PMMA polymer electrolyte by TiO2 nanoparticles

Solid State Ionics 282 (2015) 31–36

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Enhanced thermal and electrochemical properties of PVDF-HFP/PMMA polymer electrolyte by TiO2 nanoparticles DaYu Song a, Chen Xu a, YuanFu Chen b,⁎, JiaRui He b, Yan Zhao a, PingJian Li b, Wei Lin b, Fei Fu b a b

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, PR China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China

a r t i c l e

i n f o

Article history: Received 4 May 2015 Received in revised form 22 September 2015 Accepted 23 September 2015 Available online xxxx Keywords: PVDF-HFP PMMA TiO2 nanoparticles Composite polymer electrolyte Lithium ion batteries

a b s t r a c t (Poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate)) PVDF-HFP/PMMA based gel polymer electrolyte comprising 0–7 wt.% TiO2 nanoparticles has been synthesized. After introducing TiO2 nanoparticles, the thermal properties of the PVDF-HFP/PMMA/TiO2 composite polymer electrolyte (CPE) are remarkably improved: when the working temperature is up to 90 °C, the weight loss of CPE with 7% TiO2 is only 12.8% of that without TiO2, suggesting much higher thermal stability; the shrinkage of the CPE at 130 °C can also obviously decrease from 23.4% down to 14.4% after introducing TiO2 nanoparticles. In addition, the CPE exhibits much higher ionic conductivity and better electrochemical stability. Furthermore, the LiCoO2/Li cells with CPE exhibit good cyclic stability and C-rate performance: the capacity maintains 92.1% of the initial capacity after 50 cycles; the capacity can reach as large as ~80 mAhg−1 even at 5 C. It is promising for PVDF-HFP/PMMA/TiO2 CPE to meet the practical demands of high-performance and thermal safety for lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Rechargeable Li-ion batteries (LIBs) are key components of both hybrid electric vehicles (HEVs) and full electric vehicles (EVs) which require high energy and power capability [1–3]. The use of polymer electrolytes is widely regarded as a promising approach for Li-ion batteries [4–7], due to their lack of leakage, high flexibility within the cell geometry and high physical and chemical stability [8,9]. Among of all the polymer electrolytes, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(methyl methacrylate) (PMMA) polymer matrixes have attracted much attention [10–14] due to the advantages of excellent mechanical, chemical stability [15,16] and considerable wettability [17]. Nevertheless, they also have shown some drawbacks such as low thermal stability, poor electrochemical stability, low cyclic and rate performances, which limit the wide applications [18–20]. To overcome the above-mentioned disadvantages [21–23], considerable research efforts have been done. Yang et al. improved the thermal property of polymer electrolyte by using core-shell structured SiO2–PMMA microspheres [17,18], while Kim used gamma ray irradiation to improve the thermal properties of the polymer electrolyte [19]. Such methods seem a little complicated. Introducing inorganic nanoparticles such as SiO2 [17,18,20,24–26], Al2O3 [27–29], SnO2 [30], TiO2 [31–35] and CaCO3 [10] into polymer electrolyte is another effective way to improve the ionic conductivity and enhance electrochemical performance of LIBs. The comprehensive effects of the composite ⁎ Corresponding author. E-mail address: [email protected] (Y. Chen).

http://dx.doi.org/10.1016/j.ssi.2015.09.017 0167-2738/© 2015 Elsevier B.V. All rights reserved.

polymer electrolyte (CPE) with various contents of inorganic nanoparticles on the porous structure, the thermal and electrochemical properties are still not so clear. In this study, we perform a facile method to prepare PVDF-HFP/PMMA based gel polymer electrolyte comprising 0–7 wt.% TiO2 nanoparticles. The effects of TiO2 nanoparticles on the porous structure, the thermal and electrochemical properties have been investigated. The results show that after introducing TiO2 nanoparticles, the composite polymer electrolyte exhibits much better thermal stability, higher ionic conductivity, and wider electrochemical window. The LiCoO2/Li cells with CPE exhibit excellent cyclic stability and C-rate performance. 2. Experimental 2.1. Materials Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw 455,000 g · mol − 1), and Poly(methyl methacrylate) (PMMA, Mw 996,000 g · mol− 1) were purchased from Sigma-Aldrich. P25 Titanium dioxide (TiO2) nanoparticles were purchased from EVONIKDEGUSSA Co., Ltd. Acetone and N,N-Dimethylformamide (DMF) were analytical grade. 2.2. Preparation of the composite polymer membrane (CPM) PVDF-HFP and PMMA (in a weight ratio of 1:1) were dissolved in a mixture of DMF and Acetone (v/v = 1:3) at 60 °C for about 2 h under

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D. Song et al. / Solid State Ionics 282 (2015) 31–36

surface morphology of the polymer membrane was performed by a scanning electron microscope (SEM,JSM-7000F, JEOL). The thermal analysis was performed using TA Instruments TGA-Q50, under a nitrogen atmosphere with a heating rate of 10 °C min−1. The thermal shrinkage of the polymer membrane was calculated based on Eq. (1):

Intensity (a.u.)

TiO2 (0 wt%) TiO2 (2 wt%)

Shrinkage ð%Þ ¼ ðSo −Si Þ=So  100%

TiO2 (5 wt%)

where So and Si are the areas of polymer membrane before and after heat treatment at 130 °C for 1 h in the oven. (So = 4 cm × 4 cm) The electrolyte uptake (U) of the polymer membrane was calculated by Eq. (2):

TiO2 (7 wt%)

TiO2

Uð%Þ ¼ ðW i −W o Þ=W o  100%

0

10

20

30

40

50

60

70

ð1Þ

ð2Þ

80

2θ (deg) Fig. 1. XRD patterns of TiO2 powders, PVDF-HFP/PMMA polymer membrane, and PVDFHFP/PMMA/TiO2 composite polymer membrane.

continuous stirring. The solution was then cast onto a glass plate, and dried in vacuum oven at 80 °C for 12 h, and the CPM was obtained. 2.3. Characterization of CPM The crystalline structure of CPM was characterized by X-ray diffraction (XRD Rigaku D/MAX-r Adiffractometer) using Cu Kα radiation. The

where Wo and Wi are the weights of the polymer membrane before and after absorbing the liquid electrolyte, respectively. 2.4. Electrochemical measurements of CPE The CPM was cut into identical round pieces and immersed in liquid electrolyte for 24 h under dry argon atmosphere in a glove box. The liquid electrolyte consists of 1 M LiPF6 electrolyte solution in a mixture of ethylene carbonate and dimethyl carbonate (v/v = 1:1). The electrochemical stability was tested by linear sweep voltammetry (LSV), at a potential scanning rate of 0.05 mV s−1. The ionic conductivity of the composite polymer electrolyte was measured by electrochemical

Fig. 2. (a) Low-magnification SEM and (b) high-magnification SEM images of PVDF-HFP/PMMA polymer membrane, and corresponding (c) low-magnification SEM and (d) high-magnification SEM images of PVDF-HFP/PMMA/TiO2 (5 wt.%) composite polymer membrane.

D. Song et al. / Solid State Ionics 282 (2015) 31–36

a

33

b

100 100.0

60

99.9

TiO2 (0 wt%) TiO2 (2 wt%) TiO2 (5 wt%) TiO2 (7 wt%)

40

TiO2 (0 wt%) TiO2 (2 wt%) TiO2 (5 wt%) TiO2 (7 wt%)

Weight (%)

Weight (%)

80

99.8 20 0 0

100 200 300 400 500 600 30

40

50

60

70

80

99.7 90

o

Temperature ( C)

Temperature ( C)

o

Fig. 3. TGA thermograms of PVDF-HFP/PMMA/TiO2 polymer membranes under the temperature ranging from 30 to 700 °C (a), and 30–90 °C (b).

impedance spectroscopy (EIS). EIS tests were carried out in the frequency range between 100 kHz and 1 Hz, using CHI660D electrochemical workstation (CHI instrument). Its ionic conductivity (σ) was calculated using Eq. (3): σ ¼ d=ðRb AÞ

ð3Þ

where σ is the ionic conductivity, d is the thickness of polymer electrolyte, Rb is the bulk resistance of polymer electrolyte, A is the area of the symmetrical electrode. 2.5. Battery preparation and electrochemical characterization The LiCoO2/Li cells with CPE were used to analyze the electrochemical properties of the CPE. Coin-type (CR2025) cells were assembled in an argon-filled M. Braun glove box with oxygen and water contents below 0.5 ppm. LiCoO2 and lithium metal were used as cathode and anode. CPE was used as separator and electrolyte. The LiCoO2 cathode was fabricated by mixing 80 wt.% active material (LiCoO2), 10 wt.%

carbon black (Super-P), and 10 wt.% PVDF in NMP solution to form homogenous slurry. After coating the above slurry on the aluminum foil (the thickness of the LiCoO2 was in the range of 20–30 μm), the electrodes were dried at 120 °C under vacuum for 24 h to remove the solvent. Galvanostatic charge–discharge cycles were tested by LAND CT2001A instrument (Wuhan Jinnuo Electronic Co. Ltd.) at vaious Crates between 2.5 V and 4.3 V (vs. Li/Li+) at room temperature. 3. Results and discussion The XRD patterns of TiO2 nanoparticles and PVDF-HFP/PMMA/TiO2 composite polymer membrane are shown in Fig. 1. The diffraction peaks at 2θ = 20.3° correspond to the (101) of PVDF β phases, while the amorphous halos (a large hump) centered at 2θ = 16.9° correspond to the PMMA amorphous phases. After introducing the TiO2, the PVDFHFP/PMMA/TiO2 composite polymer membrane exhibits three peaks at 2θ = 25.23°, 48.1° and 55.1°, corresponding to the (101), (200) and (211) of the anatase TiO2 phase. In addition, with increasing TiO2 content, the intensities of corresponding TiO2 diffraction peaks become

Fig. 4. Thermal shrinkage photographs of PVDF-HFP/PMMA/TiO2 polymer membranes before (a) and after (b) storing at 130 °C for 1 h.

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D. Song et al. / Solid State Ionics 282 (2015) 31–36

1.0

0.5

5.1

Current onset potential (V)

Current (mA)

1.5

TiO2 (0 wt%) TiO2 (2 wt%) TiO2 (5 wt%) TiO2 (7 wt%)

5.0 4.9 4.8 4.7 4.6 4.5 0%

2%

5%

7%

TiO2 Content

0.0 3.0

3.5

4.0

4.5

5.0

5.5

+

Potential vs. Li/Li (V) Fig. 5. Linear sweep voltammograms for PVDF-HFP/PMMA/TiO2 polymer electrolyte. The inset shows TiO2 content dependence of current onset potential.

strong. It suggests that polycrystalline TiO2 nanoparticles have been introduced into the PVDF-HFP/PMMA polymer membrane. The morphology of the PVDF-HFP/PMMA polymer membrane without and with TiO2 nanoparticles are characterized by SEM. As shown in Fig. 2a and b, the PVDF-HFP/PMMA polymer membrane without TiO2 has low porosity with small pore size, resulting in lower electrolyte absorption ability. It is noted that the polymer membrane with TiO2 (5 wt.%) has reticular porous fabric and the porosity is high with suitable pore size of 100–200 nm, as shown in Fig. 2c and d. The thermal stability of PVDF-HFP/PMMA-based polymer membranes was characterized by thermogravimetric analyzer (TGA). Fig. 3 illustrates the TGA data of PVDF-HFP/PMMA-based polymer membranes with and without TiO2 nanoparticles. As shown in Fig. 3a, one can observe that the PVDF-HFP/PMMA/TiO2 polymer membranes exhibit one-step weight loss with higher residual production at high temperature, while the PVDF-HFP/PMMA polymer membrane shows a twostep weight loss with lower residual production. As shown in Fig. 3b, one can observe that with increasing TiO2 content, the weight loss at low temperature dramatically decrease: when the working temperature is up to 90 °C, the weight losses of CPE with 0, 2%, 5%, and 7% TiO2 are 0.204%, 0.124%, 0.035%, and 0.0261%, respectively; the weight loss of CPE with 7% TiO2 is only 12.8% of that without TiO2, suggesting much higher thermal stability at work temperature less than 90 °C. As we known, the work temperature of LIBs is less than 100 °C, so the enhanced thermal stability of CPE by 5–7% TiO2 nanoparticles can meet the practical demands of lithium ion batteries in terms of the thermal safety well.

As the polymer membrane can be acted as separator too, the thermal and shape stability of polymer electrolyte is crucial to prevent internal short circuit between the electrodes. Here the thermal shrinkage of the composite polymer membrane is investigated. Fig. 4 shows the thermal shrinkage of PVDF-HFP/PMMA-based polymer membranes without and with various contents of TiO2 after storing them at 130 °C for 1 h. Comparing the images of the CPE samples before (Fig. 4a) and after (Fig. 4b) storing at 130 °C for 1 h, one can conclude that the thermal and shape stability can be significantly improved after introducing TiO2 nanoparticles: serious crimp can be observed in the CPE without TiO2, while the flat shape is almost kept for the membranes with TiO2; the membrane without TiO2 exhibits high degree of shrinkage (~23.4%), while the membranes with 2–7% TiO2 exhibit much lower shrinkage (~14.4%). This improvement of the thermal and shape stability can be attributed to the introduction of heat-resistant and hard TiO2 nanoparticles. The result indicates that PVDF-HFP/PMMA/TiO2 is capable of working at the elevated temperatures, which is usually caused by larger current density operation or improper use such as overcharging, over-discharging, and short-circuiting. As mentioned above, the composite polymer membrane comprising TiO2 nanoparticles exhibits excellent thermal performance. As we know, the electrochemical stability of the composite polymer membrane is also very important. After activated by soaking in liquid electrolyte, the electrochemical properties of the composite polymer electrolyte are characterized. Fig. 5 shows the linear sweep voltammograms (LSV) using the CPE without and with TiO2. Stainless steel was used as the working electrode, and Li foil was used as the reference and counter electrode. The current onset potentials of the CPE with 0, 2%, 5%, 7% TiO2 are 4.5, 4.6,4.75 and 5.0 V, which increase with increasing TiO2 content, as shown in the inset of Fig. 5. This suggests that the incorporation of TiO2 nanoparticles can be effective to improve the electrochemical stability and enhance the energy density of the LIBs. The effects of TiO2 nanoparticles on the electrochemical performance are systematically investigated. Firstly, the EIS spectra of the CPE without and with various contents of TiO2 are shown in Fig. 6. The values of bulk resistance (Rb) were obtained from AC impedance spectra of polymer electrolytes, which are summarized in Table 1. The electrolyte uptake and the ionic conductivity of polymer electrolyte are calculated using Eqs. (2) and (3). It is found that after introducing TiO2, the room-temperature ionic conductivity of the CPE is significantly enhanced. PVDF-HFP/PMMA with 5 wt.% TiO2 exhibits the highest conductivity of 2.49 mS cm−1 and the highest electrolyte uptake is 267%, while the ionic conductivity and the electrolyte uptake of PVDF-HFP/PMMA without TiO2 is only 1.35 mS cm−1 and 160%, respectively. The significant enhancement in ionic conductivity is attributed to the reticular porous morphology and suitable pore size of the polymer electrolyte, which also enhances the electrolyte uptake and transmission of Li ions. The cyclic performances of the cells (LiCoO2/Li) are examined using PVDF-HFP/PMMA/TiO2 composite polymer electrolytes. The cells are cycled between 2.5 and 4.3 V at a constant charge/discharge rate of 0.2 C. Based on the charge and discharge capacity in Fig. 7a, the coulombic efficiency of all the samples can be calculated. It can be found that the coulombic efficiency of the all the samples are 98.2%, 99.2%, 99.5% and 99.3% corresponding the various weight contents of TiO2 (0, 2, 5 and 7 wt.%) in PVDF-HFP/PMMA. This may be ascribed to the introduced TiO2 that can keep interfacial stability between polymer electrolyte and electrodes and decrease interfacial resistance. The cells deliver the initial capacities of 143.6, 180.5, 188.1 and 163.6 mAh g−1 when used PVDF-

Table 1 Ionic conductivity and electrolyte uptake of polymer electrolyte.

Fig. 6. AC impedance plots of the PVDF-HFP/PMMA/TiO2 polymer electrolyte.

TiO2 (wt.%)

0

2

5

7

Rb (Ω) Ion conductivity σ (mS cm−1) Electrolyte uptake (%)

3.671 1.35 160

3.075 1.62 211

1.996 2.49 267

2.574 1.93 240

D. Song et al. / Solid State Ionics 282 (2015) 31–36

35

Fig. 7. (a) Charge/discharge profile and (b) cyclic performances for the LiCoO2/Li cells using the composite polymer electrolytes at 0.2 C.

HFP/PMMA/TiO2 composite polymer electrolytes with various ratios of TiO2 (0, 2, 5 and 7 wt.%). After 50 cycles, the corresponding capacities decrease to 114, 154.3, 173.2 and 139.9 mAh g−1 (Fig. 7), remaining 79.4%, 85.5%, 92.1% and 85.5%. The cell using PVDF-HFP/PMMA/TiO2 (5 wt.%) exhibits excellent cycling performance. The rate performances of the cells (LiCoO2/Li) are investigated at 0.2 C, 0.5 C, 1 C, 2 C, 5 C and 0.2 C in the voltage range between 2.5 and 4.3 V. The discharge capacities of the cells using CPE with 0– 7 wt.% TiO2 are shown in Fig. 8. The cell using CPE with 5 wt.% TiO2 shows a very stable and high capacity even at the high rate of 5 C (~ 80.3 mAh g− 1 ); however, the capacity of PVDF/PMMA at 5 C is only 2.5 mAh g− 1. In order to understand the effect of polymer electrolyte introducing TiO2 on the kinetic performance of LiCoO2/Li half cell, the EIS of LiCoO2/ Li half cell with CPE (with 0, 2%, 5%, and 7% TiO2) have been tested as shown in Fig. 9. The lithium ion diffusion coefficient can be calculated according to the following equation: D ¼

R2 T 2 2A2 n4 F 4 C 2 σ 2

where the mean-

ings of n is the number of electrons per molecule during oxidization, A the surface area of the cathode, D the diffusion coefficient of lithium ion, R the gas constant, T the absolute temperature, F the Faraday constant, C the concentration of lithium ion, and σ is the Warburg factor. Based on the equation, the apparent diffusion coefficients of Li-ion for CPE with 0, 2%, 5%, and 7% TiO2 are 1.32 × 10− 13, 2.88 × 10− 13, 3.98 × 10−13, and 3.17 × 10−13 cm2 s−1 respectively. The high Li-ion diffusion coefficient for the LiCoO2/Li half cell with TiO2 nanoparticles in PVDF-HFP/PMMA polymer electrolyte also demonstrated the advantage of CPE with 5 wt.% TiO2.

As mentioned above, the cell using CPE with TiO2 exhibits much better electrochemical performance than that without TiO2. The main reasons might be as follows. The CPE has reticular porous morphology and suitable pore size, which also enhances the electrolyte uptake and transmission of Li ions. This makes the CPE with TiO2 having better thermal and electrochemical stability, higher ionic conductivity and lower impedance, which results in better electrochemical performance. 4. Conclusions The PVDF-HFP/PMMA-based composite polymer electrolyte comprising various contents of TiO2 nanoparticles was synthesized by a facile method. The effects of TiO2 nanoparticles on morphology, the thermal and electrochemical stability, and the electrochemical performance were systematically investigated. With the incorporation of TiO2, the composite polymer electrolyte exhibits improved thermal and electrochemical properties. In particular, the CFP with 5 wt.% TiO2 nanoparticles has uniform and interconnected pore structure and shows excellent performance. The ionic conductivity can be up to 2.49 mS cm−1 at room temperature,as well as a high electrochemical stability up to 5 V vs. Li/ Li+. The specific capacity of assembled Li/LiCoO2 cells using PVDF/ PMMA/TiO2 (5 wt.%) is 188.1 mAh g−1 at 0.2 C, and maintains 92.1% of the initial capacity after 50 cycles. Particularly, the high-rate discharge performance shows a high capacity even at the high rate of 5 C (~80.3 mAh g−1). It is promising for PVDF-HFP/PMMA/TiO2 CPE to meet the practical demands of high-performance and thermal safety for lithium ion batteries.

Fig. 8. (a) Charge/discharge profile of the LiCoO2/Li cells using PVDF-HFP/PMMA/TiO2 (5 wt.%) and (b) rate performances of the LiCoO2/Li cells using the composite polymer electrolytes at various C-rates.

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D. Song et al. / Solid State Ionics 282 (2015) 31–36

-300 TiO2 (0 wt%) TiO2 (2 wt%) TiO2 (5 wt%) TiO2 (7 wt%)

Z'' (ohm)

-200

-100

0

0

100

200

300

400

500

Z' (ohm) Fig. 9. AC impedance plots of the LCO/Li half cell with various contents of TiO2.

Acknowledgments The research was supported by the National Natural Science Foundation of China (Grant Nos. 51202022, 51372033 and 61378028), the Specialized Research Fund for the Doctoral Program of Higher Education (Gran No. 20120185120011), the 111 Project (Grant No. B13042), Sichuan Youth Science and Technology Innovation Research Team Funding (Grant No. 2011JTD0006), the International Science and Technology Cooperation Program of China (Gran No. 2012DFA51430), and the Sino-German Cooperation PPP Program of China (Grant No. 201400260068). References [1] [2] [3] [4]

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