In a common salt-in-polymer electrolyte, a polymer which has polar groups in the molecular chain is necessary because the polar groups dissolve lithium salt and coordinate cations. Based on the above point of view, polystyrene [PS] that has nonpolar groups is not suitable for the polymer matrix. However, in this PS-based composite polymer-in-salt system, the transport of cations is not by segmental motion but by ion-hopping through a lithium percolation path made of high content lithium salt. Moreover, Al2O3 can dissolve salt, instead of polar groups of polymer matrix, by the Lewis acid-base interactions between the surface group of Al2O3 and salt. Notably, the maximum enhancement of ionic conductivity is found in acidic Al2O3 compared with neutral and basic Al2O3 arising from the increase of free ion fraction by dissociation of salt. It was revealed that PS-Al2O3 composite solid polymer electrolyte containing 70 wt.% salt and 10 wt.% acidic Al2O3 showed the highest ionic conductivity of 9.78 × 10-5 Scm-1 at room temperature.
A lithium secondary battery using solid polymer electrolyte [SPE] is an attractive energy source for portable devices since the use of SPE makes the fabrication of safe batteries possible and permits the development of thin batteries with design flexibility. Most of the efforts to date have focused on poly(ethylene oxide) [PEO] as the host material for SPE [1–3]. However, it has a major drawback of having a low ionic conductivity (10-8 to 10-5 Scm-1) at room temperature . Thus, many researchers [5–7] have focused on the SPE consisting of the polymer with low glass transition temperature [Tg] and moderate concentrations of salt in order to overcome the low ionic conductivity of SPE, but high ambient conductivity has not yet been reached. Low ionic conductivity can be achieved from the fact that the ionic mobility strongly depends on the polymer segmental motion and that the cation transport number is low in the SPE at a high salt concentration. Therefore, new materials with unconventional conduction mechanisms are clearly needed .
In common SPEs, a polymer which has polar groups in the chain is necessary for electrolyte formation. The polar groups dissolve lithium salt and coordinate cations. The cations can move between coordinating sites in one chain or in neighboring chains, promoted by the segmental motion . From this point of view, a polymer which has nonpolar groups is not suitable for the polymer matrix in common SPEs. However, in this new composite SPE consisting of polystyrene [PS] and having nonpolar groups, LiCF3SO3 and Al2O3 with polymer-in-salt system, the transport of cations is done by ion-hopping through an ion percolation path made of high content lithium salt instead of segmental motion. Moreover, Al2O3 can cause conductivity enhancement depending on the nature of the filler surface group . In this work, the ionic conductivity of PS-Al2O3 composite SPE according to the salt content was checked, and the effect of Al2O3 type and content on ion conduction properties in PS-based composite SPE was investigated.
Polystyrene (Sigma-Aldrich Corporation, St. Louis, MO, USA) with a number average molecular weight (M n ) of 170,000 was used as received without undergoing further purification process. As salt, LiCF3SO3 (Sigma-Aldrich Corporation) was dried and stored in a desiccator under nitrogen. Three types of aluminum oxides [Al2O3] (Sigma-Aldrich Corporation) with acidic, neutral, and basic surface groups as fillers were also used. As an organic solvent, N-butyl acetate (Junsei Chemical Co., Ltd., Chuo-ku, Tokyo, Japan) was used in order to dissolve the materials.
Preparation of PS-based composite SPE films
An appropriate amount of PS was introduced into N-butyl acetate and stirred for 24 h; after that, a definitive amount of LiCF3SO3 was added to the solution and stirred again for 24 h. At the same time of the PS/LiCF3SO3 solution preparation, 5, 10, 15, and 20 wt.% Al2O3 were added to N-butyl acetate. Then, the solution was sonicated for 10 min and stirred for 24 h for dispersion. PS-based SPE was prepared by mixing the PS/LiCF3SO3 and Al2O3 solutions for 4 days. The solutions were directly cast on 3 × 3 cm2 stainless steel plates after mixing and then allowed to dry in a vacuum oven for 5 days at 40°C.
Ionic conductivity of the sample was measured by Gamry Instruments' (Warminster, PA, USA) Reference 600 impedance analyzer. Deconvolution of the composite bands of the Fourier transform infrared [FT-IR] spectra was accomplished by the best fits of constituent Gaussian peaks, and the fractions of salt forms were calculated by the peak fitting program of Origin 7.0 software (OriginLab Corporation, Northampton, MA, USA) to analyze the change of salt forms in SPEs. Scanning electron microscopy [SEM] was also used to observe the morphology of the specimen.
Results and discussion
Ionic conductivities of PS-based composite SPEs with salt content
Figure 1 shows the ionic conductivities of PS-based SPEs with 0, 5, 10, 15, and 20 wt.% Al2O3 and various salt contents. In the common SPEs, the ionic conductivity increased with the salt content up to its peak and then the ionic conductivity decreased because the polymer mobility decreased and Tg of the polymer increased as the salt content increased. However in PS-based composite SPEs, there is no decrease in the ionic conductivity because PS does not contribute to the dissociation of salts and transport of cations. The transport of cations is done by ion-hopping through the ion percolation path made of high content of salt instead of segmental motion, so the ionic conductivities increased with salt content.
Moreover, Al2O3 can dissolve the lithium salt instead of the polar groups in the polymer matrix using Lewis acid-base interactions between the surface group of Al2O3 and salt . The sample consisting 70 wt.% salt and 10 wt.% Al2O3 shows the highest ionic conductivity of 5.83 × 10-5 Scm-1.
Ionic conductivity of PS-based composite SPE according to Al2O3 type and content
Figure 2 shows the ionic conductivities of PS-based composite SPEs consisting 70 wt.% salt and different types of Al2O3. The ionic conductivity increased up to 10 wt.% Al2O3 and then decreased, irrespective of the Al2O3 type. As the content of Al2O3 increased over 10 wt.%, the aggregates of Al2O3 was observed in all types of Al2O3. This is related to the decrease in ionic conductivity at above 10 wt.% Al2O3. Among the samples, the maximum ionic conductivity was found for SPE with acidic Al2O3, and the ionic conductivity decreased in the order of SPEs with acidic, neutral, and basic Al2O3. This tendency may be related to the number of free ions by dissociation of salt. Salt can be dissociated by the interaction between salt anions and surface OH groups of Al2O3. Acidic Al2O3 has had the most OH groups which interact with salt, so the SPE having acidic Al2O3 can have the highest free-ion numbers. Neutral Al2O3 has had the second amount of surface OH groups, and basic Al2O3 has had the least surface OH groups. Thus, the ionic conductivity decreased in that order, and the highest ionic conductivity of 9.78 × 10-5 Scm-1 could be obtained at 10 wt.% acidic Al2O3.
FT-IR evidence of dissociated ions
In the polymer-in-salt system, ion clouds made of ion aggregates and ion pairs play an important role in ion conduction , so it is necessary to investigate the change of salt form of the SPE films as salt concentrations increase. FT-IR spectra have confirmed the presence of ion pairs and aggregates in SPEs based upon poly(propylene oxide) [12–16] and PEO [17–24]. A clear distinction between free (dissociated) ions, contact ion pairs, and more aggregates may be observed in the vibrational spectra of the internal mode of anions, such as the triflate anion [Tf-]. Ion association occurs at the SO3 end of the anion; thus, the symmetric SO3 stretching mode is highly sensitive to change in the coordination state of the anion. Band fitting of these regions has provided information pertaining to the types of aggregation and strengths of ionic interactions occurring in the SPEs.
Above a certain salt concentration, the symmetric SO3 stretching mode is found to consist of two or more peaks. The different anion environments may be attributed to ion association, in consideration of the nondegenerate A1 symmetry of this mode. The symmetry of an anion is lowered by coordination to a cation. Band fitting of the SO3 regions reveals the peak components which arise from various ion aggregates. Higher frequency components, corresponding to higher aggregates, may be observed with further increase in salt concentration . Assignments for bands observed in the symmetric SO3 stretching regions are summarized in Table 1 [18, 26].
When Al2O3 is added, the anions have greater affinity toward the Al2O3 surface acid groups than the cations. Due to the polarizability of the Tf-, a strong affinity can be expected between the Tf- and the Al2O3 surface acid groups. It results in the dissociation of the salt and makes the cations free [10, 27]. The free-ion and ion aggregate fractions of PS-based composite SPEs having 70 wt.% salt with various types of Al2O3 are shown in Figure 3. As shown in Figure 3, for all cases, the free-ion fractions of SPE consisting 70 wt.% salt increased until the content of Al2O3 reached 10 wt.%. By adding more Al2O3, the free-ion fraction decreased. SPE with acidic Al2O3 having the most OH groups which interact with salt had the highest free-ion fraction. The free-ion fraction decreased in the order of SPEs with acidic, neutral, and basic Al2O3. This tendency was similar to that of the ionic conductivity. From this result, it could be known that the ionic conductivity was mainly influenced by the free-ion fraction. Also, the highest ionic conductivity could be obtained in the case of SPE with 10 wt.% acidic Al2O3 whose free-ion fraction was the highest and ion aggregate fraction was the lowest.
Figure 4 shows the SEM images of PS-based composite SPEs with Al2O3 content. From the SEM images, we can see that the fillers are well dispersed, and there are no aggregates of Al2O3 until 10 wt.% Al2O3 is reached, but as more Al2O3 was added, the aggregates of fillers which might disturb the ion transport were observed. Thus, the ionic conductivity of PS-based composite SPEs, as shown in Figure 2, increased up to 10 wt.% Al2O3, and then the ionic conductivity decreased.
Composite SPEs based on PS, LiCF3SO3, and Al2O3 were prepared, and the effect of the Al2O3 type and content on ion conduction properties of SPEs was investigated. As the salt content increased, the ionic conductivities increased continuously. In contrary of the common SPE, PS-based composite SPE has no decrease in ionic conductivity because PS does not contribute to the dissociation of salts and transport of cations. As the Al2O3 content increased, the ionic conductivity increased until the content of Al2O3 reached 10 wt.%. Then, the sample consisting 70 wt.% salt and 10 wt.% Al2O3 shows the highest ionic conductivity of 5.83 × 10-5 Scm-1. The maximum ionic conductivity was found in SPE with acidic Al2O3. The ionic conductivity decreased in the order of SPEs with acidic, neutral, and basic Al2O3. This tendency may be related to the number of free ions by dissociation of salt. The SEM images show that the fillers are well dispersed, and there is no aggregate of fillers until 10 wt.% Al2O3 is reached. On the other hand, as more Al2O3 contents were added, the aggregates of fillers appeared. It seems that the aggregates of fillers disturb the ion transport, so the ionic conductivity increased up to 10 wt.% Al2O3, and then the ionic conductivity decreased. The highest ionic conductivity of 9.78 × 10-5 Scm-1 could be obtained at 10 wt.% acidic Al2O3.
Fenton DE, Parker JM, Wright PV: Complexes of alkali metal ions with poly(ethylene oxide). Polymer 1973, 14: 589.
Wright PV: Electrical conductivity in ionic complexes of poly (ethylene oxide). Br Polym J 1975, 7: 319–327. 10.1002/pi.4980070505
Armand MB, Chabagno JM, Duclot MJ: Fast Ion Transport in Solids. Amsterdam: Elsevier; 1979.
Kim S, Park SJ: Interlayer spacing effect of alkylammonium-modified montmorillonite on conducting and mechanical behaviors of polymer composite electrolytes. J Colloid Interface Sci 2009, 332: 145–150. 10.1016/j.jcis.2008.12.006
Lee JY, Lee YM, Bhattacharya B, Nho YC, Park JK: Solid polymer electrolytes based on crosslinkable polyoctahedral silsesquioxanes (POSS) for room temperature lithium polymer batteries. J Solid State Electrochem 2010, 14: 1445–1449. 10.1007/s10008-009-0968-1
Jia J, Lia B, Zhong WH: Simultaneously enhancing ionic conductivity and mechanical properties of solid polymer electrolytes via a copolymer multi-functional filler. Electrochim Acta 2010, 55: 9075–9082. 10.1016/j.electacta.2010.08.036
Jeong SK, Jo NJ: The effect of silicate layers on electrochemical properties in nanocomposite solid polymer electrolytes. Key Eng Mater 2007, 336–338: 526–529.
Wei X, Shriver DF: Highly conductive polymer electrolytes containing rigid polymers. Chem Mater 1998, 10: 2307–2308. 10.1021/cm980170z
Gray FM: Polymer Electrolytes: Electroactive Polymer Materials. Biggleswade: Turpin Distribution Services Ltd; 1997.
Jayathilaka PARD, Dissanauake MAKL, Albinsson I, Mellander BE: Effect of nano-porous Al2O3on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system. Electrochim Acta 2002, 47: 3257–3268. 10.1016/S0013-4686(02)00243-8
Jeong SK, Jo YK, Jo NJ: Decoupled ion conduction mechanism of poly(vinyl alcohol) based Mg-conducting solid polymer electrolyte. Electrochim Acta 2006, 52: 1549–1555. 10.1016/j.electacta.2006.02.061
Bernson A, Lindgren J: Free ions and ion pairing/clustering in the system LiCF3SO3---PPOn. Solid State Ionics 1993, 60: 37–41. 10.1016/0167-2738(93)90271-4
Schanta S, Torell LM: Evidence of dissolved ions and ion pairs in dilute poly (propylene oxide)-salt solutions. Solid State Ionics 1993, 60: 47–53. 10.1016/0167-2738(93)90273-6
Kakihani M, Schanta S, Torell LM: Raman spectroscopic study of ion-ion interaction and its temperature dependence in a poly(propylene-oxide)-based NaCF3SO3-polymer electrolyte. J Chem Phys 1990, 92: 6271–6277. 10.1063/1.458351
Lundin A, Jacobsson P: Effect of high pressure on volume and ion-ion interaction in poly (propylene glycol) complexed with LiCF3SO3. Solid State Ionics 1993, 60: 43–46. 10.1016/0167-2738(93)90272-5
Schantz S, Sandfhal J, Borjesson L, Torell LM, Stevens JR: Ion pairing in polymer electrolytes; a comparative Raman study of NaCF3SO3complexed in poly(propylene-glycol) and dissolved in acetonitrile. Solid State Ionics 1988, 28–31: 1047–1053.
Wensjo A, Lindgren J, Thomas JO, Farrington GC: The effect of temperature and concentration on the local environment in the system M(CF3SO3)2PEOnfor M = Ni, Zn and Pb. Solid State Ionics 1992, 53–56: 1077–1082.
Bernson A, Lindgren J: Free ions and ion pairing/clustering in the system LiCF3SO3--PPOn. Solid State Ionics 1993, 60: 37–41. 10.1016/0167-2738(93)90271-4
Yang L, Lin J, Wang Z, Zhou R, Liu Q: Effects of plasticizers on properties of poly(ethylene oxide) complex electrolytes. Solid State Ionics 1990, 40–41: 616–619.
Kakihani M, Schanta S, Torell LM: Dissociated ions and ion-ion interactions in poly(ethylene oxide) based NaCF3SO3complexes. Solid State Ionics 1990, 40–41: 641–644.
Petersen G, Torell LM, Panero S, Scosati B, da Silva CJ, Smith M: Ionic interactions in MCF3SO3-polyether complexes containing mono-, di- and trivalent cations. Solid State Ionics 1993, 60: 55–60. 10.1016/0167-2738(93)90274-7
Papke BL, Ratner MA, Shriver DF: Vibrational spectroscopic determination of structure and ion pairing in complexes of poly(ethylene oxide) with lithium salts. J Electrochem Soc 1982, 129: 1434–1438. 10.1149/1.2124179
Huang W, Frech R: Dependence of ionic association on polymer chain length in poly(ethylene oxide)-lithium triflate complexes. Polymer 1994, 35: 235–242. 10.1016/0032-3861(94)90684-X
Huang W, Frech R, Wheeler RA: Molecular structures and normal vibrations of trifluoromethane sulfonate (CF3SO3-) and its lithium ion pairs and aggregates. J Phys Chem 1994, 98: 100–110. 10.1021/j100052a018
Bishop AG, MacFarlane DR, McNaughton D, Forsyth M: FT-IR investigation of ion association in plasticized solid polymer electrolytes. J Phys Chem 1996, 100: 2237–2243. 10.1021/jp9520456
Lindin A, Jacobsson P: Effect of high pressure on volume and ion-ion interaction in poly (propylene glycol) complexed with LiCF3SO3. Solid State Ionics 1993, 60: 43–46. 10.1016/0167-2738(93)90272-5
Kumar B, Scanlon LG: Polymer-ceramic composite electrolytes: conductivity and thermal history effects. Solid State Ionics 1999, 124: 239–254. 10.1016/S0167-2738(99)00148-4
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2011-0026644).
The authors declare that they have no competing interests.
YJL and YHA carried out the measurements and analysis of data. NJJ commented on various points and drafted the manuscript. All authors read and approved the final manuscript.
Yu-Jeong Lim, Yu-Ha An contributed equally to this work.
Authors’ original submitted files for images
Below are the links to the authors’ original submitted files for images.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License ( https://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Lim, YJ., An, YH. & Jo, NJ. Polystyrene-Al2O3 composite solid polymer electrolyte for lithium secondary battery. Nanoscale Res Lett 7, 19 (2012). https://doi.org/10.1186/1556-276X-7-19
- solid polymer electrolyte
- polymer-in-salt system
- lithium secondary battery