J Solid State Electrochem (2013) 17:1377–1382 DOI 10.1007/s10008-013-2000-z
ORIGINAL PAPER
Physical, thermal, and electrochemical characterization of stretched polyethylene separators for application in lithium-ion batteries K. Prasanna & Chang Woo Lee
Received: 6 August 2012 / Revised: 1 January 2013 / Accepted: 8 January 2013 / Published online: 25 January 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract The single-layered microporous polyethylene separator is prepared by dry process and has been stretched in uni-axial direction to two different ratios namely 180 and 300 % in order to create highperformance and cost-effective separator for practical application in lithium-ion batteries. In this study, the structures of the microporous polyethylene separator prepared by dry process and uni-axially stretched to two different ratios of 180 and 300 % were characterized. The physical structure of the stretched separator is characterized by key factors such as thickness, mean pore size, porosity, Gurley value, ionic resistivity, MacMullin number and tortuosity. The thermal behavior of the stretched separator is explained by using differential scanning calorimeter (DSC). DSC explains the melting and shutdown behavior of the separator. Electrochemical property is studied by linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and cyclic performance. EIS is performed to explain, in elaborate, the resistance of separator and the specific discharge capacity is observed using the cyclic performance. Three hundred percent stretched separator is observed to have comparatively less resistance and higher discharge capacity than the 180 % stretched separator. Keywords Dry process . Uni-axial . Stretching ratio . Polyethylene . Lithium-ion batteries
K. Prasanna : C. W. Lee (*) Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1732 Deogyeong-daero, Gihung, Yongin, Gyeonggi 446-701, South Korea e-mail:
[email protected]
Introduction Lithium-ion battery is believed to be the most capable power source of electric vehicles that does not use gasoline and has no carbon emission [1]. Application of lithium-ion battery faces several problems one of which is safety issues. Among the several other factors separators play a major role in the safety aspect. In order to select a separator for lithium-ion batteries, several factors must be considered namely, the separators should have lower ionic resistance, robust mechanical and dimensional stability, sufficient physical strength to sustain battery assembly process, and durable chemical resistance against electrode and electrolyte [2]. At present microporous polyolefin membranes like polyethylene (PE), polypropylene (PP), and combinations of the two are widely used in lithium-ion batteries [3]. Despite the widespread use of separators, a great need still exists for improving the performance, increasing the life, and reducing the cost of separator. This paper discusses aspects of improving the PE separator properties, based on reasonable changes in physical properties. The primary function of the separator is to keep the positive and negative electrodes physically apart for preventing flow of any electronic current directly passing between them and on the other hand to allow the optimum flow of ionic current [4]. This function can be characterized by measuring the ionic resistivity of the separator. Ionic resistivity of the separator is essentially the resistivity of the electrolyte that is embedded in the pores of the separator [5]. Determination of MacMullin and tortuosity factors provides clear view on the porous nature of the separator. The thickness of the membrane and the extent to which the electrolyte wets the pores of the separator also plays a major role in the resistive nature of the separator [6].
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In this work, PE separator prepared by dry process and uni-axially stretched to two different ratios of 180 and 300 % were tested over its physical, thermal, and electrochemical properties for its application in lithium-ion batteries. Physical characters namely thickness, pore size, porosity, Gurley value, ionic resistivity, MacMullin number, and tortuosity have been measured and explained in detail. DSC is performed to study its thermal character, specific to its shutdown behavior. Regarding the electrochemical characterization linear sweep voltammetry, EIS analysis, and cyclic performance have been performed to prove its application in lithium-ion batteries.
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Mean pore size All measurements were carried out over the pressure range 0.6×105 to 410×105 Pa using a Micrometrics® Autopore IV 9200 series. The pore diameters were measured by intrusion and extrusion of mercury. In this method, the size and volume of pores in a material are measured by determining the quantity of mercury, which can be forced into the pores at increasing pressure. Pd ¼
2g cosðθÞ ΔP
ð1Þ
Experimental
Where Pd is the pore diameter, ΔP the mercury pressure, γ the surface tension equal to 480×10−5 Ncm−1, and θ is the contact angle equal to 130° for all samples [8].
Materials
Porosity
Separator
Control of porosity is very important regarding the battery separators. The porosity is also important for high permeability and also for providing a reservoir of electrolyte. The porosity is measured using liquid or gas absorption methods according to American society for testing and materials (ASTM) D-2873 [9].
The microporous PE separator prepared by dry process and stretched to different ratios in uni-axial direction is provided by CS Tech, South Korea. The provided separator is of two kinds namely 180 % stretched and 300 % stretched. Liquid electrolyte The electrolyte used is non-aqueous 1 M LiPF6 in ethylene carbonate (EC): diethylcarbonate (DEC) (1/1 vol%; StarLyte, Ukseung Chemical Co., Ltd). The electrolyte is stored in a glove box under argon atmosphere. Electrode The cathode electrode fabrication is done by mixing 94 wt.% of layered Li[Ni1/3Co1/3Mn1/3]O2 active material, 3 wt.% of conductive carbon agent, and 3 wt.% of polyvinylidene fluoride which acts as a binder. Commercially available lithium metal is used as an anode electrode. Physical characterization Thickness The separators being developed for EV/HEV applications will require thicker (≈40 μm) separators. In the current technologies, ≤25 μm seems to be the standard thickness for consumer rechargeable batteries. The thinner separator takes up less space and allows the utilization of longer electrodes. The thinness also makes it a low resistance separator [7]. The thickness of the separator is measured by the method of electrical thickness gauge.
Gurley value Gurley value expresses the time required for a specific amount of air to pass through a specific area of separator under a specific pressure. Air permeability measurement is commonly denoted as Gurley. Gurley value is directly proportional to electrical resistance, for a given separator morphology. Lower Gurley value means higher porosity, lower tortuosity, and accordingly lower electrical resistance. Gurley value is measured by standard test method described in ASTM-D726 (B) [6]. Ionic resistivity Ionic resistivity measurement is a more prevalent measure of permeability than the Gurley value, since the measurements are performed in the presence of actual electrolyte solution. For the separator resistance, Rs, measurement CR 2032 coin cell is assembled with the separator immersed in the 1 M LiPF6 in EC/DEC (1/1 vol%) electrolyte and placed between two stainless steel (SS) electrodes (16 mm diameter). Cell has been kept with no run for the separator to get saturated with the electrolyte for 24 h. Measurements were carried out using EIS (IVIUM technologies instruments) over the frequency range from 0.1 Hz to 100 kHz at 10 mV scan rate from this the resistance offered by the separator and electrolyte, R, is measured. Rs is measured using the Eq. 2, where Re refers to the resistance of electrolyte [4]. Based on the values obtained, specific resistivity of
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the separator (ρs) and electrolyte (ρe) had been determined by using the Eqs. 3 and 4. Rs ðseparatorÞ ¼ Rðseparator þ electrolyteÞ Re ðelectrolyteÞ
ρs ¼
ρe ¼
Rs A 1 Re A 1
ð2Þ
ð3Þ
ð4Þ
Where “A” is the area of the electrode and “I” is the thickness of the separator.
MacMullin number is the ratio of the resistivity of the separator to that of the electrolyte. It describes the relative contribution of a separator to cell resistance. It assumes that the separator wets completely in the electrolyte. MacMullin is free of the used electrolyte but associated with the thickness of the material [6, 8]. It has been calculated from the following Eq. 5, ρs ρe
Electrochemical characterization Linear sweep voltammetry Linear sweep voltammetry is performed using an IVIUM technologies instruments. For the measurement of linear sweep voltammetry 2032 coin cell is prepared using SS and lithium electrode. SS acts as a working electrode and lithium acts as a counter and reference electrode. The prepared coin cell is scanned at the rate of 1.0 mVs−1 [11, 12]. Electrochemical impedance spectroscopy
MacMullin number
Nm ¼
temperature sweep rate. Here the sample is scanned between 60 and 150 °C by heating at a rate of 20 °C per minute. DSC provides information regarding the melting point and the shutdown behavior [10] of the sample.
ð5Þ
Where Nm is the MacMullin number. Tortuosity Tortuosity depends upon the ionic path length of the separator and thickness of the separator. It provides information regarding the transport of ions on the effect of pore blockage [6, 8]. Tortuosity is calculated by the value obtained from the electrical resistivity measurement of separator and electrolyte, porosity value using the following Eq. 6. rffiffiffiffiffiffiffi ρs " t¼ ð6Þ ρe Where τ is the tortuosity and ε is the porosity.
EIS measurements were carried out for half cell (2032 coin type), which consist of [Ni1/3Co1/3Mn1/3] O2 as cathode, lithium metal as anode, sample separator, and 1 M LiPF6 in EC/DEC (1/1 vol%) electrolyte. Measurement is carried by a.c. complex impedance analysis using IVIUM technologies instruments over the frequency range from 0.1 Hz to 100 kHz at 10 mV scan rate. Using the EIS the change in charge transfer resistance (Rct) is measured on certain intervals namely 24, 48, 72, and 120 h [13]. Cyclic performance The cyclic performance (charge–discharge) is observed in galvanostatic mode using BT2000 Arbin cycler. Charge–discharge test is carried out for the 2032 coin cell assembled by sandwiching the stretched separator between the [Ni1/3Co1/3Mn1/3] O2 cathode and lithium metal anode along with the 1 M LiPF6 in EC/DEC (1/1 vol%) electrolyte. The cell is cycled between the cut-off voltage of 2.7 and 4.2 V at constant 0.1 C rate at room temperature. By this the specific discharge capacity for 30 cycles is observed.
Results and discussion Physical characterization
Thermal characterization DSC DSC helps in studying the thermal property of the stretched separators. DSC experiment is carried out using TA Instruments, Model 2920; the samples are placed in sample pan made of aluminum. DSC measures the difference of energy required to heat a reference pan and a sample pan at a fixed
Physical characters such as thickness, mean pore size, porosity, Gurley value, ionic resistivity, MacMullin number, and tortuosity have been studied with respect to the two different stretching ratios. Thickness is the essential factor to enhance the mechanical strength of the separator to reduce the probability of punctures during the fabrication of the cell. Separator should also be relatively thin to provide sufficient place for active materials to increase the efficiency of the
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battery. Thinner separator will also provide lower resistance inside the cell as shown in Fig. 1. Increase in the stretching ratio reasonably reduces the thickness and increases the pore size and porosity as shown in Table 1. Three hundred percent stretched separator is observed to have lower thickness, correspondingly large pore size, and higher porosity. Sub-micron dimensions of pores are likely to be chosen for the lithium-ion batteries to avoid the internal shorts. Higher and uniform porosity is preferred for unhindered ionic current flow. Increase in porosity decreases the Gurley value as displayed in Table 1. Increase in porosity also provides a better reservoir for the electrolyte in the cell. Hence 300 % stretched separator with higher porosity value provides more space for the electrolyte and accordingly reduces the ionic resistivity [9]. Separators are the key factor for the movement of ions inside the cell. Parameters such as MacMullin number and tortuosity are used to characterize this behavior. Both are defined by studying the ionic resistivity of separator saturated in the electrolyte and placed between two SS electrode. Electrolyte 1 M LiPF6 in EC/DEC (1/1 vol%) is used commonly for both the separator. MacMullin number provides information regarding the increased resistance of the inter-electrode medium, caused by the presence of a separator in relation to the resistance of the liquid electrolyte. As the MacMullin number increases the ionic resistance also increases related to the porosity structure and poor affinity between the separator and electrolyte. Here lower MacMullin number is obtained for the 300 % stretched separator, indicating its high porous volume and good affinity towards electrolyte. Tortuosity is used to provide information regarding the ionic transport on the effect of pore blockage. Tortuosity factor τ=1 describes an ideal porous body with cylindrical and parallel pores. Whereas τ>1 is good for dendrite resistance but can lead to higher separator resistance [6]. Here the τ value is observed to be >1 for both the 900
300% Stretched 180% Stretched
800
Table 1 Thickness, mean pore size, porosity, and Gurley value of stretched separators Property
Unit
Hot stretching ratio 180 %
Hot stretching ratio 300 %
Material Thickness Mean pore size Porosity Gurley value
– μm μm % s/100 cc
PE 22 0.04 40 400
PE 20 0.08 45 250
300 and 180 % stretched separator. Three hundred percent stretched separator is observed to encompass lower tortuosity value compared to 180 % stretched separator favoring the dendrite resistance as well as reducing the separator resistance reasonably. Values of ionic resistivity, MacMullin number, and tortuosity are shown in Table 2. Thermal characterization The shutdown property of the separator is a very useful safety feature for preventing the thermal runaway reactions in lithium-ion batteries. Hence the shutdown property of the separator becomes mandatory to be determined. DSC is the efficient method for determining the shutdown property of the separator. Shutdown usually takes place close to the melting temperature of the separator. Shutdown property refers to the change of porous ionically conductive polymer film into a non-porous insulating layer between the electrodes. Due to the non-porous nature of the separator at the shutdown point, the resistance increases dramatically and the passage of current through the cell is restricted, by this explosion of the cell is avoided. The commercially used PE separators have shutdown temperature of 135 °C [14]. As observed in Fig. 2 different ratios of stretched PE separators are also observed to possess similar shutdown temperature of around 135 °C, which means that the increase in stretching ratio and reduction in thickness does not have any effect on the melting temperature.
700
Electrochemical characterization
600
-Z" /
500
Electrochemical studies were performed with the help of linear sweep voltammetry and EIS to prove its application
400 300 200
Table 2 Ionic resistivity, MacMullin, and tortuosity value of stretched separators
100
PE separator stretching ratio
ρe(Ω cm)
Nm
τ
180 % 300 %
1.68 1.02
14 8.5
2.3 1.9
0 0
0.005
0.01
0.015
Z'/
Fig. 1 EIS of separators sandwiched between SS plates
0.02
ρe =0.12 Ωcm at 25 °C
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4
a
180% Stretched
300
300% Stretched
24 hours
2
48 hours
250
120 hours 200
-Z" /
Heat Flow / wg-1
72 hours
0
-2
150
100
-4
50
-6 0
-8 60
80
100
120
0
100
200
300
400
500
600
Z'/
140
Temperature / °C
Fig. 2 DSC profiles of 180 and 300 % stretched polyethylene separator
b
160 24 hours 48 hours 72 hours 120 hours
140
0.0045 300% Stretched
0.0040
120
-Z" /
100 80 60 40 20 0
0
50
100
150
200
250
300
350
Z' /
c 25 300% Stretched 180% Stretched 20
Increase in Rct / %
in lithium-ion batteries. Linear sweep voltammetry is carried out using the prepared cell and it shows no decomposition below 5 V vs. Li+/Li as displayed in Fig. 3, which proves its application in high-voltage lithium-ion batteries [11]. The EIS analysis have been carried out over the 2032 coin cell consisting of the sample separator along with the Li[Ni1/3Co1/3Mn1/3] O2 cathode, pure lithium anode, and 1 M LiPF6 in EC/DEC (1/1 vol%) electrolyte. The change in Rct obtained for different intervals over the 180 % stretched and 300 % stretched separators is shown in Fig. 4a and b, respectively [13]. From the figure the increase in Rct is observed and to be more specific the percentage of increase in Rct is shown in Fig. 4c. Three hundred percent stretched separator is observed to have lower percentage of increase in Rct than the 180 % stretched separator due to its higher porosity, lower Gurley value, lower tortuosity, and MacMullin number. Specific discharge capacity of prepared cells is
15
10
180% Stretched
0.0035
5
Current / mA
0.0030 0
0.0025
0
20
40
60
80
100
120
0.0020
Time / hour
0.0015
Fig. 4 EIS obtained based on specified time intervals for the Li[Ni1/ 3Co1/3Mn1/3]O2/Li cells containing: a 180 % stretched separator, b 300 % stretched separator, and c percentage of increase in Rct based on specified time intervals
0.0010 0.0005 0.0000
0
1
2
3
4
5
6
7
8
9
10
Voltage / V
Fig. 3 Linear sweep voltammogram of 180 and 300 % stretched polyethylene separator at room temperature and the scan rate of 1.0 mVs−1
observed at constant 0.1 C rate, with the cut-off voltage range of 2.7 and 4.2 V. Initial discharge capacity of 165.74 and 167.70 mAhg−1 is observed for 180 and 300 % stretched separator respectively. From Fig. 5, it is observed that the
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Specific Discharge Capacity / mAhg-1
180
lithium-ion batteries in practical applications. Electrochemical analysis acts as a proof for application in lithium-ion batteries. In EIS 300 %, stretched separator is observed with less percentage of increase in resistance based on aging, by linear sweep voltammetry confirms the application of 180 and 300 % separators in high-voltage lithium-ion batteries and by cyclic performance higher discharge capacity is observed for 300 % stretched separator. By these analysis 300 % stretched separator is observed to posses better properties than the 180 % stretched separator and hence it is suggested to be a better option for application in lithium-ion batteries.
170 160 150 140
180% Stretched
130
300% Stretched
120 110 100
0
5
10
15
20
25
30
Cycle Number
Fig. 5 Specific discharge capacity of Li/Li[Ni1/3Co1/3Mn1/3] O2 cell based on the 180 % stretched and 300 % PE separators
Acknowledgments This research was supported by the Collaborative Research Program among industry, academia, and research institutes through the Korea Industrial Technology Association (KOITA) funded by the Ministry of Education, Science, and Technology (KOITA-2012).
specific discharge capacity of the 300 % stretched separator is higher and also stable than that of 180 % stretched separator. One hundred eighty percent stretched separator have less discharge capacity due to the increase in resistance of ion migration in the separator. Hence 300 % stretched separator is suggested to be used in lithium-ion batteries for better performance.
References
Conclusions Uni-axially stretched PE separator had been investigated on its physical, thermal, and electrochemical properties for its application in lithium-ion batteries. The physical properties had been clearly analyzed to prove its enhancement in electrochemical efficiency. Physical factors specifically MacMullin and tortuosity seem to be a relevant indicator to define the efficiency of the separator inside the battery. The 300 % stretched separator is observed to have better physical properties than the 180 % stretched separator. Thermal property of 180 and 300 % stretched separator is similar compared to the commercially used PE separator, which ensures the proper thermal shutdown operation of
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