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Novel PEO-based composite solid polymer electrolytes incorporated with active inorganic–organic hybrid polyphosphazene microspheres
Materials Chemistry and Physics 121 (2010) 511–518
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Novel PEO-based composite solid polymer electrolytes incorporated with active inorganic–organic hybrid polyphosphazene microspheres Jiawei Zhang a,b , Xiaobin Huang a,∗ , Jianwei Fu a,b , Yawen Huang a,b , Wei Liu a,b , Xiaozhen Tang b a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China School of Chemistry and Chemical Engineering, National Key Laboratory of Metallic Matrix Composite Material, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b
a r t i c l e
i n f o
Article history: Received 3 September 2009 Received in revised form 11 January 2010 Accepted 9 February 2010 Keywords: Composite polymer electrolytes Polyphosphazene microspheres Hydroxyl groups Ionic conductivity Lithium ion transference number
a b s t r a c t Inorganic–organic hybrid poly(cyclotriphosphazene-co-4,4 -sulfonyldiphenol) (PZS) microspheres with active hydroxyl groups were incorporated in poly(ethylene oxide) (PEO), using LiClO4 as a dopant salt, to form a novel composite polymer electrolyte (CPE). The polymer chain flexibility and crystallinity properties are studied by DSC. The effects of active PZS microspheres on the electrochemical properties of the PEO-based electrolytes, such as ionic conductivity, lithium ion transference number, and electrochemical stability window are studied by electrochemical impedance spectroscopy and steady-state current method. Maximum ionic conductivity values of 3.36 × 10−5 S cm−1 at ambient temperature and 1.35 × 10−3 S cm−1 at 80 ◦ C with 10 wt.% content of active PZS microspheres were obtained and the lithium ion transference number was 0.34. The experiment results showed that the inorganic–organic hybrid polyphosphazene microspheres with active hydroxyl groups can enhance the ionic conductivity and increase the lithium ion transference number of PEO-based electrolytes more effectively comparing with traditional ceramic fillers such as SiO2 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid polymer electrolytes (SPEs) play an important role in the development of new energy sources, like solid-state batteries, fuel cells, sensors and electrochemical displays [1–3]. Poly(ethylene oxide) (PEO) is one of the most promising SPEs [4]. However, due to the poor ionic conductivity of PEO-based polymer electrolytes at ambient operating temperature, various modifications have been done [5]. A group of materials, such as ceramic powders [6,7], organic acid [8], and organic/inorganic composites [9], was added to PEO to prepare composite solid polymer electrolytes. These inert fillers such as SiO2 [10–12] and Al2 O3 [5–7] were incorporated to polymer electrolytes to improve the properties such as ionic conductivity and mechanical properties [13]. A great enhancement in the conductivity of the PEO-based composite polymer electrolytes was reported by using the combined action of a large surface area and the Lewis acid character of the ceramic additive [14]. The Lewis acid–base interaction plays a vital role in the enhancement of ionic conductivity, electrochemical stability and interfacial stability of
∗ Corresponding author. Tel.: +86 21 54747142; fax: +86 21 54741297. E-mail address: [email protected] (X. Huang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.02.016
the composite polymer electrolytes. But those ceramic fillers are mostly inorganic and easy to aggregate because of the poor compatibility between PEO chains and fillers. Hexachlorocyclotriphosphazenes (HCCPs), as very important inorganic rings, play a crucial role in the development of new polymers because of the excellent tunability of the backbone and the unprecedented structural diversity [15,16]. Recently, our group demonstrates for the first time the preparation of poly(cyclotriphosphazene-co-4,4 -sulfonyldiphenol) (PZS) nanotubes [17] and fully crosslinked PZS microspheres via precipitation polymerization [18]. The PZS microspheres with active hydroxyl groups have also been prepared by our group [19]. In our previous studies, PEO-based solid polymer electrolytes with plain PZS microspheres showed better electrochemical properties compared to traditional ceramic fillers such as SiO2 . The inorganic–organic hybrid structure of active PZS microspheres also offers good compatibility between the fillers and PEO polymer chains. Together with the active hydroxyl groups on the surface, PZS microspheres may have higher ionic conductivity and become a new class of fillers in the development of composite solid polymer electrolytes. In this study, composite polymer electrolytes based on PEO were prepared by using LiClO4 as a dopant salt and PZS microspheres with active hydroxyl groups as fillers in order to study the effects of active PZS microspheres on performances of CPEs.
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Scheme 1. Synthetic route and chemical structure of active PZS microspheres.
2. Experimental 2.1. Materials PEO, Mw = 200,000 (Shanghai Chemical Regent Company) and LiClO4 , A.R. (Shanghai Chemical Regent Company), were vacuum dried for 24 h at 50 ◦ C and 120 ◦ C, respectively. Acetonitrile, A.R. (Shanghai Chemical Regent Company), anhydrous by 4A molecular sieves before use. HCCP (synthesized as described in literature [20]) was recrystallized from dry hexane followed by two times sublimation. The melting point of purified HCCP was 113–114 ◦ C. 4,4 -Sulfonyldiphenol (BPS), triethylamine (TEA) and SiO2 (average diameter 500 nm) were purchased from Shanghai Chemical Regent Company and used without further purification. 2.2. Preparation of PZS microspheres with active hydroxyl groups via one-step precipitation polymerization
stored in an oven under nitrogen atmosphere coupled with a temperature controller and for each temperature at least 30 min was allowed before the impedance response was recorded. Then the impedance tests were carried out in the 1 MHz to 1 Hz frequency range using a Solartron 1260 Impedance/Gain-Phase Analyzer coupled with a Solartron 1287 Electrochemical Interface and the signal amplitude for ac impedance spectroscopy is 10 mV. The equivalent circuit of the electrochemical cell impedance measurement is shown in Fig. 1a, where R1 is the resistance of cell besides the composite polymer electrolytes. Rp and Cp are the resistance and capacitance of the polymer electrolytes. Cd is the interface capacitance at the stainless steel blocking electrodes. The equation for calculating the conductivity is = (1/Rp) × (d/S), where d is the thickness of the polymer electrolytes and S is the area of the stainless steel blocking electrodes. The bulk resistance (Rp) was obtained from the equivalent circuit by fitting the data of the ac impedance using ZPlot (Scribner Associates, Inc.) software. Fig. 1b shows the ac impedance spectroscopy for SS/PEO10 –LiClO4 /SS cell at 25 ◦ C. Lithium ion transference number (TLi+ ) measurements were evaluated using the method of ac impedance combined with steady-state current technique proposed by Vincent and Bruce [21–23], which involves a combination of ac- and
The preparation of PZS microspheres with active hydroxyl groups (PZSMS–OH) was carried out as follows (Scheme 1) [19]: triethylamine (0.87 g, 8.64 mmol) was added to a solution of BPS (1.29 g, 5.18 mmol) in acetone (80 ml), and HCCP (0.50 g, 1.44 mmol) in 20 ml acetone was added drop by drop into the solution. The reaction mixtures were stirred in an ultrasonic bath at room temperature for 3 h. The resultant particles were obtained by centrifugation and then were washed three times using acetone and deionized water. The solid was dried under vacuum to yield PZS active microspheres directly as white powders. 2.3. The preparation of CPEs The preparation of CPEs involved first the dispersion of the fillers and LiClO4 by ultrasonic in anhydrous acetonitrile at room temperature and the addition of the PEO/acetonitrile solution. The resulting slurry was cast onto a Teflon plate and then the plate was under the sweep of air in order to let the solvent evaporate. Finally, the obtained films were dried under vacuum at 50 ◦ C for 24 h to remove the residue solvent. These procedures yielded transparent homogenous films. The average thickness of the polymer electrolytes is about 100 m. All the CPE films were stored in an argon atmosphere glove box before test. The CPE samples used in this study were denoted as PEO10 –LiClO4 /x% filler, in which the EO/Li ratio was fixed to 10 for all samples and the content of filler, x, ranged from 0 wt.% to 30 wt.% of the PEO weight. 2.4. Characterization and instruments SEM images of the surface of CPEs were observed on a JEOL JSM-7401F field emission SEM instrument with gold sputtered-coated films. Differential scanning calorimeter (DSC) measurements were carried out on a PerkinElmer Pyris-1 analyzer. The measurements were carried out at a heating rate of 10 ◦ C min−1 from −60 ◦ C to 100 ◦ C in the heating cycle and from 100 ◦ C to −20 ◦ C in the cooling cycle. Ionic conductivity of the CPEs was determined by ac impedance spectroscopy. The film was sandwiched between two stainless steel (SS) blocking electrodes (with a diameter of 1 cm) to form a symmetrical SS/CPE/SS cell. The stack SS/CPE/SS was
Fig. 1. (a) Equivalent circuit of the electrochemical cell impedance measurement and (b) ac impedance spectra for SS/PEO10 –LiClO4 /SS cell at 25 ◦ C.
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dc-measurements. The method consists of initial measurement of the lithium interfacial resistance (R0 ) by impedance spectroscopy in the 100 kHz to 0.1 Hz frequency range, then application of a small voltage (<10 mV) until a steady current (Iss ) was obtained (time = 3000 s), and finally measurement of the interfacial resistance (Rss ) by impedance spectroscopy in the 100 kHz to 0.1 Hz frequency range. According to this method, the lithium ion transference number could be calculated with the following equation TLi+ =
Iss I0
V − I0 R0 V − Iss Rss
where V is the voltage pulse, R0 is the initial lithium interfacial resistance, and Rss is the secondary lithium interfacial resistance (as the steady-state polarization current is reached). I0 is the initial current and Iss is the current reached in the steady-state. The parameters for the calculation can be obtained from the impedance response and steady-state current response. Measurements were performed using a Solartron 1260 Impedance/Gain-Phase Analyzer coupled with a Solartron 1287 Electrochemical Interface. The CPE was sandwiched between two lithium-unblocking electrodes to form a symmetrical Li/CPE/Li cell and was closed in a coin cell. The cell was assembled and sealed in an argon-filled UNILAB glove box (O2 < 1 ppm; H2 O < 0.1 ppm). Electrochemical stability window of the CPEs were determined by running a linear sweep voltammetry in three electrode cells using stainless steel as the blocking working electrode, lithium as both the counter and the reference electrode and the CPE film as the electrolyte. A Solartron 1287 Electrochemical Interface was used to run the voltammetry at a scan rate of 1 mV s−1 . Cyclic voltammogram (CV) of the composite polymer electrolytes was run with a symmetrical Li/CPE/Li cell closed in a coin cell and recorded using a Solartron 1287 Electrochemical Interface at a scan rate of 5 mV s−1 .
3. Results and discussion 3.1. Surface morphology Fig. 2 shows the SEM microphotograph of the PZS microspheres with active hydroxyl groups. It can be seen from the SEM images
Fig. 2. SEM image of PZSMS–OH microspheres.
that the average diameter of the microspheres is about 0.5 m and the size of the microspheres is almost the same. Fig. 3 shows the SEM microphotographs of various CPEs. The pristine PEO10 –LiClO4 film exhibits a rough surface with some rumples in Fig. 3a. After blended with PZSMS–OH, the surface morphology of CPEs changes from rough to smooth and a reduction of rumples is found in Fig. 3b. This may be related to the reduction of PEO crystallinity and an increase in amorphous PEO content upon blending with active PZS microspheres, which could be further confirmed by DSC. Another reason is that the inorganic–organic hybrid structure of active PZS microspheres makes good compatibility between the PEO polymer matrix and PZSMS–OH. The morphology
Fig. 3. Surface SEM images of (a) PEO10 –LiClO4 , (b) PEO10 –LiClO4 /10% PZSMS–OH, and (c) PEO10 –LiClO4 /30% PZSMS–OH composite polymer electrolytes.
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Fig. 4. Thermal properties of PEO10 –LiClO4 and PEO10 –LiClO4 /x% PZSMS–OH: (a) glass transition temperature Tg , (b) melting temperature Tm and (c) crystallinity Xc .
of the electrolytes becomes rough when the content of PZSMS–OH further increased (Fig. 3c), which results from the aggregation of PZSMS–OH at excess high loading content. 3.2. Thermal analysis of composite polymer electrolytes Fig. 4 shows the thermal properties of PEO10 –LiClO4 /x% PZSMS–OH composite polymer electrolytes obtained from DSC analysis. The relative percentage of crystallinity (Xc ) has been sample ∗ ) × 100, where calculated with the equation Xc = (Hm /Hm sample
Hm is the melting enthalpy of the composite polymer elec∗ is the melting enthalpy of a completely trolytes sample and Hm crystalline PEO sample [24]. Fig. 4a shows Tg for PEO10 –LiClO4 /x% PZSMS–OH. It shows that Tg is lowered when PZSMS–OH is added to PEO10 –LiClO4 . As Tg lowers, the amorphous phase becomes more flexible and the ionic conductivity should be enhanced. It can be seen from Fig. 4b and c that both the melting temperature (Tm ) and the crystallinity of PEO (Xc ) decrease obviously when active PZS microspheres are added in PEO10 –LiClO4 complex. Tm of PEO10 –LiClO4 /x% PZSMS–OH decreases from 60.5 ◦ C to 48.8 ◦ C when the content of active PZS microspheres increases from 0 wt.% to 30 wt.% and Xc of PEO10 –LiClO4 /x% PZSMS–OH also decreases from 44.0% to 25.5% with the increase of PZSMS–OH content.
The decrease of Tg and Xc will correspond to an increase of the flexibility of the PEO chains and of the ratio of amorphous state PEO. It is suggested that the good affinity between the inorganic–organic hybrid structure of PZSMS–OH fillers and PEO chains may cause these results. The coordination among the ether O atoms of PEO chains and Li+ cations, and the Lewis acid–base interactions between the ether O atoms of PEO chains and Lewis acid sites on the surfaces of PZSMS–OH, resulting in a reduction in recrystallization of PEO, are responsible for the above results too. 3.3. Ionic conductivity The influence of temperature on the ionic conductivity of the composite polymer electrolytes containing different amounts of PZSMS–OH is shown in Fig. 5. According to these observations, the addition of PZSMS–OH has increased the ionic conductivity of the composite polymer electrolytes throughout the temperatures range studied, giving the maximum conductivity value when PZSMS–OH concentration is at about 10 wt.%. When the content of PZSMS–OH increases further, the ionic conductivity decreases. The blocking effect on the transport of charge carriers, which maybe results from the aggregation of the PZSMS–OH, can decrease the ionic conductivity.
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Table 1 Lithium ion transference numbers of pristine PEO10 –LiClO4 and PEO10 –LiClO4 /10% filler composite polymer electrolytes at 70 ◦ C.
Fig. 5. Temperature dependence of ionic conductivity for PEO10 –LiClO4 /x% PZSMS–OH composite polymer electrolytes.
Sample
PEO10 –LiClO4
PEO10 –LiClO4 /10% SiO2
PEO10 –LiClO4 /10% PZSMS–OH
TLi+
0.19
0.24
0.34
different temperatures. It can be seen that in the temperature range studied the conductivity of PEO10 –LiClO4 /10% PZSMS–OH is higher than PEO10 –LiClO4 /10% SiO2 . As SiO2 and PZSMS–OH used in this study have similar diameter, the reason for the difference of enhancement of conductivity is attributed to the different surface chemical structures of the two fillers. The OH groups on the surface of PZSMS–OH can act as Lewis acid sites to enhance the dissolvability of LiClO4 and the S, O and N atoms of PZSMS–OH structures can coordinate with Li+ ions and also enhance the dissolvability of LiClO4 in PEO matrix. These two factors makes PZSMS–OH induce more free ions than SiO2 and the ionic conductivity is enhanced higher by PZSMS–OH than SiO2 . 3.4. Lithium ion transference number
Fig. 6. Temperature dependence of ionic conductivity for PEO10 –LiClO4 , PEO10 –LiClO4 /10% SiO2 , PEO10 –LiClO4 /10% PZSMS–OH composite polymer electrolytes.
One reason for the enhancement of the conductivity of PEO10 –LiClO4 /x% PZSMS–OH compared with PEO10 –LiClO4 is the lowering of Tg and Xc caused by the presence of PZSMS–OH, which means the increase of flexibility of PEO chains and the ratio of amorphous state PEO. The increases of flexibility of PEO chains and the ratio of amorphous state PEO are beneficial for the transport of Li+ cations and thus enhance the conductivity. Another reason of PZSMS–OH helping to increase the ionic conductivity of composite polymer electrolytes is believed to be caused by Lewis acid–base interactions among active OH groups at the filler surface and the ether oxygen atoms of PEO chains and LiClO4 . The H atoms on the surface of PZSMS–OH can coordinate with ClO4 − and weaken the interactions between Li+ and ClO4 − , thus enhance the dissolvability of LiClO4 . On the other side the oxygen atoms, nitrogen atoms and sulfur atoms on the surface of PZSMS–OH can compete with oxygen atoms in PEO backbone for complexation with Li+ ions, weaken the attraction between Li+ cations and PEO chains, resulting in a more relaxed coordination between oxygen atoms and Li+ ions and thereby facilitating the transport of Li+ ions through the polymer [16]. Fig. 6 shows the conductivity of PEO10 –LiClO4 , PEO10 –LiClO4 /10% SiO2 and PEO10 –LiClO4 /10% PZSMS–OH at
Lithium ion transference number, TLi+ , is one of the most important parameters for rechargeable lithium ion batteries because a relative high TLi+ can eliminate the concentration gradients within the batteries and ensure the battery operation at high current density [14,24–26]. Fig. 7 shows the ac- and dc-measurements for the lithium transference numbers and TLi+ of the PEO10 –LiClO4 before and after the addition of PZSMS–OH fillers are compared in Table 1. It can be seen that the TLi+ of the composite polymer electrolytes can be obviously increased by the addition of PZSMS–OH fillers. In PEO10 –LiClO4 complex, Li+ can coordinate not only with the ether O in PEO but also with the O atoms in ClO4 − , and then its transport ability is restricted, resulting in a very low TLi+ value. After the addition of PZSMS–OH, TLi+ can be increased obviously through the well-known Lewis acid–base interactions. The Lewis acid sites on the surface of PZSMS–OH can interact with O atoms in PEO and ClO4 − , and hence weaken the interactions between these O atoms and Li+ ions. On the other hand, the interface between the inorganic–organic hybrid framework of PZSMS–OH and PEO chains may provide novel conducting pathway for Li+ ions. The addition of PZSMS–OH fillers could provide a highly conducting layer at the electrolyte/filler interface [27–32]. This interface layer could be an amorphous polymer layer surrounding PZSMS–OH and a space-charge layer [33,34]. The existence of the conductive layers between the inorganic–organic hybrid surface of PZSMS–OH and PEO chains could act as the special conducting pathway of the charge carriers and enhance the lithium ion transference number with the addition of PZSMS–OH fillers. 3.5. Electrochemical stability window For an application of composite solid polymer electrolytes to a secondary battery, the polymer electrolytes should not be decomposed by an oxidation/reduction reaction in the range of operating voltage. Electrochemical stability window of the composite polymer electrolytes can be obtained by the method of linear voltage sweep. Fig. 8 displays the linear voltage sweep curves of pristine PEO10 –LiClO4 and PEO10 –LiClO4 /10% PZSMS–OH composite polymer electrolyte at 80 ◦ C. The irreversible onset of the current determines the electrolytes breakdown voltage. In the case of PEO10 –LiClO4 , the maximum working voltage (Vmax ) only extends to about 4.5 V versus Li. The addition of PZSMS–OH widens the electrochemical stability window and the Vmax of PEO10 –LiClO4 /10%
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PZSMS–OH exceeds to 4.8 V, indicating that PZSMS–OH can obviously improve the electrochemical stability of the composite polymer electrolytes. Fig. 9 shows the cyclic voltammogram (CV) of the composite polymer electrolytes PEO10 –LiClO4 /10% PZSMS–OH between
−0.8 V and 0.3 V versus Li+ /Li, which are recorded with a symmetrical Li/CPE/Li cell at a scan rate of 5 mV s−1 . It displayed an anodic peak for Li at about 0.12 V and a corresponding cathodic peak at −0.12 V. The redox peaks in the CV are resulted from the electrochemically reversible redox reaction by enhancing the diffusion of
Fig. 7. Ac- and dc-measurements for the lithium transference number measurements of composite polymer electrolytes: (a) PEO10 –LiClO4 , (b) PEO10 –LiClO4 /10% SiO2 , and (c) PEO10 –LiClO4 /10% PZSMS–OH.
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Fig. 8. Current–voltage response of PEO10 –LiClO4 and PEO10 –LiClO4 /10% PZSMS–OH composite polymer electrolytes at 80 ◦ C on stainless steel working electrode at a scanning rate of 1 mV s−1 .
other electrochemical properties such as lithium ion transference number, electrochemical stability window of the composite polymer electrolytes due to the Lewis acid–base interactions and inorganic–organic hybrid structure of PZSMS–OH. Scanning electron microscope (SEM) and DSC studies shows improved interaction between PZSMS–OH and PEO chains. The excellent electrochemical properties such as high room temperature ionic conductivity and lithium ion transference number combined with wide electrochemical stability window ensures the use of PEO10 –LiClO4 /PZSMS–OH composite polymer electrolytes as candidate electrolyte materials for all solid-state rechargeable lithium polymer batteries. References
Fig. 9. Cyclic voltammogram (CV) of the composite polymer electrolytes PEO10 –LiClO4 /10% PZSMS–OH versus Li+ /Li at a scanning rate of 5 mV s−1 .
Li+ in the polymer matrix which is due to doping of the PZSMS–OH fillers into the PEO film. 4. Conclusions Novel PEO-based all solid-state composite polymer electrolytes, by using inorganic–organic hybrid polyphosphazene microspheres with active hydroxyl groups as fillers are obtained and the effects of PZSMS–OH are studied. Compared with traditional ceramic fillers such as SiO2 , PZSMS–OH in PEO-based polymer electrolytes leads to higher enhancement in ionic conductivity and enhances
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Novel PEO-based composite solid polymer electrolytes incorporated with active inorganic–organic hybrid polyphosphazene microspheres Jiawei Zhang a,b , Xiaobin Huang a,∗ , Jianwei Fu a,b , Yawen Huang a,b , Wei Liu a,b , Xiaozhen Tang b a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China School of Chemistry and Chemical Engineering, National Key Laboratory of Metallic Matrix Composite Material, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b
a r t i c l e
i n f o
Article history: Received 3 September 2009 Received in revised form 11 January 2010 Accepted 9 February 2010 Keywords: Composite polymer electrolytes Polyphosphazene microspheres Hydroxyl groups Ionic conductivity Lithium ion transference number
a b s t r a c t Inorganic–organic hybrid poly(cyclotriphosphazene-co-4,4 -sulfonyldiphenol) (PZS) microspheres with active hydroxyl groups were incorporated in poly(ethylene oxide) (PEO), using LiClO4 as a dopant salt, to form a novel composite polymer electrolyte (CPE). The polymer chain flexibility and crystallinity properties are studied by DSC. The effects of active PZS microspheres on the electrochemical properties of the PEO-based electrolytes, such as ionic conductivity, lithium ion transference number, and electrochemical stability window are studied by electrochemical impedance spectroscopy and steady-state current method. Maximum ionic conductivity values of 3.36 × 10−5 S cm−1 at ambient temperature and 1.35 × 10−3 S cm−1 at 80 ◦ C with 10 wt.% content of active PZS microspheres were obtained and the lithium ion transference number was 0.34. The experiment results showed that the inorganic–organic hybrid polyphosphazene microspheres with active hydroxyl groups can enhance the ionic conductivity and increase the lithium ion transference number of PEO-based electrolytes more effectively comparing with traditional ceramic fillers such as SiO2 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid polymer electrolytes (SPEs) play an important role in the development of new energy sources, like solid-state batteries, fuel cells, sensors and electrochemical displays [1–3]. Poly(ethylene oxide) (PEO) is one of the most promising SPEs [4]. However, due to the poor ionic conductivity of PEO-based polymer electrolytes at ambient operating temperature, various modifications have been done [5]. A group of materials, such as ceramic powders [6,7], organic acid [8], and organic/inorganic composites [9], was added to PEO to prepare composite solid polymer electrolytes. These inert fillers such as SiO2 [10–12] and Al2 O3 [5–7] were incorporated to polymer electrolytes to improve the properties such as ionic conductivity and mechanical properties [13]. A great enhancement in the conductivity of the PEO-based composite polymer electrolytes was reported by using the combined action of a large surface area and the Lewis acid character of the ceramic additive [14]. The Lewis acid–base interaction plays a vital role in the enhancement of ionic conductivity, electrochemical stability and interfacial stability of
∗ Corresponding author. Tel.: +86 21 54747142; fax: +86 21 54741297. E-mail address: [email protected] (X. Huang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.02.016
the composite polymer electrolytes. But those ceramic fillers are mostly inorganic and easy to aggregate because of the poor compatibility between PEO chains and fillers. Hexachlorocyclotriphosphazenes (HCCPs), as very important inorganic rings, play a crucial role in the development of new polymers because of the excellent tunability of the backbone and the unprecedented structural diversity [15,16]. Recently, our group demonstrates for the first time the preparation of poly(cyclotriphosphazene-co-4,4 -sulfonyldiphenol) (PZS) nanotubes [17] and fully crosslinked PZS microspheres via precipitation polymerization [18]. The PZS microspheres with active hydroxyl groups have also been prepared by our group [19]. In our previous studies, PEO-based solid polymer electrolytes with plain PZS microspheres showed better electrochemical properties compared to traditional ceramic fillers such as SiO2 . The inorganic–organic hybrid structure of active PZS microspheres also offers good compatibility between the fillers and PEO polymer chains. Together with the active hydroxyl groups on the surface, PZS microspheres may have higher ionic conductivity and become a new class of fillers in the development of composite solid polymer electrolytes. In this study, composite polymer electrolytes based on PEO were prepared by using LiClO4 as a dopant salt and PZS microspheres with active hydroxyl groups as fillers in order to study the effects of active PZS microspheres on performances of CPEs.
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Scheme 1. Synthetic route and chemical structure of active PZS microspheres.
2. Experimental 2.1. Materials PEO, Mw = 200,000 (Shanghai Chemical Regent Company) and LiClO4 , A.R. (Shanghai Chemical Regent Company), were vacuum dried for 24 h at 50 ◦ C and 120 ◦ C, respectively. Acetonitrile, A.R. (Shanghai Chemical Regent Company), anhydrous by 4A molecular sieves before use. HCCP (synthesized as described in literature [20]) was recrystallized from dry hexane followed by two times sublimation. The melting point of purified HCCP was 113–114 ◦ C. 4,4 -Sulfonyldiphenol (BPS), triethylamine (TEA) and SiO2 (average diameter 500 nm) were purchased from Shanghai Chemical Regent Company and used without further purification. 2.2. Preparation of PZS microspheres with active hydroxyl groups via one-step precipitation polymerization
stored in an oven under nitrogen atmosphere coupled with a temperature controller and for each temperature at least 30 min was allowed before the impedance response was recorded. Then the impedance tests were carried out in the 1 MHz to 1 Hz frequency range using a Solartron 1260 Impedance/Gain-Phase Analyzer coupled with a Solartron 1287 Electrochemical Interface and the signal amplitude for ac impedance spectroscopy is 10 mV. The equivalent circuit of the electrochemical cell impedance measurement is shown in Fig. 1a, where R1 is the resistance of cell besides the composite polymer electrolytes. Rp and Cp are the resistance and capacitance of the polymer electrolytes. Cd is the interface capacitance at the stainless steel blocking electrodes. The equation for calculating the conductivity is = (1/Rp) × (d/S), where d is the thickness of the polymer electrolytes and S is the area of the stainless steel blocking electrodes. The bulk resistance (Rp) was obtained from the equivalent circuit by fitting the data of the ac impedance using ZPlot (Scribner Associates, Inc.) software. Fig. 1b shows the ac impedance spectroscopy for SS/PEO10 –LiClO4 /SS cell at 25 ◦ C. Lithium ion transference number (TLi+ ) measurements were evaluated using the method of ac impedance combined with steady-state current technique proposed by Vincent and Bruce [21–23], which involves a combination of ac- and
The preparation of PZS microspheres with active hydroxyl groups (PZSMS–OH) was carried out as follows (Scheme 1) [19]: triethylamine (0.87 g, 8.64 mmol) was added to a solution of BPS (1.29 g, 5.18 mmol) in acetone (80 ml), and HCCP (0.50 g, 1.44 mmol) in 20 ml acetone was added drop by drop into the solution. The reaction mixtures were stirred in an ultrasonic bath at room temperature for 3 h. The resultant particles were obtained by centrifugation and then were washed three times using acetone and deionized water. The solid was dried under vacuum to yield PZS active microspheres directly as white powders. 2.3. The preparation of CPEs The preparation of CPEs involved first the dispersion of the fillers and LiClO4 by ultrasonic in anhydrous acetonitrile at room temperature and the addition of the PEO/acetonitrile solution. The resulting slurry was cast onto a Teflon plate and then the plate was under the sweep of air in order to let the solvent evaporate. Finally, the obtained films were dried under vacuum at 50 ◦ C for 24 h to remove the residue solvent. These procedures yielded transparent homogenous films. The average thickness of the polymer electrolytes is about 100 m. All the CPE films were stored in an argon atmosphere glove box before test. The CPE samples used in this study were denoted as PEO10 –LiClO4 /x% filler, in which the EO/Li ratio was fixed to 10 for all samples and the content of filler, x, ranged from 0 wt.% to 30 wt.% of the PEO weight. 2.4. Characterization and instruments SEM images of the surface of CPEs were observed on a JEOL JSM-7401F field emission SEM instrument with gold sputtered-coated films. Differential scanning calorimeter (DSC) measurements were carried out on a PerkinElmer Pyris-1 analyzer. The measurements were carried out at a heating rate of 10 ◦ C min−1 from −60 ◦ C to 100 ◦ C in the heating cycle and from 100 ◦ C to −20 ◦ C in the cooling cycle. Ionic conductivity of the CPEs was determined by ac impedance spectroscopy. The film was sandwiched between two stainless steel (SS) blocking electrodes (with a diameter of 1 cm) to form a symmetrical SS/CPE/SS cell. The stack SS/CPE/SS was
Fig. 1. (a) Equivalent circuit of the electrochemical cell impedance measurement and (b) ac impedance spectra for SS/PEO10 –LiClO4 /SS cell at 25 ◦ C.
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dc-measurements. The method consists of initial measurement of the lithium interfacial resistance (R0 ) by impedance spectroscopy in the 100 kHz to 0.1 Hz frequency range, then application of a small voltage (<10 mV) until a steady current (Iss ) was obtained (time = 3000 s), and finally measurement of the interfacial resistance (Rss ) by impedance spectroscopy in the 100 kHz to 0.1 Hz frequency range. According to this method, the lithium ion transference number could be calculated with the following equation TLi+ =
Iss I0
V − I0 R0 V − Iss Rss
where V is the voltage pulse, R0 is the initial lithium interfacial resistance, and Rss is the secondary lithium interfacial resistance (as the steady-state polarization current is reached). I0 is the initial current and Iss is the current reached in the steady-state. The parameters for the calculation can be obtained from the impedance response and steady-state current response. Measurements were performed using a Solartron 1260 Impedance/Gain-Phase Analyzer coupled with a Solartron 1287 Electrochemical Interface. The CPE was sandwiched between two lithium-unblocking electrodes to form a symmetrical Li/CPE/Li cell and was closed in a coin cell. The cell was assembled and sealed in an argon-filled UNILAB glove box (O2 < 1 ppm; H2 O < 0.1 ppm). Electrochemical stability window of the CPEs were determined by running a linear sweep voltammetry in three electrode cells using stainless steel as the blocking working electrode, lithium as both the counter and the reference electrode and the CPE film as the electrolyte. A Solartron 1287 Electrochemical Interface was used to run the voltammetry at a scan rate of 1 mV s−1 . Cyclic voltammogram (CV) of the composite polymer electrolytes was run with a symmetrical Li/CPE/Li cell closed in a coin cell and recorded using a Solartron 1287 Electrochemical Interface at a scan rate of 5 mV s−1 .
3. Results and discussion 3.1. Surface morphology Fig. 2 shows the SEM microphotograph of the PZS microspheres with active hydroxyl groups. It can be seen from the SEM images
Fig. 2. SEM image of PZSMS–OH microspheres.
that the average diameter of the microspheres is about 0.5 m and the size of the microspheres is almost the same. Fig. 3 shows the SEM microphotographs of various CPEs. The pristine PEO10 –LiClO4 film exhibits a rough surface with some rumples in Fig. 3a. After blended with PZSMS–OH, the surface morphology of CPEs changes from rough to smooth and a reduction of rumples is found in Fig. 3b. This may be related to the reduction of PEO crystallinity and an increase in amorphous PEO content upon blending with active PZS microspheres, which could be further confirmed by DSC. Another reason is that the inorganic–organic hybrid structure of active PZS microspheres makes good compatibility between the PEO polymer matrix and PZSMS–OH. The morphology
Fig. 3. Surface SEM images of (a) PEO10 –LiClO4 , (b) PEO10 –LiClO4 /10% PZSMS–OH, and (c) PEO10 –LiClO4 /30% PZSMS–OH composite polymer electrolytes.
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Fig. 4. Thermal properties of PEO10 –LiClO4 and PEO10 –LiClO4 /x% PZSMS–OH: (a) glass transition temperature Tg , (b) melting temperature Tm and (c) crystallinity Xc .
of the electrolytes becomes rough when the content of PZSMS–OH further increased (Fig. 3c), which results from the aggregation of PZSMS–OH at excess high loading content. 3.2. Thermal analysis of composite polymer electrolytes Fig. 4 shows the thermal properties of PEO10 –LiClO4 /x% PZSMS–OH composite polymer electrolytes obtained from DSC analysis. The relative percentage of crystallinity (Xc ) has been sample ∗ ) × 100, where calculated with the equation Xc = (Hm /Hm sample
Hm is the melting enthalpy of the composite polymer elec∗ is the melting enthalpy of a completely trolytes sample and Hm crystalline PEO sample [24]. Fig. 4a shows Tg for PEO10 –LiClO4 /x% PZSMS–OH. It shows that Tg is lowered when PZSMS–OH is added to PEO10 –LiClO4 . As Tg lowers, the amorphous phase becomes more flexible and the ionic conductivity should be enhanced. It can be seen from Fig. 4b and c that both the melting temperature (Tm ) and the crystallinity of PEO (Xc ) decrease obviously when active PZS microspheres are added in PEO10 –LiClO4 complex. Tm of PEO10 –LiClO4 /x% PZSMS–OH decreases from 60.5 ◦ C to 48.8 ◦ C when the content of active PZS microspheres increases from 0 wt.% to 30 wt.% and Xc of PEO10 –LiClO4 /x% PZSMS–OH also decreases from 44.0% to 25.5% with the increase of PZSMS–OH content.
The decrease of Tg and Xc will correspond to an increase of the flexibility of the PEO chains and of the ratio of amorphous state PEO. It is suggested that the good affinity between the inorganic–organic hybrid structure of PZSMS–OH fillers and PEO chains may cause these results. The coordination among the ether O atoms of PEO chains and Li+ cations, and the Lewis acid–base interactions between the ether O atoms of PEO chains and Lewis acid sites on the surfaces of PZSMS–OH, resulting in a reduction in recrystallization of PEO, are responsible for the above results too. 3.3. Ionic conductivity The influence of temperature on the ionic conductivity of the composite polymer electrolytes containing different amounts of PZSMS–OH is shown in Fig. 5. According to these observations, the addition of PZSMS–OH has increased the ionic conductivity of the composite polymer electrolytes throughout the temperatures range studied, giving the maximum conductivity value when PZSMS–OH concentration is at about 10 wt.%. When the content of PZSMS–OH increases further, the ionic conductivity decreases. The blocking effect on the transport of charge carriers, which maybe results from the aggregation of the PZSMS–OH, can decrease the ionic conductivity.
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Table 1 Lithium ion transference numbers of pristine PEO10 –LiClO4 and PEO10 –LiClO4 /10% filler composite polymer electrolytes at 70 ◦ C.
Fig. 5. Temperature dependence of ionic conductivity for PEO10 –LiClO4 /x% PZSMS–OH composite polymer electrolytes.
Sample
PEO10 –LiClO4
PEO10 –LiClO4 /10% SiO2
PEO10 –LiClO4 /10% PZSMS–OH
TLi+
0.19
0.24
0.34
different temperatures. It can be seen that in the temperature range studied the conductivity of PEO10 –LiClO4 /10% PZSMS–OH is higher than PEO10 –LiClO4 /10% SiO2 . As SiO2 and PZSMS–OH used in this study have similar diameter, the reason for the difference of enhancement of conductivity is attributed to the different surface chemical structures of the two fillers. The OH groups on the surface of PZSMS–OH can act as Lewis acid sites to enhance the dissolvability of LiClO4 and the S, O and N atoms of PZSMS–OH structures can coordinate with Li+ ions and also enhance the dissolvability of LiClO4 in PEO matrix. These two factors makes PZSMS–OH induce more free ions than SiO2 and the ionic conductivity is enhanced higher by PZSMS–OH than SiO2 . 3.4. Lithium ion transference number
Fig. 6. Temperature dependence of ionic conductivity for PEO10 –LiClO4 , PEO10 –LiClO4 /10% SiO2 , PEO10 –LiClO4 /10% PZSMS–OH composite polymer electrolytes.
One reason for the enhancement of the conductivity of PEO10 –LiClO4 /x% PZSMS–OH compared with PEO10 –LiClO4 is the lowering of Tg and Xc caused by the presence of PZSMS–OH, which means the increase of flexibility of PEO chains and the ratio of amorphous state PEO. The increases of flexibility of PEO chains and the ratio of amorphous state PEO are beneficial for the transport of Li+ cations and thus enhance the conductivity. Another reason of PZSMS–OH helping to increase the ionic conductivity of composite polymer electrolytes is believed to be caused by Lewis acid–base interactions among active OH groups at the filler surface and the ether oxygen atoms of PEO chains and LiClO4 . The H atoms on the surface of PZSMS–OH can coordinate with ClO4 − and weaken the interactions between Li+ and ClO4 − , thus enhance the dissolvability of LiClO4 . On the other side the oxygen atoms, nitrogen atoms and sulfur atoms on the surface of PZSMS–OH can compete with oxygen atoms in PEO backbone for complexation with Li+ ions, weaken the attraction between Li+ cations and PEO chains, resulting in a more relaxed coordination between oxygen atoms and Li+ ions and thereby facilitating the transport of Li+ ions through the polymer [16]. Fig. 6 shows the conductivity of PEO10 –LiClO4 , PEO10 –LiClO4 /10% SiO2 and PEO10 –LiClO4 /10% PZSMS–OH at
Lithium ion transference number, TLi+ , is one of the most important parameters for rechargeable lithium ion batteries because a relative high TLi+ can eliminate the concentration gradients within the batteries and ensure the battery operation at high current density [14,24–26]. Fig. 7 shows the ac- and dc-measurements for the lithium transference numbers and TLi+ of the PEO10 –LiClO4 before and after the addition of PZSMS–OH fillers are compared in Table 1. It can be seen that the TLi+ of the composite polymer electrolytes can be obviously increased by the addition of PZSMS–OH fillers. In PEO10 –LiClO4 complex, Li+ can coordinate not only with the ether O in PEO but also with the O atoms in ClO4 − , and then its transport ability is restricted, resulting in a very low TLi+ value. After the addition of PZSMS–OH, TLi+ can be increased obviously through the well-known Lewis acid–base interactions. The Lewis acid sites on the surface of PZSMS–OH can interact with O atoms in PEO and ClO4 − , and hence weaken the interactions between these O atoms and Li+ ions. On the other hand, the interface between the inorganic–organic hybrid framework of PZSMS–OH and PEO chains may provide novel conducting pathway for Li+ ions. The addition of PZSMS–OH fillers could provide a highly conducting layer at the electrolyte/filler interface [27–32]. This interface layer could be an amorphous polymer layer surrounding PZSMS–OH and a space-charge layer [33,34]. The existence of the conductive layers between the inorganic–organic hybrid surface of PZSMS–OH and PEO chains could act as the special conducting pathway of the charge carriers and enhance the lithium ion transference number with the addition of PZSMS–OH fillers. 3.5. Electrochemical stability window For an application of composite solid polymer electrolytes to a secondary battery, the polymer electrolytes should not be decomposed by an oxidation/reduction reaction in the range of operating voltage. Electrochemical stability window of the composite polymer electrolytes can be obtained by the method of linear voltage sweep. Fig. 8 displays the linear voltage sweep curves of pristine PEO10 –LiClO4 and PEO10 –LiClO4 /10% PZSMS–OH composite polymer electrolyte at 80 ◦ C. The irreversible onset of the current determines the electrolytes breakdown voltage. In the case of PEO10 –LiClO4 , the maximum working voltage (Vmax ) only extends to about 4.5 V versus Li. The addition of PZSMS–OH widens the electrochemical stability window and the Vmax of PEO10 –LiClO4 /10%
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PZSMS–OH exceeds to 4.8 V, indicating that PZSMS–OH can obviously improve the electrochemical stability of the composite polymer electrolytes. Fig. 9 shows the cyclic voltammogram (CV) of the composite polymer electrolytes PEO10 –LiClO4 /10% PZSMS–OH between
−0.8 V and 0.3 V versus Li+ /Li, which are recorded with a symmetrical Li/CPE/Li cell at a scan rate of 5 mV s−1 . It displayed an anodic peak for Li at about 0.12 V and a corresponding cathodic peak at −0.12 V. The redox peaks in the CV are resulted from the electrochemically reversible redox reaction by enhancing the diffusion of
Fig. 7. Ac- and dc-measurements for the lithium transference number measurements of composite polymer electrolytes: (a) PEO10 –LiClO4 , (b) PEO10 –LiClO4 /10% SiO2 , and (c) PEO10 –LiClO4 /10% PZSMS–OH.
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Fig. 8. Current–voltage response of PEO10 –LiClO4 and PEO10 –LiClO4 /10% PZSMS–OH composite polymer electrolytes at 80 ◦ C on stainless steel working electrode at a scanning rate of 1 mV s−1 .
other electrochemical properties such as lithium ion transference number, electrochemical stability window of the composite polymer electrolytes due to the Lewis acid–base interactions and inorganic–organic hybrid structure of PZSMS–OH. Scanning electron microscope (SEM) and DSC studies shows improved interaction between PZSMS–OH and PEO chains. The excellent electrochemical properties such as high room temperature ionic conductivity and lithium ion transference number combined with wide electrochemical stability window ensures the use of PEO10 –LiClO4 /PZSMS–OH composite polymer electrolytes as candidate electrolyte materials for all solid-state rechargeable lithium polymer batteries. References
Fig. 9. Cyclic voltammogram (CV) of the composite polymer electrolytes PEO10 –LiClO4 /10% PZSMS–OH versus Li+ /Li at a scanning rate of 5 mV s−1 .
Li+ in the polymer matrix which is due to doping of the PZSMS–OH fillers into the PEO film. 4. Conclusions Novel PEO-based all solid-state composite polymer electrolytes, by using inorganic–organic hybrid polyphosphazene microspheres with active hydroxyl groups as fillers are obtained and the effects of PZSMS–OH are studied. Compared with traditional ceramic fillers such as SiO2 , PZSMS–OH in PEO-based polymer electrolytes leads to higher enhancement in ionic conductivity and enhances
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