Electrospun Trilayer Polymeric Membranes as Separator for Lithium–ion Batteries

Electrochimica Acta 127 (2014) 167–172

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Electrospun Trilayer Polymeric Membranes as Separator for Lithium–ion Batteries N. Angulakshmi, A. Manuel Stephan ∗ CSIR-Network Institutes of Solar Energy(CSIR-NISE), Electrochemical Power Systems Division, Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi 630 006, India

a r t i c l e

i n f o

Article history: Received 4 November 2013 Received in revised form 21 January 2014 Accepted 28 January 2014 Available online 12 February 2014 Keywords: Electrospinning Porous membrane Electrolyte uptake Thermal stability Charge-discharge studies.

a b s t r a c t Poly(vinylidene fluoride- hexafluoropropylene) (PVdF-HFP)/poly (vinyl chloride) (PVC)/(PVdF-HFP) based- trilayer porous polymeric membrane (PM) was prepared by electrospinning for lithium batteries. The formation of beads was significantly reduced by increasing the concentration and by reducing the surface tension of the polymer solutions. Although, single layer PVdF-HFP membrane exhibited high porosity and uptake of electrolyte, its mechanical integrity was found to be poor (not free-standing). On the other hand, electrospinning of PVC over PVdF-HFP enhanced the mechanical integrity of the membrane. The prepared membranes were subjected to SEM, ionic conductivity, electrolyte uptake and shrinkage analyses. A 2032-type coin cell composed of Li/PM/LiFePO4 has been assembled and its cycling profile was examined at different C-rates. The (PVdF-HFP)/PVC/(PVdF-HFP) trilayer membrane can be a strong contender for lithium battery applications. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Lithium batteries are identified as the ultimate choice of power to energize portable electronic devices such as laptop computers, digital cameras, cellular phones etc.,[1]. They are the technology of choice for future hybrid electric vehicles, which are urgently needed for addressing energy and environmental issues [2]. The state-of-art lithium-ion battery comprises a graphitic electrode (anode) and a positive electrode (cathode) obtained from layered/olivine lithium transition metal oxides separated by a poly (olifine) separator soaked in a non- aqueous liquid electrolyte [3,4].The key role of a separator is to prevent electrical short circuits between the electrodes with a rapid admission of ionic charge carriers [5]. The ionic conductivity of the porous membranes mainly depends on the conductivity of liquid electrolyte, membrane’s porosity, tortuosity of the pores, thickness and its wettability [6,7]. Microporous polyolefin membranes which are made up of poly ethylene (PE) or poly (propylene) (PP) are commonly used for lithium-ion battery applications. Although, these membranes provide excellent chemical and mechanical properties, the low

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A.M. Stephan). http://dx.doi.org/10.1016/j.electacta.2014.01.162 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

porosity (about 40%) and poor wettability, remain a problem area [8]. Consequently, these factors restrict the performance of the batteries [9,10]. Therefore, in order to circumvent these problems numerous attempts are being made to develop porous polymeric separators for lithium-ion batteries. The commercially available Celgard (2325) membrane is composed of poly (propylene) (PP)/poly(ethylene) (PE)/poly (propylene)(PP) trilayer structure. The low melting point of PE enables its use as a thermal fuse. When the temperature approaches (due to unexpected chemical reactions in a battery system) the melting point of the polymer 135 ◦ C, for PE and 165 ◦ C for PP, the shutdown process takes by losing its porosity [5]. However, these membranes possess only 50% porosity and the wettability of the membranes is also low. Very recently, the electrospinning method has drawn attention due to its versatility and simple preparative methods [11,12]. These are made up of thin fibres from micron to submicron diameters. Another advantage is the inter-laying fibres generate large porosity (>90%) with fully interconnected pore structure and large surface area to volume ratio facilitating high electrolyte uptake and easy transport of ions [13]. PVdF-HFP has slightly lower critical surface tension value ‘␥c ’ (25 mN m−2 ) than commercially available poly (propylene) separator (29 mNm−2 ) which facilitates for better wettability of the non-aqueous electrolytes [14]. On the other hand, PVC is mechanically robust, inexpensive and compatible with a large number of carbonate plasticizers, and has been reported for lithium-ion battery applications [15,16].

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Fig. 1. SEM image of PVdF-HFP (a) membranes with beads (b) membranes without beads (c) Histogram of the electrospun membrane.

In the present work, an attempt has been made to prepare a trilayer polymeric membrane by electrospinning in order to enhance the uptake of electrolyte solution and to improve the mechanical integrity and thermal stability of the polymeric membrane. The cycling performance of the trilayer membrane was analysed by assembling a 2032-type coin cell with Li/LiFePO4 configuration and the obtained results are compared with the commercially available Celgard membrane. 2. Experimental setup The electrospinning equipment (Plastomek, India) consists of a high voltage supplier (25 kV), and a syringe pump with a plastic syringe equipped with a 22 gauge stainless steel needle. Aluminum foil was used to collect the membrane. PVdF-HFP (88:12) (Kynar 2801, Alf Chem, Japan), poly (vinyl chloride) (Aldrich, USA) were used as received. The distance between the orifice and the aluminum collector was 10 cm and the applied voltage was 12 kV. The solution feeding speed was fixed as 1 ml/h at 25 ◦ C. The polymer solution composed of PVdF-HFP and acetone was electrospun on an aluminum collector. Then PVC was electrospun over PVdFHFP. The same procedure was adopted to coat PVdF-HFP over PVC in order to get a trilayer configuration. The overall thickness of the membrane was 70 microns. In order to maintain the adhesiveness among the membranes, the coating process was completed within 30mins. Morphological examination of the films was made by a scanning electron microscope (FE-SEM, S-4700, Hitachi) under a vacuum condition (10−1 Pa) after sputtering gold on one side of the films. The histogram of the electrospun membranes were generated from the SEM images using the Image J software. TG measurements were performed at a rate of 10 ◦ C min−1 between temperature ranges from 20 to 300 ◦ C in a nitrogen atmosphere. The ionic conductivity of the membranes sandwiched between two stainless steel blocking electrodes (1 cm2 diameter) was measured using an electrochemical impedance analyzer (IM6-Bio Analytical Systems) between 50 mHz and 100 kHz frequency range at ambient temperature. The mechanical strength of the electrospun membrane was determined using a tensile

machine (Tinius Olsen, Germany) according to ASTM D882-09 standards with a constant cross-head speed of 10 mm min−1 . The stretching of Celgard membrane was in machine direction (MD). The composite cathode was prepared by blending LiFePO4 as active material with acetylene black carbon as electronic conductor and poly(vinylidene fluoride) as binder in the 70:20:10 wt.% ratio respectively as reported earlier[17,18].The electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate(DMC) with a 1:2 volume ratio (Merk, Germany). The lithium metal foil (Aldrich, USA) was used as anode. 3. Results and discussions Fig. 1 shows the SEM image of PVdF-HFP electrospun single layer membrane. The SEM image appears with lot of beads (Fig. 1a) with an average fibre diameter of less than 200 nm. The formation of beads along with the nanofibers is an undesirable property. The exact reason for the formation of beads is yet to be understood. According to Shui and James[19] the bead formation is a complex process which competes with solidification. The formation of beads can be avoided by increasing the viscosity of the solution (higher polymer concentration) and increasing the charge density or reducing the surface tension. In the present study, the formation of beads was avoided by increasing the polymer concentration and also reducing the surface tension by adding dimethyl formamide as an additional solvent (Fig. 1b). Fong et al. [20] eliminated the formation of beads in poly (ethylene oxide), PEO system by reducing its surface tension with the addition of ethanol in water. Similarly, Lin et al. [21] formed a beads-free poly (styrene) membrane by adding a small amount of carbon surfactants during electrospinning. It is also observed from the histogram of the electrospun membrane (Fig. 1c) that the average fibre diameter varies from 600 nm to 1.6 microns. Fig. 2 (a) depicts the surface morphology of PVdFHFP/PVC/PVdF-HFP trilayer membrane. The cross-sectional SEM image of the PVdF-HFP/PVC/PVdF-HFP trilayer membrane is shown in Fig. 2(b) which implies a hairy rod structure with lot of pores which may facilitate to entrap huge amount of liquid electrolyte and thereby lithium ion conduction. The histogram

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169

Fig. 2. (a) Surface morphology of PVdF-HFP/PVC/PVdF-HFP trilayer membrane (b) cross-sectional SEM of tri-layer membrane (c) Histogram of the electrospun membrane.

layer of PVC on to the PVdF-HFP mat a bilayer configuration is obtained. The fine nanofibers of PVC fill the interstices of the PVdFHFP mat [25,26]. Upon further electrospinning of a third layer of PVdF-HFP on to a bilayer leads to the sandwich configuration. It is also evident from the Fig. 5 that the trilayer membrane exhibits higher tensile value than the single layer (PVdF-HFP) membrane. However, the tensile strength of the trilayer membrane is inferior to the Celgard membrane [27,28]. According to Choi et al. [29] the existence of the solvents such as chloroform and acetone on the surface of the electrospun polymeric membrane may induce physical cross linking and eventually deteriorate the surface morphology. Therefore, in the present study, all the electrospun membranes were dried at 50 ◦ C in a vacuum oven for 24 h before characterization in order to remove the solvents. TG-Single layer TG-Trilayer TG-Celgard

100 90

Weight (%)

of the trilayer membrane Fig. 2 (c) shows that the electrospun membrane has an average fibre diameter between 800 nm to 1.9 microns. The maximum number of fibres has an average diameter of 1.45 micron. In order to determine the thermal stability of electrospun polymeric membranes TG-DTA analysis was made. Generally, the heating process brings a lot of changes in the polymeric membranes and finally leaving behind inert residue. The TG traces of Celgard, PVdF-HFP(single layer) and PVdF-HFP/PVC/PVdF-HFP (trilayer) polymeric membranes are displayed in Fig. 3(a). A weight loss of around 2% was observed for the PVdF-HFP single layer membrane around 50 ◦ C and is attributed to the removal of residual solvents and moisture absorbed at the time of loading the sample. Further weight loss between 125-130 ◦ C is attributed to the melting point of PVdF-HFP [14,22].The irreversible decomposition starts around 160 ◦ C. For the trilayer membrane the onset decomposition starts around 230 ◦ C and is attributed to the higher melting point of PVC [15]. The electrospinning of PVC resulted in a strong barrier effect preventing the PVdF-HFP from the thermal degradation to a certain extent and this observation is an indication of the fact that the trilayer membrane is stable up to a temperature of 230 ◦ C in nitrogen atmosphere [23,24]. On the other hand, the observed weight loss which corresponds to 8 wt.% at 135 ◦ C for the Celgard membrane is attributed to the melting point of poly ethylene. Moreover it meets out a weight loss of nearly 10% at 135 ◦ C. The photographs of the trilayer and Celgard membrane before and after heat treatment at 150 ◦ C are shown as Fig. 4. In order to quantify the mechanical strength, the stress-strain properties have been measured and are shown in Fig. 5. The PVdFHFP (single layer) system shows a typical behaviour of flexible amorphous thermoplastic material whose deformation characteristic is very similar to rubber [25]. Upon electrospinning a second

80 70 60 50 40 30 50

100 150 200 250 300 350 400 450 500 550 0

Temperature ( C) Fig. 3. TG–traces of single, trilayer and Celgard membranes.

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Fig. 4. Shrinkage test on ceramic membrane. (i) Before heat treatment. (a) Trilayer membrane (b) Celgard membrane (ii) after heat treatment at 150 ◦ C (c) Trilayer membrane (d) Celgard membrane

The prepared membranes were soaked in a non-aqueous electrolyte for 1 h and its porosity was determined using the equation [30]: P = Ma/a

(1)

Ma/a + Mp/p

Where Mp, Ma is the mass of the dry and electrolyte absorbed in the membrane, respectively and p is the density of the polymer. The electrolyte leakage ratio of the solution “Rl ” is defined as the ratio of electrolyte absorbed by the membrane and retained after the test [31]. Rl =

Raf − Rai

(2)

Rai

Where Rai and Raf respectively denotes initial and final absorbed weight of the electrolyte. The tortuosity of the membrane was calculated from the ionic conductivity and porosity data [5,32]; T=

 x P 1/2 0

(3)

Where ␴, ␴0 and P respectively represents the conductivity of the liquid electrolyte, polymer electrolyte and porosity of the membrane. The tortuosity of the membrane was measured as 0.8. 2.0 Single layer Trilayer

Stress (MPa)

1.6

1.2

0.8

0.4

0.0 0

50

100

150

200

250

300

Strain (%) Fig. 5. The stress vs. strain behavior of electrospun membrane and Celgard membranes.

Based on the above experiments the porosity of the single layer was measured as 70% and showed an electrolyte uptake of 247% with a solution leakage of 0.75%. The membrane exhibited an ionic conductivity of the order of 3.2 × 10−3 S cm−1 at 25 ◦ C. On the other hand, upon coating PVC over PVdF-HFP, the porosity of the trilayer membrane has been reduced significantly to 62% with a remarkable reduction in the uptake (230%) of electrolyte solution. The solution leakage and ionic conductivity were calculated as 0.5% and 1.58 × 10−3 S cm−1 respectively. The reduction in the porosity and uptake of the electrolyte may be attributed to higher fibre diameter of trilayer membrane (Fig. 1c above 800 nm). Table 1 compares the physical properties of single, trilayer and Celgard membranes. Fig. 6(a) illustrates the typical charge and discharge (potential vs. time) profiles obtained at 25 ◦ C between 2.5 and 4.0 V vs. Li/Li+ at different current regimes. The cell showed a highly reproducible and well defined flat potential charge/discharge plateaus, around at 3.45 V vs. Li/Li+ , which is a typical characteristic LiFePO4 -based composite electrode [23,33–35]. The discharge capacity as a function of cycle number is also depicted in Fig. 6(b).First, five formation cycles were performed at low C- rate (0.1 C). At its first cycle, a specific discharge capacity of about 125mAh g−1 was observed, for the cell with trilayer membrane while the cell with the Celgard delivered 120 mAh g−1 . Upon cycling at C/5-rate the cell delivered a discharge capacity of 123 mAh g−1 . An abrupt decrease in capacity was observed at 2 C and 5C- rates. The reduction in the discharge capacity at higher current regime is a typical characteristic of LiFePO4 material which is attributed to its low electronic conductivity and limited diffusion of Li+ - ion into its structure that causes electrode polarization [17,18]. Further, the declining discharge capacity at higher C-rates may be due to the solid electrolyte interface (SEI) film formation with electrolyte decomposition [36]. Recent study also revealed that, the increase in interfacial resistance value which originates from parameters related to the electrode design such as thickness and density can cause capacity fading at higher rates [37]. In the present study, irrespective of the C-rates, the Celgard membranes delivered a lower discharge capacity when compared to trilayer membrane. While comparing the performance of trilayer membrane with the reduction in the discharge capacity of Celgard membrane is attributed to the lower uptake of electrolyte solution which arises due to the lower porosity of the polymeric membrane. In a similar way, Kim et al.,[38,39] reported better cycling performance for SiO2 /Al2 O3 coated PVdF-HFP membranes than poly propylene separator with Li [Ni1/3 Co1/3 Mn1/3 ]O2 as cathode.

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Table 1 Physical properties of (PVdF-HFP), (PVdF-HFP)/PVC/(PVdF-HFP) and Celgard membranes. Type of membrane

Porosity (%)

Electrolyte uptake(%)

Ionic conductivity S cm−1

Solution leakage (%)

Shrinkage (%)

Single layer (PVdF-HFP) Trilayer (PVdF-HFP)/PVC/(PVdF-HFP) Celgard*

70 62 48

247 230 120

3.2 × 10−3 1.58 × 10−3 1.01 × 10−3

0.75 0.5 0.3

>0.5 >0.2 Rolled off

*

at our laboratory condition

a

4.2 4.0 3.8

Voltage (V)

membrane has been attributed to higher porosity and uptake of the electrolytes over commercially available Celgard membrane.

Celgard (charging) Celgard (discharging) Trilayer (charging) Trilayer (discharging)

Acknowledgements

3.6

The authors gratefully acknowledge Council of Scientific and Industrial Research (CSIR, India) for financial support through TAPSUN program.

3.4 3.2

References

3.0 2.8 2.6 2.4 0

20

40

60

80

100

120

140

Capacity mAh g -1

-1

Discharge capacity (mAh g )

b

140 C/10

C/5

120

C/2 1C

1C

100 Celgard Trilayer

80 2C

60 40 5C

20 0

10

20

30

40

50

Cycle number Fig. 6. (a.) The cycling profile of LiFePO4 /polymeric membrane (trilayer)/Li cell at their. 11th cycle. Fig. 6 (b.) The discharge capacity as a function of cycle number for the cell. Li/PM/LiFePO4 at different C-rates.

The cell operated with the expected voltage vs. capacity profiles, and thus accounting for a good interfacial contact between electrodes and trilayer membrane. It is also quite obvious from the Figure that the cell retained its original capacity when cycled again at 1 C rate after 45 complete charge/discharge cycles. The results here discussed, although preliminary, demonstrate the feasibility of employing trilayer polymeric membranes with better electrochemical properties.

4. Conclusions A trilayer polymeric membrane composed of PVdFHFP/PVC/PVdF-HFP was electrospun successfully and it exhibited better electrochemical properties than the commercially available Celgard membrane. The better performance of the electrospun

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