Effect of Pressure On Separator Ionic Conductivity
IEST Instruments
Word-leading Innovative Lithium Battery Testing Solutions Provider and Professional Instruments Manufacturer
1. Background
The intercalation/deintercalation or deposition/stripping of lithium ions, along with side reactions such as continuous growth of the SEI film and gas generation, can induce internal pressure in batteries. This pressure influences various battery performance characteristics through interfacial effects[1]. Due to the porous structure and material properties of separators, significant deformation occurs under pressure, leading to pressure-dependent changes in separator ionic conductivity. Studies have shown that the deformation-pressure relationship curve of porous separators during compression can be divided into three stages: elastic (Ⅰ), plastic (Ⅱ), and densification (Ⅲ) stages, as illustrated in Figure 1.
In Stage Ⅰ, the porous separator undergoes substantial elastic deformation under pressure but rapidly restores its original morphology upon pressure release, maintaining unchanged ion transport properties. During Stage Ⅱ, the separator experiences plastic compression, where the pore structure becomes irreversibly deformed. Ion transport capability cannot be fully preserved, and the separator fails to fully recover its initial state after pressure removal. In Stage Ⅲ, the porous structure collapses and densifies, with pores completely closing to form a compact structure. At this stage, the separator loses its ion transport functionality[2-3].
Data show that the membrane deformation is 0.5 at 0-20 MPa, and increases from 0.5 to 0.55 at 20-30 MPa. At 0-12 MPa, the ionic conductivity of the membrane decreases with the increase of stress, and the two are linearly related. When the stress is higher than 12 MPa, the separator ionic conductivity decreases rapidly and gradually tends to be stable [4]. The different separator ionic conductivity decays differently with the increase of pressure. This paper tests the changes in the two separator ionic conductivity at 0-3 MPa to explore the effect of pressure on the separator ionic conductivity.
2. Test conditions & methods
2.1 Test Instrument
The Multi-channel Separator Ionic Conductivity Test System (EIC2400M) developed by IEST is used as shown in Figure 2. The equipment contains 4 test channels and can provide a high-purity argon atmosphere to achieve electrochemical impedance spectroscopy testing of multi-channel symmetrical batteries. The pressure range is 10~50Kg and the frequency range is 100KHz~0.01Hz.
1.2 Test Samples
Separator A and Separator B.
1.3 Test Procedure & Calculation Method for Separator Ionic Conductivity
Figure 3. EIS impedance spectra of different diaphragm layers (a); R value fitting diagram (b)
The diaphragm ionic conductivity is obtained by substituting the obtained ionic impedance R into Equation 1.
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σ=d /( R * S)? （1）
Among them, σ is the ionic conductivity, d is the thickness of separator , R is the ionic resistance, and S is the reaction area of the separator.
2. Results Analysis
Figure 4 and 5 shows the EIS spectra of separators A and B measured under different pressures. Using the obtained EIS as the baseline for linear fitting, the intersection values of the fitted line and the X-axis are recorded to obtain the impedance values R1, R2, R3, and R4 for 1 to 4 layers of the separator. Then, with the number of layers on the X-axis and R1, R2, R3, and R4 on the Y-axis, linear fitting is performed to obtain the ionic impedance R of the separators under different pressures, as shown in Tables 1 and 2. Substituting the R values into formula (1), the ionic conductivity of the separators under corresponding pressures is calculated, as illustrated in Figure 5. From the figure, it can be observed that as the pressure increases, the ionic conductivity of the separators decreases linearly, and the rate of decrease for separator B is significantly greater than that for separator A.
The porosity of the separator is a critical factor determining mass transport, as it ensures sufficient Li+ ion conductivity. The data from this experiment demonstrate that mechanical pressure alters the microstructure of the separator, hindering ion migration and thereby reducing the Li+ conductivity of the separator.
4. Summary
This study utilized the multi-channel separator ionic conductivity testing system independently developed by IEST to measure the different separator ionic conductivity under various pressures. It was found that the separator ionic conductivity decreases with increasing pressure, and different separators exhibit varying rates of decline. When a separator is subjected to pressure in the thickness direction, its microporous structure inevitably changes, further affecting lithium ion transport and the overall performance of the battery. This highlights the importance of comprehensively understanding the relationship between the microporous structure of the separator and pressure for producing separators better suited to the internal environment of lithium-ion batteries. We can improve battery performance by testing the separator ionic conductivity and selecting the appropriate pressure.
5. References
[1] Cui Jin, Shi Chuan, Zhao Jinbao. Research progress on the effect of mechanical pressure on the performance of lithium batteries[J]. Journal of Chemical Industry, 2021, 72(7): 3511-3523.
[2] Gioia G, Wang Y, Cuiti?o A M. The energetics of heterogeneous deformation in open-cell solid foams[J]. Proceedings of the Royal Society of London Series A: Mathematical, Physical and Engineering Sciences, 2001, 457(2009): 1079-1096.
[3] Sarkar A, Shrotriya P, Chandra A. Modeling of separator failure in lithium-ion pouch cells under compression[J]. Journal of Power Sources, 2019, 435: 226756.
[4] Peabody C, Arnold C B. The role of mechanically induced separator creep in lithium-ion battery capacity fade[J]. Journal of Power Sources, 2011, 196(19): 8147-8153.