lyte materials for rechargeable lithium polymer batteries.
Conclusion
A PEO-based NCPE has been obtained by the addition of nano-sized Li0.1Ca0.9TiO3 as the filler. Analysis by XRD, DSC, and IR shows that the nanosized Li0.1Ca0.9TiO3can reduce the crystalliza-tion of PEO very effectively, resulting in an obvious enhancement of the ionic conductivity for PEO12–LiClO4–Li0.1Ca0.9TiO3 NCPEs. The ionic conductivity of the NCPEs reaches a maximum value for
a 15 wt % loading of Li0.1Ca0.9TiO3, with a value of 1.02
⫻ 10−5 S cm−1 at room temperature. The presence of nanosized
Li0.1Ca0.9TiO3can also enhance the lithium ion transference number of the NCPEs. The good lithium transport properties, combined with
a high decomposition voltage, suggest that
PEO12–LiClO4–Li0.1Ca0.9TiO3 NCPE is a viable candidate for the electrolyte material in solid-state rechargeable lithium polymer bat-teries.
Acknowledgments
This work was supported by the National Natural Science Foun-dation of China, the Ministry of Science and Technology High
Tech-nology Development共863兲 Plan 共grant no. 2006AA03Z343兲, the 111
Project共grant no. B07004兲, and the Program for New Century
Ex-cellent Talents in Universities共grant no. NCET-08-0713兲.
Beijing University of Chemical Technology assisted in meeting the pub-lication costs of this article.
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Table II. Lithium ion transference number of PEO12–LiClO4− x wt % Li0.1Ca0.9TiO3NCPEs with x = 0, 5, 10, 15, and 20 at 70°C.
x wt % 0 5 10 15 20
Lithium ion transference number共TLi+兲 0.242 0.384 0.471 0.533 0.457
Figure 6. Current–voltage response of PEO12–LiClO4
− x wt % Li0.1Ca0.9TiO3NCPEs at 70°C on an SS electrode as the working electrode:共a兲 x = 5, 共b兲 x = 10, 共c兲 x = 15, and 共d兲 x = 20. Scanning rate: 1 mV s−1.
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