Introduction

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 [4]. 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 [8].

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 [9]. 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 [10]. 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.

Experimental section

Materials

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.

Characterization

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 conductivity

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.

Figure 1
figure 1

Ionic conductivities of PS-based composite SPEs. PS-based composite SPEs with 0, 5, 10, 15, and 20 wt.% Al2O3 and various salt contents.

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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 [10]. 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.

Figure 2
figure 2

Ionic conductivities of PS-based composite SPEs with different types of Al 2 O 3 .

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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 [11], 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 [25]. Assignments for bands observed in the symmetric SO3 stretching regions are summarized in Table 1 [18, 26].

Table 1 Some band assignments for triflate species
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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 3
figure 3

Fraction types of PS-based composite SPEs. (a) Free-ion fractions and (b) ion aggregate fractions of PS-based composite SPEs having 70 wt.% salt with various types of Al2O3.

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SEM images

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.

Figure 4
figure 4

SEM images of PS-based composite SPEs. PS-based composite SPEs with (a) 5 wt.%, (b) 10 wt.%, (c) 15 wt.%, and (d) 20 wt.% Al2O3 contents at a magnification of ×300.

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Conclusions

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.