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The compression behavior, microstructure evolution and properties variation of three kinds of commercial battery separators under compression load
Journal of Power Sources 451 (2020) 227819
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The compression behavior, microstructure evolution and properties variation of three kinds of commercial battery separators under compression load Lei Ding a, Chao Zhang a, Tong Wu b, *, Feng Yang a, Ya Cao a, Ming Xiang a a b
State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, China College of Polymer Science and Engineering, Sichuan University, Chengdu, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Compression behavior of three kinds of separators were systematically analyzed. � Detailed microporous structure evolu tion of separators was exhibited firstly. � Good connection between microstruc ture evolution and properties variation was made.
A R T I C L E I N F O
A B S T R A C T
Keywords: Li-ion battery Separator Compression behavior Microporous structure evolution Macro-properties variation
In this article, the compression behavior, microstructure evolution and properties variation of three kinds of commercial polyolefin separator under compression load are systematically analyzed. The compression testing shows that three kinds of separators present two different deformation modes. The separator made from dry process by uniaxial stretching has good resistance to compression load due to the parallel lamellae aligned along thickness direction on cross-section. And all properties variation with stress display striking uniformities with compression curves. While the other two separators with biaxial stretching, both dry and wet process, have abundant multilayered structure, causing weaker resistance to compression load and flatter compression curves. And all properties show gentle trend for biaxial stretching separator. However, three kinds of separators also share some common characteristics. The separators compact violently along thickness direction under lower strain firstly, accompanied by the dramatic decline of porosity but slightly change connectivity along thickness direction. And subsequently the connectivity along thickness direction is severely destroyed at higher strain due to different reasons. The violent shear or glide of microporous structure occurs perpendicular to thickness di rection for separator with uniaxial stretching and micropores direct blocking by non-corresponding micropores position between multilayered structure for separator with biaxial stretching.
* Corresponding author. E-mail addresses: [email protected] (L. Ding), [email protected] (C. Zhang), [email protected] (T. Wu), [email protected] (F. Yang), [email protected] (Y. Cao), [email protected] (M. Xiang). https://doi.org/10.1016/j.jpowsour.2020.227819 Received 6 September 2019; Received in revised form 26 December 2019; Accepted 26 January 2020 Available online 31 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction
the NMC layer was likely the cause as it presents a greater constraint to the deformation of the separator at large strains. Lagadec [13] used real microstructures of polyethylene and polypropylene separators acquired with focused ion beam scanning electron microscopic (FIB-SEM) to mography and simulated how these structures deform under compres sive strain. At present, three most commonly used commercial separators are manufactured by polyolefin, namely the dry process by uniaxial stretching of hard elastic iPP [34,35], the dry process by biaxial stretching of β nucleated polypropylene (β-iPP) [36,37] and the wet process used ultra-high molecular weight polyethylene (UHMWPE) [38, 39]. All these three processes include an extrusion step to make cast films and employ one or more orientation steps to impart porosity. For the dry process with uniaxial stretching, the superhigh die draw ratio (DDR) is applied during extruding to form the parallel arranged lamellae (namely shish-kebab structure). Then after annealing the cast film at high temperature about 140 � C, the cast film is stretched uniaxially to prepare the separator. While as for the dry process with biaxial stretching, the β-nucleating agent is added into polypropylene to form β nucleated polypropylene (β-iPP) cast film during extruding. Then the β-iPP cast film is stretched directly along mechanical direction (MD) at the temperature about 80–100 � C and transverse direction (TD) at the temperature about 120–140 � C successively. The wet process was based on the mechanism of thermally induced phase separation, the cast film is composed of 30% ultrahigh molecular weight polyethylene (UHMWPE) and 70% paraffin oil. Then the synchronously or asynchronously biaxial stretching is imposed on cast film, following by extracting the paraffin oil with a volatile solvent to form the microporous structure. But un fortunately, the present researches on LIBs separator compressive property failed to provide a good contrast of the three kinds of com mercial separators, but only focused on one of them. Our research team [40] recently had tried to evaluate the three types of separators, and found that the dry process separator with uniaxial stretching had a better resistance to deformation at the early stage of compression. But the three types of separator had not been studied thoroughly owning to the limitation of current characterization techniques. In addition, pre sent references mainly focused on the characterization of compression performance, but failed to quantify the ability to resist compression deformation and provide the intuitive evidence of the three-dimensional microporous structures evolution during compression process. In this article, the compression behavior of three most commonly used separators based on polyolefin was explored, and the ability of separator to resist compression deformation was evaluated by comparing the variation of recovery rate under different compression condition. Moreover, the evolution of micropores shrinkage and closure, both on surface and cross-section, throughout the whole compression process was observed directly by SEM to better complement the short board in this aspect. Furthermore, various key properties changes (porosity, electrolyte upkate and Gurley value, etc.) of the separator were tested. Therefore, a comprehensive theoretical data system from membrane micropore structure to the macroscopic performances was shown clearly in order to get a better understanding about the separator under compression load.
With the rapid development of things like portable electronic devices and electric vehicles (EVs), the rechargeable lithium-ion batteries (LIBs) have attracted considerable attention due to the low self-discharge rate, evidently high specific energy and energy density, and thus are world wide used secondary batteries [1–8]. The LIBs are mainly composed of two electrodes, electrolyte and separator. The porous separator is the key material to prevent the anode and cathode from contacting and meanwhile, conduct li-ion between two electrodes. At present, the most commonly used commercial separator is mainly prepared from poly olefin due to the low cost, relatively uniform pore sizes and mature technology [9–12]. During the LIBs application process, the separators must meet the requirement of sufficient mechanical properties to withstand various forces inside and outside the batteries. especially the compressive stress. Actually, a dynamic pressure environment can be detected inside the battery throughout the whole life cycle [13]. The stack pressure is imposed on the battery during assembly to maintain intimate contact between all components [14]. External loading and collision in daily use process also cause fluctuations in the internal pressure [15–17]. Mean while, the formation and growth of SEI is another reason [18,19]. Most importantly, during each charging and discharging cycle of li-ion bat tery, the intercalation and de-intercalation of li-ion are not just an electrochemical process, but also a mechanical evolution of battery components, leading substantial volume expansion of electrode mate rials [20,21]: 2% for lithium cobalt oxide and 10% for graphite electrode [22], even as high as 400% expansion rate for the latest high capacity anode materials such as silicon [23]. Consequently, the expanding electrodes inevitably squeezes the limited interior space within battery and certainly compresses the soft separator, causing severe micropores deformation and subsequently a lower ionic conductivity along the thickness direction [24–26]. It can be seen that the compression stress inside the battery has a particularly important effect on the performance of the porous separator and even the battery. During the operation of a lithium ion battery, the average compress stress in the battery is even up to 5Mpa [13,27]. Due to the coarse and rough electrode surface of two electrodes, the local pressure on the separator is much higher than this level. Therefore, it is crucial to explore the impact of compressive stress on the typically commercialized separator. Arnold [28–30] explored the compressive mechanical properties of polypropylene separator (Celgard 3501) over a range of strain rates and in different fluid environments. They found a high strain rate depen dence due to the viscoelasticity behavior of the polymer skeleton and the poroelastic behavior of flow of the fluid in the pores. More interestingly, they disassembled the used battery and based on the postmortem analysis they considered that the compressive stress dose not just cause the mechanical evolution, but a coupling between mechanics and elec trochemistry in which higher levels of mechanical stress leaded to a faster chemical degradation [27]. Zhang et al. [16] characterized four types of battery separators and compared the mechanical performance, strength, and failure mode. Meanwhile, they found that the compression tests also revealed the anisotropic nature of the dry processed separators by the oval shape of separators after compression [31]. This phenome non had also been found by Arnold [29]. Kalnaus [32] researched the compressive property of separator (Celgard 2325) with different tem perature and rate by recording data through digital image correlation. The results showed significant dependence of mechanical properties on temperature. The strain rate strengthening also decreased with higher temperatures while the temperature softening remained independent of the applied strain rate. Yan et al. [33] computed the compressive stress-strain curve for the stack from that of the separator (Celgard 2500) and NMC cathode layer to investigate the interaction between the separator and the electrode under compressive stress. The results sug gested that, in the stack, the compressive deformation of the separator was influenced by the NMC cathode significantly. The rough surface of
2. Experimental 2.1. Materials Three classic manufacturing processes of the polyolefin separator included the dry process by uniaxial stretching of hard elastic iPP cast film (PP-D-U, ‘D’ means dry process and ‘U’ means uniaxial stretching), the dry process by sequential biaxial stretching of β nucleated poly propylene (PP-D-B, ‘B’ means biaxial stretching) and the wet process by biaxial stretching used UHMWPE (PE-W-B, ‘W’ means wet process). Information aggregation of three kinds of separators used in this study was shown in Table 1 and all three separators had uniform thickness of 2
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16 μm. Φi ¼
2.2. Measurements
ε1
ε2
(2)
2.2.6. Electrolyte uptake testing The separator was immersed in mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 1:1 for 6 h. Then separator was took out and the electrolyte solvent remaining on the surface was removed by filter paper. Finally, the dry and immersed separators were weighed and the weight ratio of electrolyte uptake (Wup) was then determined by following formula [42,43]. Wup ¼
W0
W W
(3)
where W and W0 were the weight of dry and immersed separator respectively. Repeat measurement five times for each sample to ensure the reliability of data. 3. Results and discussion
(1)
ε1
WÞρ0 � 100% ðρ ρ0 ÞW
where W and W0 were the dry and immersed sample weight. ρ(0.808 g/ cm) and ρ0(0.91 g/cm) were the density of butanol and PP. Repeat measurement five times for each sample to ensure the reliability of data.
2.2.1. Compression testing Mechanical testing was performed on a universal testing machine (MTS) equipped with a displacement meter (circled in Fig. 1a and detailed exhibited in Fig. 1b and 1c). 32 layers stacked separators with the diameter of 14.5 mm was placed between two pressure plates to increase the accuracy of strain measurements. Meanwhile, the selfleveling base plate at bottom (Fig. 1d) was used to decrease the effects of any misalignment of the platens. By combining the displacement signal from the displacement meter and the pressure signal from the force sensor through the computer (Fig. 1e), the stress-strain curve of the lithium-ion battery separator could be recorded accurately. A precompression of 1 MPa was used to ensure a compactness stack be tween layers and the compression rate was 0.5 mm/min [29,31]. When the stress raised up to 100Mpa, the compression load was withdrawn with the same rate and the recovery curve of the separator was recorded (Fig. 1f). Then strain recovery rate at specific stress and corresponding strain could be calculated by the following formula: Rð%Þ ¼
ðW0
ρ0 W0
3.1. The fundamental structure characterization
where ε1 was the strain compressed to specific stress, ε2 was the strain when the stress withdrew to zero.
The detailed surface and cross-section morphology were shown in Fig. 2 to obtain the basic structure of the separators from a more comprehensive perspective. It should be pointed out that the SEM im ages were exhibited with different magnification due to different structural characteristics of micropores. Three types of separators dis played two different unique structures due to the different preparation processes. Both the surface and the cross-section of the PP-D-U displayed the completely identical microporous structures (Fig. 2a), namely slitlike microporous structures arranged parallel along the MD, which was formed by the direct separation of the stacked lamellae structure formed during extrusion process and therefore, PP-D-U had the maximum orientation degree in 2D-WAXD spectra. On the other hand, the circular micropores can be seen from the surface for the separators prepared by biaxial stretching process, both dry and wet process. Meanwhile, abundant multilayered structure can be observed on crosssection, which was evolved from the biaxial stretching of spherulites [44,45]. Moreover, the orientation degree of the two kinds of separators prepared by biaxial stretching was much weaker owing to the transverse stretching along the direction perpendicular to MD (TD). In addition, PP-D-B still had plenty coarse microfibrils or even the dense areas without any micropores due to the polydispersity of β-lamellae in β-iPP, causing poorer pore size distribution compared with the other two separators [46]. In order to further quantify the orientation degree, the azimuthal intensity distribution of β (300) diffraction ring of different separators, which could characterize the crystal phase orientation along and across the loading direction, was presented in Fig. 2d. Obviously, the highest azimuthal intensity and the sharpest intensity peak of PP-D-U illustrated the highest orientation degree, while the PP-D-B and PE-W-B exhibited lower orientation degree, which is corresponded to previous SEM images.
2.2.2. Scanning electron microscopy (SEM) The separators were immersed in liquid nitrogen for 24 h and then fractured along mechanical direction (MD) to obtain the intact crosssection morphology. The surface and cross-section morphology of sep arators were inspected directly by FEI Inspect F scanning electron mi croscope with the acceleration voltage of 5 KV. 2.2.3. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) The 2D-WAXD experiments were carried out by D8 DISCOVER twodimensional X-ray diffractometer (Bruker, Germany) equipped with a Cu Kα radiation (λ ¼ 0.154 nm) to characterize the orientation degree of three separators. The operating voltage was 40 kV and the operating current was 20 mA. 2.2.4. Air permeability measurement (Gurley value) The air permeability or Gurley value was examined by Gurley Pre cision Instruments (Model No. 4110 N) with an automatic digital timer. 100 mL air was forced to penetrate the separator by constant pressure and the corresponding time was recorded to evaluate the air perme ability of separator. 2.2.5. Porosity determination The separator was soaked in butanol for 12 h and then taken out quickly. The residual butanol was removed carefully using filter paper. Finally, the dry and immersed separators were weighed and the porosity was calculated by the following equation [41].
Table 1 Summary of three separators used in this study.
3.2. Compression behavior
Sample
Porosity (%)
Air permeability (s/ 100 mL)
Manufacturing company
PP-D-U
44.37
234.3
PP-D-B
41.31
328.4
PE-WB
43.69
271.2
Shenzhen Senior Technology Material Co., Ltd Kunming Yuntianhua NewmiTech Co., Ltd Chognqing Yuntianhua NewmiTech Co., Ltd
A typical stress-strain curves of three kinds of separators under compression during loading and subsequent unloading were shown in Fig. 3. Three kinds of separators exhibited two completely different compression behaviors due to the different microporous structures along the thickness direction. For PP-D-U, the cross-section had an identical morphology with surface, but completely different function. The parallel lamellae aligned 3
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Fig. 1. (a) and (b) the pictures of compression module; the elaborated exhibition of (c) displacement meter and (d) self-leveling lower-plate; (e) the schematic of compressive test setup and (f) the schematic to calculate the recovery rate at specific stress.
along thickness direction on cross-section appeared as columnar struc ture, which had good resistance to compression load because the columnar structure could play critical bracing function for the separator. Therefore, the compression stress skyrocketed with little change in strain at the initial stage of compression (ε<5%), which is corresponded to elastic deformation of the separator. Subsequently, PP-D-U presented obvious yield platform with higher stress (about 40 MPa), indicating that the internal microporous structure produced the nonrecoverable plastic deformation. The stress associated with the plateau is below the yield stress that would be expected for bulk polypropylene owing to the porous structure of the material, which dictates that the stress on the actual polymer material is higher than the stress calculated based on the nominal area of the sample [29]. At higher stress level, the gradual closure of micropores caused a higher stiffness of the material and allowed more materials to support the compression loading, making an upward slope ultimately. In addition, the separator deformed aniso tropically in macroscopic view (Fig. 3b) after compression. Further statistical measurement in Fig. 3c displayed that the separator expanded visibly in the TD and contracted slightly in MD. On the other hand, the separators prepared by biaxial stretching had abundant multilayered structure along thickness direction, absenting the structure to resist deformation under compression load. Therefore, the characteristic yield phenomenon cannot be detected for PP-D-B and PE-W-B. However, due to the similar porosity and the same material, PPD-B finally reached to 100 MPa at similar strain compared with PP-D-U. But the strain of PE-W-B at 100 MPa was much larger due to softer material for PE. The statistical measure of compressed samples indicated that there was no anisotropic change in the size of the two kinds of separators. More interestingly, PE-W-B had become translucent state after compression (Fig. 3b), illustrating that the micropores of PE-W-B had closed almost entirely under greater mechanical strain. In order to further explore the microporous structure evolution of different kinds of separators during the whole compression process, the stress-strain curves compressed to specific stress and corresponding re covery were recorded in Fig. 4. Clearly, PP-D-U showed a recovery curve that almost overlapped with the compression curve before yield
(detailed enlarged in Fig. 4a). However, once the stress reached the yield platform (about 40 MPa), the recovery dropped sharply and the recovery decreased to a lower level when the plateau ended. Even though the stress-strain curve exhibited a higher upward slope in the final stages (about 70–100 MPa), the recovery decreased slightly. On the contrary, PP-D-B and PE-W-B separators without characteristic yield behavior showed a resemblance in strain recovery. A higher recovery rate (about 80%) can be achieved only under minimal stress (5 MPa or less). Moreover, the recovery rate decreased rapidly at the early stage of compression, which is completely different from PP-D-U with a sudden change after yield, reducing to a low level only at 20 MPa and remaining at this level until the end of compression. The recovery at specific stress and corresponding strain was calcu lated by Eq. (1) to further quantify the recovery variation during the compression process (Fig. 4d and 4f). Fig. 4d showed that PP-D-U had the highest recovery with increasing stress during the whole compres sion process, which indicating a better resistance to pressure. In addi tion, the recovery of PP-D-U depressed few under lower stress (R>90% when σ < 30 MPa), illustrating the excellent resistance to pressure for PP-D-U before yield. But a cliff-like drop can be inspected once yield occurred, which corresponded the yielding plateau in compressive curves. When the stress reached to about 50–60 MPa, the descent rate of recovery suddenly slowed down, which was coincidently corresponding to the beginning of strain hardening in compressive curves. Finally, the recovery maintained the lower decline rate until the end of compression. However, the recovery of PP-D-B and PE-W-B decreased sharply under lower stress and declined to about 30% only at stress ¼ 20 MPa. Then the recovery remained at a very low level until the end of compression, which was also in accordance with the compression curves of corre sponding separators. And due to softer material for PE, PE-W-B had the lowest recovery during the whole compression process. Clearly, the re covery variation trend with increasing stress was highly consistent with the compression stress-strain curves, which maybe indicate a correlation between the internal structure change and the recovery vs stress curves or the compression curves. But the recovery of three kinds of separators with corresponding strain (Fig, 4e) had almost identical variation trend 4
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Fig. 2. The morphology and crystal structure characterization of three kinds of separators: (a) PP-D-U, (b) PP-D-B, (c) PE-W-B; (d) the azimuth intensity of (300) plane diffraction ring of three separators (2θ ¼ 14.1� for PP and 22.5� for PE).
Fig. 3. (a) The stress-strain curves of three kinds of separators under compression during loading and subsequent unloading; (b) the photograph of the compressed separators after 100 MPa; (c) the statistical diameter of circular sample along MD and TD direction after compression.
due to the almost overlapping recovery-strain curves, although the recovery-strain curve of PP-D-U had a weak consistency with stressstrain curves.
3.3. Macroscopic performance characterization In order to get a better understanding about the compression process, the typical performances variation of three kinds of separators with 5
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Fig. 4. The stress-strain and corresponding recovery curves of three kinds of separators under specific pressure stress: (a) PP-D-U, (b) PP-D-B, (c) PE-W-B; the calculated recovery under (d) specific stress and (e) corresponding strain.
specific stress and corresponding strain were characterized and the experimental results were exhibited in Fig. 5. The porosity of the three types of separators showed a continuous decline with the increase of stress (Fig. 5a). However, the porosity of PP-D-U was well maintained at low stress and was always the highest during the whole compression process. And the porosity decreased rapidly after yield due to the plastic deformation, then following by a slower decreased porosity when compressed to strain hardening stage. While the porosity of the other two types with biaxial stretching decreased rapidly at the initial stage, especially, PE-W-B had the lowest porosity and finally almost dropped to 0%, which was very similar to the recovery variation curves with stress. However, the porosity-strain curves (Fig. 5d) showed that the porosity of all three types of separators decreased almost linearly with strain. but
the porosity-strain curve of PP-D-U still had a weak relevance with stress-strain curves. More interestingly, the electrolyte uptake with increasing stress and strain (Fig. 5b and 5e) exhibited the same variation trend with recovery and porosity. It can be seen that some key param eters variation caused by compression (porosity, suction volume) was highly consistent with the recovery rate. And the variation trend of these parameters with stress can better describe the internal structure evolu tion during compression. In addition, the Gurley value had a strict negative with air perme ability, namely a lower Gurley value corresponded to a higher air permeability and smaller the internal resistance (Fig. 5c). Clearly, the Gurley value of three kinds of separators varies greatly with the different stress. PP-D-U showed the lowest Gurley value during the whole 6
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Journal of Power Sources 451 (2020) 227819
Fig. 5. The representative performances variation with specific stress: (a) porosity, (b) electrolyte uptake, (c) the Gurley value; the representative performances variation with corresponding strain: (d) porosity, (e) electrolyte uptake, (f) the Gurley value.
compression process, indicating the best air permeability and optimal ability to resisted compression load. While the Gurley value of PE-W-B increased fastest, reaching about 2600s (�296s) under the pressure of 50 MPa. More interestingly, the Gurley value variation curves of three kinds of separators with corresponding strain (Fig. 5f) were almost coincide, showing two sequential parts. In the case of low strain (σ<35%), Gurley value of the three types of separators had been maintained at a low level (<600s), but the porosity of separators had almost halved. With the further increase of strain (σ>40%), Gurley value increases rapidly. And this higher strain corresponded roughly to the strain hardening stage of the three separators compression curves, indicating that there may be sequential two different structure changes within three kinds of separators during compression, both uniaxial and biaxial stretching separator. In order to further explore the structural evolution of different kinds of separators along thickness direction, the morphology evolution during the whole compression process, both on surface and cross-section, were inspected by SEM directly.
gradually, causing the deflection of partial fibrils bridges and slit-like micropores. Meanwhile, the pore size along MD and TD was also decreased slightly. But when the stress exceeded 60 MPa (strain hard ening section), the vertical lamellae along thickness direction folded and sheared violently because the stress was mainly concentrated in the thickness direction. As a result, a characteristic multilayered structure with serious shear and glide between layers formed under higher stress (at the stage of stress hardening), causing the enormous change of pore size on cross-section at stress hardening section (Fig. 6e and 6f). While the pore size on surface had a gradual change (Fig. 6a and 6b), which further indicated that the connectivity along thickness direction was caused by further shearing and gliding of multilayered structure, leading to the rapid increase of Gurley value at strain hardening stage. Fig. 6g presented the mechanism diagram of PP-D-U during compression process. For the other two types of separators prepared by biaxial stretching, circular micropores on surface and stacked-layer structure along crosssection were typical characteristics. Therefore, the compression pro cess and morphology evolution were very similar, namely the gradual compaction of the stacked-layer structure and the gradual closure of the surface microporous structure during the whole compression process. The significant decrease of recovery in the early compression means the sharp decline of porosity in the early stage of compression because there was no support structure along thickness direction like PP-D-U, but the surface micropores were basically remain unchanged while the previ ously loose stacked-layer structure was compacted more compactly from the cross-section view, indicating that the compaction of stacked-layer structure mainly occurred in the early stage of compression. Mean while, the pore area on surface changed slightly when σ < 40 MPa, which can further the above conclusion. However, at the later stage of compression (σ > 40 MPa), there was little space left between layers for compression, causing the significant increase of the compression stressstrain curve slope. And reflected in the separator, the dense stackedlayer structure does not change much, but the surface micropores were obviously closed (Fig. 7e and 7f). The closure of micropores may come from two aspects. One was the autogenous shrinkage of
3.4. Microporous structure evolution during compression process The detailed compression testing manifested that the three kinds of separators had two completely different compression behavior due to the different stacked structure along thickness direction. Moreover, the Gurley value variation with different strain showed that there may be sequential two different structure changes within the all kinds of sepa rators during compression. However, there is still some speculation about the internal structure evolution and the current literature also lacked the visualized evidence of structural evolution during compres sion process. Therefore, we conducted a comprehensive SEM charac terization of the microporous structure evolution during the whole compression process, both on surface and cross-section, to obtain the most direct evidence. The surface and cross-section morphology of PP-DU did not change substantially before yield behavior (20 MPa in Fig. 6a and 6b). However, plastic deformation occurred when the stress increased from 40 MPa (yield stress) to 60 MPa (the initial stress of strain hardening) and the parallel aligned lamellae on surface distorted 7
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Fig. 6. The SEM images of PP-D-U under specific pressure stress: (a) surface, (b) cross-section; the pore size variation during compression process: (c) pore size along MD on surface, (d) pore size along TD on surface, (e) pore size along MD on cross-section, (f) pore size along ND on cross-section; (g) the mechanism diagram of PP-DU during compression process.
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evolution during compression can be well demonstrated. The compres sion process of all kinds of separators mainly consisted of two parts. The dramatic decline of porosity but slightly change of connectivity along thickness direction under lower strain firstly. And secondly, the con nectivity along thickness direction decreases sharply at higher strain. However, the reasons for the sharp decline of connectivity of different kinds of separators were also different due to two completely different microporous structures. For PP-D-U, the vertical lamellae along thick ness direction bended and folded violently and a characteristic multi layered structure with serious shear and glide between layers formed under higher stress. Therefore, the connectivity of PP-D-U decreased rapidly by further shearing and gliding of multilayered structure at higher strain. But for the other two separators, expect for the shrinkage of micropores itself, the dense area of the next layer would severely block the microporous structure of the previous layer at higher strain because the micropores did not corresponded to each other between layers.
micropores on the surface layer. Another and perhaps the most impor tant reason was that the dense area of the next layer would block the microporous structure of the previous layer during compression because the micropores did not corresponded to each other between layers, causing the rapid increase of Gurley value at this stage. Therefore, the residual trace of micropores can be observed although the micropores closed, which can be clearly observed on the separator surface at 100 MPa, especially on the surface of PE-W-B due to softer material for PE, accompanied by the much more violent shrinking of pore area at higher stress (60–100 MPa) compared with PP-D-B (Fig. 7f). In addition, a little experiment was designed to further verify the structure evolution during compression process. The separators were cut into 0.5 mm*50 mm along MD direction and then compressed to specific stress. Then the samples were vertically suspended in the different beakers with 150 mL mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 1:1 (small amount of red pigment was added to enhance contrast) and a vertical scale (The schematic diagrams were shown in Fig. 8). Finally, recording the electrolyte level elevation of samples after 60 min. When the samples were placed in a sealed beaker with the bottom immersed in the electrolyte (Fig. 8a), the elec trolyte level elevation of three kinds of separators rose gradually with the increase of stress or strain, following by a maximum and finally decline rapidly. This is a very interesting phenomenon. The previous literature verified that the suction height of the separators to electrolyte would decrease in the open system at the early stage of compression, while the upward and downward trend of the height during compression had not been discovered and demonstrated. The above data showed that the violent decrease of porosity inside the separators mainly occurred at the initial stage of compression, while the micropores connectivity changed slightly. Later the connectivity has plummeted at the end of compression process. Therefore, the sequential change inside the sepa rators was probably the main reason and further experiments were carried out to verify this phenomenon. In the unsealed beaker, the suction height of the separators with bottom immersed in electrolyte (Fig. 8b) showed a monotonous down ward trend with the increase of stress or strain, meaning that the ab sorption or diffusion rate of separators perpendicular to thickness direction decreased strictly due to the compaction along thickness di rection. In addition, the suction height of separators hanged over the electrolyte in sealed beaker (Fig. 8c) could indicate the adsorption ca pacity of the electrolyte vapor in the sealed beaker parallel to thickness direction t. Clearly, Fig. 8c exhibited the exactly the same variation trend compared with Fig. 8a, the electrolyte level elevation increased initially and decrease rapidly afterwards. By adding the electrolyte level elevation of Fig. 8b and 8c numerically (shown in Fig. d), it can be found that the sum showed an amazing consistency with Fig. 8a, which also indicated that the suction capacity of the separators with the bottom immersed in electrolyte in the sealed environment was composed of two parts, namely the adsorption capacity for electrolyte liquid perpendic ular to thickness direction (Fig. 8b) and the ability to absorb electrolyte vapor along thickness direction (Fig. 8c). According to the previous data, it is not difficult to understand that the surface micropore structure hardly changed at the initial stage of compression, and the Gurley value of the membrane also changed slightly, which indicated that the connectivity of micropores along thickness direction did not change significantly, therefore, the absorp tion ability of the separators to electrolyte vapor actually does not change much. But due to the significant decrease of porosity, the sepa rator only needed to adsorb lesser electrolyte to fill the micropores, therefore, the separators showed faster apparent absorption capacity and higher suction height at the early stage of compression process. Subsequently, the micropores on surface closed or blocked drastically at the later stage of compression and the separator cannot absorb the electrolyte vapor, causing a significant decline of suction height at the end of compression process. It can be seen from this simple experiment that the mode of structure
4. Conclusions In this article, the deformation mode, microstructure evolution and performances variation of three kinds of commercial polyolefin sepa rator under compression load were systematically analyzed, which was important to explore the long-term behavior of batteries. The compression results indicated that three kinds of separator presented two entirely different deformation modes due to the different microporous structure. There were abundant parallel lamellae aligned along thickness direction on cross-section for PP-D-U, leading to excel lent ability to resist compression loads. Therefore, PP-D-U presented the unique compression behavior, which had a striking yield behavior firstly. Then the compression plateau can be observed, accompanied by compaction along thickness and formation of multilayered structure due to the bending or folding of parallel lamellae aligned along thickness direction on cross-section. Finally the multilayered structure further sheared or flowed violently, causing the strain hardening in compression curve and rapid increase of Gurley value. More interestingly, all prop erties variation with stress displayed striking uniformities with compression curves. On the other hand, the separator made by biaxial stretching, both dry and wet process, had numerous stacked-layer structure with poor supporting structure along thickness direction, therefore, there was no obvious yield behavior on compression curve. And all properties showed a gentle trend for biaxial stretching separator, which did not exhibit sudden changes like PP-D-U. However, three kinds of separator also shared some common char acteristics, which the compression process mainly consisted of two similar parts. The dramatic decline of porosity but slightly change of connectivity along thickness direction under lower strain firstly. And secondly, the connectivity along thickness direction decreases sharply at higher strain. However, the reasons for the sharp decline of connectivity of different kinds of separators were also different due to two completely different microporous structures. For PP-D-U, the vertical lamellae along thickness direction bended and folded violently, but the microporous connectivity in the thickness direction was affected slightly under lower stress. While a characteristic multilayered structure with serious shear and glide between layers formed under higher stress. Therefore, the connectivity of PP-D-U decreased rapidly by further shearing and gliding of multilayered structure at higher strain. But for the other two sepa rators, expect for the shrinkage of micropores itself, the dense area of the next layer would severely block the microporous structure of the pre vious layer at higher strain because the micropores did not corresponded to each other between layers, causing the sudden change of Gurley value. Declaration of competing interest The authors declare that they have no known competing financial 9
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Fig. 7. The SEM images of PP-D-B and PE-W-B under specific pressure stress: (a) surface morphology of PP-D-B, (b) cross-section morphology of PP-D-B, (c) surface morphology of PE-W-B, (d) cross-section morphology of PE-W-B; the area variation of pores on surface: (e) PP-D-B, (f) PE-W-B; (e) the mechanism diagram of two kinds of separator during compression process.
10
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Fig. 8. The electrolyte level elevation of three kinds of separators at different cases after 60min: (a) in sealed beaker and bottom of samples (10 mm) were immersed in electrolyte; (b) in non-sealed beaker and bottom of samples were immersed in electrolyte; (c) in sealed beaker and suspended samples over electrolyte level; (d) the sum of electrolyte level elevation of (b) and (c).
interests or personal relationships that could have appeared to influence the work reported in this paper.
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11
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
The compression behavior, microstructure evolution and properties variation of three kinds of commercial battery separators under compression load Lei Ding a, Chao Zhang a, Tong Wu b, *, Feng Yang a, Ya Cao a, Ming Xiang a a b
State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, China College of Polymer Science and Engineering, Sichuan University, Chengdu, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Compression behavior of three kinds of separators were systematically analyzed. � Detailed microporous structure evolu tion of separators was exhibited firstly. � Good connection between microstruc ture evolution and properties variation was made.
A R T I C L E I N F O
A B S T R A C T
Keywords: Li-ion battery Separator Compression behavior Microporous structure evolution Macro-properties variation
In this article, the compression behavior, microstructure evolution and properties variation of three kinds of commercial polyolefin separator under compression load are systematically analyzed. The compression testing shows that three kinds of separators present two different deformation modes. The separator made from dry process by uniaxial stretching has good resistance to compression load due to the parallel lamellae aligned along thickness direction on cross-section. And all properties variation with stress display striking uniformities with compression curves. While the other two separators with biaxial stretching, both dry and wet process, have abundant multilayered structure, causing weaker resistance to compression load and flatter compression curves. And all properties show gentle trend for biaxial stretching separator. However, three kinds of separators also share some common characteristics. The separators compact violently along thickness direction under lower strain firstly, accompanied by the dramatic decline of porosity but slightly change connectivity along thickness direction. And subsequently the connectivity along thickness direction is severely destroyed at higher strain due to different reasons. The violent shear or glide of microporous structure occurs perpendicular to thickness di rection for separator with uniaxial stretching and micropores direct blocking by non-corresponding micropores position between multilayered structure for separator with biaxial stretching.
* Corresponding author. E-mail addresses: [email protected] (L. Ding), [email protected] (C. Zhang), [email protected] (T. Wu), [email protected] (F. Yang), [email protected] (Y. Cao), [email protected] (M. Xiang). https://doi.org/10.1016/j.jpowsour.2020.227819 Received 6 September 2019; Received in revised form 26 December 2019; Accepted 26 January 2020 Available online 31 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction
the NMC layer was likely the cause as it presents a greater constraint to the deformation of the separator at large strains. Lagadec [13] used real microstructures of polyethylene and polypropylene separators acquired with focused ion beam scanning electron microscopic (FIB-SEM) to mography and simulated how these structures deform under compres sive strain. At present, three most commonly used commercial separators are manufactured by polyolefin, namely the dry process by uniaxial stretching of hard elastic iPP [34,35], the dry process by biaxial stretching of β nucleated polypropylene (β-iPP) [36,37] and the wet process used ultra-high molecular weight polyethylene (UHMWPE) [38, 39]. All these three processes include an extrusion step to make cast films and employ one or more orientation steps to impart porosity. For the dry process with uniaxial stretching, the superhigh die draw ratio (DDR) is applied during extruding to form the parallel arranged lamellae (namely shish-kebab structure). Then after annealing the cast film at high temperature about 140 � C, the cast film is stretched uniaxially to prepare the separator. While as for the dry process with biaxial stretching, the β-nucleating agent is added into polypropylene to form β nucleated polypropylene (β-iPP) cast film during extruding. Then the β-iPP cast film is stretched directly along mechanical direction (MD) at the temperature about 80–100 � C and transverse direction (TD) at the temperature about 120–140 � C successively. The wet process was based on the mechanism of thermally induced phase separation, the cast film is composed of 30% ultrahigh molecular weight polyethylene (UHMWPE) and 70% paraffin oil. Then the synchronously or asynchronously biaxial stretching is imposed on cast film, following by extracting the paraffin oil with a volatile solvent to form the microporous structure. But un fortunately, the present researches on LIBs separator compressive property failed to provide a good contrast of the three kinds of com mercial separators, but only focused on one of them. Our research team [40] recently had tried to evaluate the three types of separators, and found that the dry process separator with uniaxial stretching had a better resistance to deformation at the early stage of compression. But the three types of separator had not been studied thoroughly owning to the limitation of current characterization techniques. In addition, pre sent references mainly focused on the characterization of compression performance, but failed to quantify the ability to resist compression deformation and provide the intuitive evidence of the three-dimensional microporous structures evolution during compression process. In this article, the compression behavior of three most commonly used separators based on polyolefin was explored, and the ability of separator to resist compression deformation was evaluated by comparing the variation of recovery rate under different compression condition. Moreover, the evolution of micropores shrinkage and closure, both on surface and cross-section, throughout the whole compression process was observed directly by SEM to better complement the short board in this aspect. Furthermore, various key properties changes (porosity, electrolyte upkate and Gurley value, etc.) of the separator were tested. Therefore, a comprehensive theoretical data system from membrane micropore structure to the macroscopic performances was shown clearly in order to get a better understanding about the separator under compression load.
With the rapid development of things like portable electronic devices and electric vehicles (EVs), the rechargeable lithium-ion batteries (LIBs) have attracted considerable attention due to the low self-discharge rate, evidently high specific energy and energy density, and thus are world wide used secondary batteries [1–8]. The LIBs are mainly composed of two electrodes, electrolyte and separator. The porous separator is the key material to prevent the anode and cathode from contacting and meanwhile, conduct li-ion between two electrodes. At present, the most commonly used commercial separator is mainly prepared from poly olefin due to the low cost, relatively uniform pore sizes and mature technology [9–12]. During the LIBs application process, the separators must meet the requirement of sufficient mechanical properties to withstand various forces inside and outside the batteries. especially the compressive stress. Actually, a dynamic pressure environment can be detected inside the battery throughout the whole life cycle [13]. The stack pressure is imposed on the battery during assembly to maintain intimate contact between all components [14]. External loading and collision in daily use process also cause fluctuations in the internal pressure [15–17]. Mean while, the formation and growth of SEI is another reason [18,19]. Most importantly, during each charging and discharging cycle of li-ion bat tery, the intercalation and de-intercalation of li-ion are not just an electrochemical process, but also a mechanical evolution of battery components, leading substantial volume expansion of electrode mate rials [20,21]: 2% for lithium cobalt oxide and 10% for graphite electrode [22], even as high as 400% expansion rate for the latest high capacity anode materials such as silicon [23]. Consequently, the expanding electrodes inevitably squeezes the limited interior space within battery and certainly compresses the soft separator, causing severe micropores deformation and subsequently a lower ionic conductivity along the thickness direction [24–26]. It can be seen that the compression stress inside the battery has a particularly important effect on the performance of the porous separator and even the battery. During the operation of a lithium ion battery, the average compress stress in the battery is even up to 5Mpa [13,27]. Due to the coarse and rough electrode surface of two electrodes, the local pressure on the separator is much higher than this level. Therefore, it is crucial to explore the impact of compressive stress on the typically commercialized separator. Arnold [28–30] explored the compressive mechanical properties of polypropylene separator (Celgard 3501) over a range of strain rates and in different fluid environments. They found a high strain rate depen dence due to the viscoelasticity behavior of the polymer skeleton and the poroelastic behavior of flow of the fluid in the pores. More interestingly, they disassembled the used battery and based on the postmortem analysis they considered that the compressive stress dose not just cause the mechanical evolution, but a coupling between mechanics and elec trochemistry in which higher levels of mechanical stress leaded to a faster chemical degradation [27]. Zhang et al. [16] characterized four types of battery separators and compared the mechanical performance, strength, and failure mode. Meanwhile, they found that the compression tests also revealed the anisotropic nature of the dry processed separators by the oval shape of separators after compression [31]. This phenome non had also been found by Arnold [29]. Kalnaus [32] researched the compressive property of separator (Celgard 2325) with different tem perature and rate by recording data through digital image correlation. The results showed significant dependence of mechanical properties on temperature. The strain rate strengthening also decreased with higher temperatures while the temperature softening remained independent of the applied strain rate. Yan et al. [33] computed the compressive stress-strain curve for the stack from that of the separator (Celgard 2500) and NMC cathode layer to investigate the interaction between the separator and the electrode under compressive stress. The results sug gested that, in the stack, the compressive deformation of the separator was influenced by the NMC cathode significantly. The rough surface of
2. Experimental 2.1. Materials Three classic manufacturing processes of the polyolefin separator included the dry process by uniaxial stretching of hard elastic iPP cast film (PP-D-U, ‘D’ means dry process and ‘U’ means uniaxial stretching), the dry process by sequential biaxial stretching of β nucleated poly propylene (PP-D-B, ‘B’ means biaxial stretching) and the wet process by biaxial stretching used UHMWPE (PE-W-B, ‘W’ means wet process). Information aggregation of three kinds of separators used in this study was shown in Table 1 and all three separators had uniform thickness of 2
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16 μm. Φi ¼
2.2. Measurements
ε1
ε2
(2)
2.2.6. Electrolyte uptake testing The separator was immersed in mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 1:1 for 6 h. Then separator was took out and the electrolyte solvent remaining on the surface was removed by filter paper. Finally, the dry and immersed separators were weighed and the weight ratio of electrolyte uptake (Wup) was then determined by following formula [42,43]. Wup ¼
W0
W W
(3)
where W and W0 were the weight of dry and immersed separator respectively. Repeat measurement five times for each sample to ensure the reliability of data. 3. Results and discussion
(1)
ε1
WÞρ0 � 100% ðρ ρ0 ÞW
where W and W0 were the dry and immersed sample weight. ρ(0.808 g/ cm) and ρ0(0.91 g/cm) were the density of butanol and PP. Repeat measurement five times for each sample to ensure the reliability of data.
2.2.1. Compression testing Mechanical testing was performed on a universal testing machine (MTS) equipped with a displacement meter (circled in Fig. 1a and detailed exhibited in Fig. 1b and 1c). 32 layers stacked separators with the diameter of 14.5 mm was placed between two pressure plates to increase the accuracy of strain measurements. Meanwhile, the selfleveling base plate at bottom (Fig. 1d) was used to decrease the effects of any misalignment of the platens. By combining the displacement signal from the displacement meter and the pressure signal from the force sensor through the computer (Fig. 1e), the stress-strain curve of the lithium-ion battery separator could be recorded accurately. A precompression of 1 MPa was used to ensure a compactness stack be tween layers and the compression rate was 0.5 mm/min [29,31]. When the stress raised up to 100Mpa, the compression load was withdrawn with the same rate and the recovery curve of the separator was recorded (Fig. 1f). Then strain recovery rate at specific stress and corresponding strain could be calculated by the following formula: Rð%Þ ¼
ðW0
ρ0 W0
3.1. The fundamental structure characterization
where ε1 was the strain compressed to specific stress, ε2 was the strain when the stress withdrew to zero.
The detailed surface and cross-section morphology were shown in Fig. 2 to obtain the basic structure of the separators from a more comprehensive perspective. It should be pointed out that the SEM im ages were exhibited with different magnification due to different structural characteristics of micropores. Three types of separators dis played two different unique structures due to the different preparation processes. Both the surface and the cross-section of the PP-D-U displayed the completely identical microporous structures (Fig. 2a), namely slitlike microporous structures arranged parallel along the MD, which was formed by the direct separation of the stacked lamellae structure formed during extrusion process and therefore, PP-D-U had the maximum orientation degree in 2D-WAXD spectra. On the other hand, the circular micropores can be seen from the surface for the separators prepared by biaxial stretching process, both dry and wet process. Meanwhile, abundant multilayered structure can be observed on crosssection, which was evolved from the biaxial stretching of spherulites [44,45]. Moreover, the orientation degree of the two kinds of separators prepared by biaxial stretching was much weaker owing to the transverse stretching along the direction perpendicular to MD (TD). In addition, PP-D-B still had plenty coarse microfibrils or even the dense areas without any micropores due to the polydispersity of β-lamellae in β-iPP, causing poorer pore size distribution compared with the other two separators [46]. In order to further quantify the orientation degree, the azimuthal intensity distribution of β (300) diffraction ring of different separators, which could characterize the crystal phase orientation along and across the loading direction, was presented in Fig. 2d. Obviously, the highest azimuthal intensity and the sharpest intensity peak of PP-D-U illustrated the highest orientation degree, while the PP-D-B and PE-W-B exhibited lower orientation degree, which is corresponded to previous SEM images.
2.2.2. Scanning electron microscopy (SEM) The separators were immersed in liquid nitrogen for 24 h and then fractured along mechanical direction (MD) to obtain the intact crosssection morphology. The surface and cross-section morphology of sep arators were inspected directly by FEI Inspect F scanning electron mi croscope with the acceleration voltage of 5 KV. 2.2.3. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) The 2D-WAXD experiments were carried out by D8 DISCOVER twodimensional X-ray diffractometer (Bruker, Germany) equipped with a Cu Kα radiation (λ ¼ 0.154 nm) to characterize the orientation degree of three separators. The operating voltage was 40 kV and the operating current was 20 mA. 2.2.4. Air permeability measurement (Gurley value) The air permeability or Gurley value was examined by Gurley Pre cision Instruments (Model No. 4110 N) with an automatic digital timer. 100 mL air was forced to penetrate the separator by constant pressure and the corresponding time was recorded to evaluate the air perme ability of separator. 2.2.5. Porosity determination The separator was soaked in butanol for 12 h and then taken out quickly. The residual butanol was removed carefully using filter paper. Finally, the dry and immersed separators were weighed and the porosity was calculated by the following equation [41].
Table 1 Summary of three separators used in this study.
3.2. Compression behavior
Sample
Porosity (%)
Air permeability (s/ 100 mL)
Manufacturing company
PP-D-U
44.37
234.3
PP-D-B
41.31
328.4
PE-WB
43.69
271.2
Shenzhen Senior Technology Material Co., Ltd Kunming Yuntianhua NewmiTech Co., Ltd Chognqing Yuntianhua NewmiTech Co., Ltd
A typical stress-strain curves of three kinds of separators under compression during loading and subsequent unloading were shown in Fig. 3. Three kinds of separators exhibited two completely different compression behaviors due to the different microporous structures along the thickness direction. For PP-D-U, the cross-section had an identical morphology with surface, but completely different function. The parallel lamellae aligned 3
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Fig. 1. (a) and (b) the pictures of compression module; the elaborated exhibition of (c) displacement meter and (d) self-leveling lower-plate; (e) the schematic of compressive test setup and (f) the schematic to calculate the recovery rate at specific stress.
along thickness direction on cross-section appeared as columnar struc ture, which had good resistance to compression load because the columnar structure could play critical bracing function for the separator. Therefore, the compression stress skyrocketed with little change in strain at the initial stage of compression (ε<5%), which is corresponded to elastic deformation of the separator. Subsequently, PP-D-U presented obvious yield platform with higher stress (about 40 MPa), indicating that the internal microporous structure produced the nonrecoverable plastic deformation. The stress associated with the plateau is below the yield stress that would be expected for bulk polypropylene owing to the porous structure of the material, which dictates that the stress on the actual polymer material is higher than the stress calculated based on the nominal area of the sample [29]. At higher stress level, the gradual closure of micropores caused a higher stiffness of the material and allowed more materials to support the compression loading, making an upward slope ultimately. In addition, the separator deformed aniso tropically in macroscopic view (Fig. 3b) after compression. Further statistical measurement in Fig. 3c displayed that the separator expanded visibly in the TD and contracted slightly in MD. On the other hand, the separators prepared by biaxial stretching had abundant multilayered structure along thickness direction, absenting the structure to resist deformation under compression load. Therefore, the characteristic yield phenomenon cannot be detected for PP-D-B and PE-W-B. However, due to the similar porosity and the same material, PPD-B finally reached to 100 MPa at similar strain compared with PP-D-U. But the strain of PE-W-B at 100 MPa was much larger due to softer material for PE. The statistical measure of compressed samples indicated that there was no anisotropic change in the size of the two kinds of separators. More interestingly, PE-W-B had become translucent state after compression (Fig. 3b), illustrating that the micropores of PE-W-B had closed almost entirely under greater mechanical strain. In order to further explore the microporous structure evolution of different kinds of separators during the whole compression process, the stress-strain curves compressed to specific stress and corresponding re covery were recorded in Fig. 4. Clearly, PP-D-U showed a recovery curve that almost overlapped with the compression curve before yield
(detailed enlarged in Fig. 4a). However, once the stress reached the yield platform (about 40 MPa), the recovery dropped sharply and the recovery decreased to a lower level when the plateau ended. Even though the stress-strain curve exhibited a higher upward slope in the final stages (about 70–100 MPa), the recovery decreased slightly. On the contrary, PP-D-B and PE-W-B separators without characteristic yield behavior showed a resemblance in strain recovery. A higher recovery rate (about 80%) can be achieved only under minimal stress (5 MPa or less). Moreover, the recovery rate decreased rapidly at the early stage of compression, which is completely different from PP-D-U with a sudden change after yield, reducing to a low level only at 20 MPa and remaining at this level until the end of compression. The recovery at specific stress and corresponding strain was calcu lated by Eq. (1) to further quantify the recovery variation during the compression process (Fig. 4d and 4f). Fig. 4d showed that PP-D-U had the highest recovery with increasing stress during the whole compres sion process, which indicating a better resistance to pressure. In addi tion, the recovery of PP-D-U depressed few under lower stress (R>90% when σ < 30 MPa), illustrating the excellent resistance to pressure for PP-D-U before yield. But a cliff-like drop can be inspected once yield occurred, which corresponded the yielding plateau in compressive curves. When the stress reached to about 50–60 MPa, the descent rate of recovery suddenly slowed down, which was coincidently corresponding to the beginning of strain hardening in compressive curves. Finally, the recovery maintained the lower decline rate until the end of compression. However, the recovery of PP-D-B and PE-W-B decreased sharply under lower stress and declined to about 30% only at stress ¼ 20 MPa. Then the recovery remained at a very low level until the end of compression, which was also in accordance with the compression curves of corre sponding separators. And due to softer material for PE, PE-W-B had the lowest recovery during the whole compression process. Clearly, the re covery variation trend with increasing stress was highly consistent with the compression stress-strain curves, which maybe indicate a correlation between the internal structure change and the recovery vs stress curves or the compression curves. But the recovery of three kinds of separators with corresponding strain (Fig, 4e) had almost identical variation trend 4
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Fig. 2. The morphology and crystal structure characterization of three kinds of separators: (a) PP-D-U, (b) PP-D-B, (c) PE-W-B; (d) the azimuth intensity of (300) plane diffraction ring of three separators (2θ ¼ 14.1� for PP and 22.5� for PE).
Fig. 3. (a) The stress-strain curves of three kinds of separators under compression during loading and subsequent unloading; (b) the photograph of the compressed separators after 100 MPa; (c) the statistical diameter of circular sample along MD and TD direction after compression.
due to the almost overlapping recovery-strain curves, although the recovery-strain curve of PP-D-U had a weak consistency with stressstrain curves.
3.3. Macroscopic performance characterization In order to get a better understanding about the compression process, the typical performances variation of three kinds of separators with 5
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Fig. 4. The stress-strain and corresponding recovery curves of three kinds of separators under specific pressure stress: (a) PP-D-U, (b) PP-D-B, (c) PE-W-B; the calculated recovery under (d) specific stress and (e) corresponding strain.
specific stress and corresponding strain were characterized and the experimental results were exhibited in Fig. 5. The porosity of the three types of separators showed a continuous decline with the increase of stress (Fig. 5a). However, the porosity of PP-D-U was well maintained at low stress and was always the highest during the whole compression process. And the porosity decreased rapidly after yield due to the plastic deformation, then following by a slower decreased porosity when compressed to strain hardening stage. While the porosity of the other two types with biaxial stretching decreased rapidly at the initial stage, especially, PE-W-B had the lowest porosity and finally almost dropped to 0%, which was very similar to the recovery variation curves with stress. However, the porosity-strain curves (Fig. 5d) showed that the porosity of all three types of separators decreased almost linearly with strain. but
the porosity-strain curve of PP-D-U still had a weak relevance with stress-strain curves. More interestingly, the electrolyte uptake with increasing stress and strain (Fig. 5b and 5e) exhibited the same variation trend with recovery and porosity. It can be seen that some key param eters variation caused by compression (porosity, suction volume) was highly consistent with the recovery rate. And the variation trend of these parameters with stress can better describe the internal structure evolu tion during compression. In addition, the Gurley value had a strict negative with air perme ability, namely a lower Gurley value corresponded to a higher air permeability and smaller the internal resistance (Fig. 5c). Clearly, the Gurley value of three kinds of separators varies greatly with the different stress. PP-D-U showed the lowest Gurley value during the whole 6
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Fig. 5. The representative performances variation with specific stress: (a) porosity, (b) electrolyte uptake, (c) the Gurley value; the representative performances variation with corresponding strain: (d) porosity, (e) electrolyte uptake, (f) the Gurley value.
compression process, indicating the best air permeability and optimal ability to resisted compression load. While the Gurley value of PE-W-B increased fastest, reaching about 2600s (�296s) under the pressure of 50 MPa. More interestingly, the Gurley value variation curves of three kinds of separators with corresponding strain (Fig. 5f) were almost coincide, showing two sequential parts. In the case of low strain (σ<35%), Gurley value of the three types of separators had been maintained at a low level (<600s), but the porosity of separators had almost halved. With the further increase of strain (σ>40%), Gurley value increases rapidly. And this higher strain corresponded roughly to the strain hardening stage of the three separators compression curves, indicating that there may be sequential two different structure changes within three kinds of separators during compression, both uniaxial and biaxial stretching separator. In order to further explore the structural evolution of different kinds of separators along thickness direction, the morphology evolution during the whole compression process, both on surface and cross-section, were inspected by SEM directly.
gradually, causing the deflection of partial fibrils bridges and slit-like micropores. Meanwhile, the pore size along MD and TD was also decreased slightly. But when the stress exceeded 60 MPa (strain hard ening section), the vertical lamellae along thickness direction folded and sheared violently because the stress was mainly concentrated in the thickness direction. As a result, a characteristic multilayered structure with serious shear and glide between layers formed under higher stress (at the stage of stress hardening), causing the enormous change of pore size on cross-section at stress hardening section (Fig. 6e and 6f). While the pore size on surface had a gradual change (Fig. 6a and 6b), which further indicated that the connectivity along thickness direction was caused by further shearing and gliding of multilayered structure, leading to the rapid increase of Gurley value at strain hardening stage. Fig. 6g presented the mechanism diagram of PP-D-U during compression process. For the other two types of separators prepared by biaxial stretching, circular micropores on surface and stacked-layer structure along crosssection were typical characteristics. Therefore, the compression pro cess and morphology evolution were very similar, namely the gradual compaction of the stacked-layer structure and the gradual closure of the surface microporous structure during the whole compression process. The significant decrease of recovery in the early compression means the sharp decline of porosity in the early stage of compression because there was no support structure along thickness direction like PP-D-U, but the surface micropores were basically remain unchanged while the previ ously loose stacked-layer structure was compacted more compactly from the cross-section view, indicating that the compaction of stacked-layer structure mainly occurred in the early stage of compression. Mean while, the pore area on surface changed slightly when σ < 40 MPa, which can further the above conclusion. However, at the later stage of compression (σ > 40 MPa), there was little space left between layers for compression, causing the significant increase of the compression stressstrain curve slope. And reflected in the separator, the dense stackedlayer structure does not change much, but the surface micropores were obviously closed (Fig. 7e and 7f). The closure of micropores may come from two aspects. One was the autogenous shrinkage of
3.4. Microporous structure evolution during compression process The detailed compression testing manifested that the three kinds of separators had two completely different compression behavior due to the different stacked structure along thickness direction. Moreover, the Gurley value variation with different strain showed that there may be sequential two different structure changes within the all kinds of sepa rators during compression. However, there is still some speculation about the internal structure evolution and the current literature also lacked the visualized evidence of structural evolution during compres sion process. Therefore, we conducted a comprehensive SEM charac terization of the microporous structure evolution during the whole compression process, both on surface and cross-section, to obtain the most direct evidence. The surface and cross-section morphology of PP-DU did not change substantially before yield behavior (20 MPa in Fig. 6a and 6b). However, plastic deformation occurred when the stress increased from 40 MPa (yield stress) to 60 MPa (the initial stress of strain hardening) and the parallel aligned lamellae on surface distorted 7
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Fig. 6. The SEM images of PP-D-U under specific pressure stress: (a) surface, (b) cross-section; the pore size variation during compression process: (c) pore size along MD on surface, (d) pore size along TD on surface, (e) pore size along MD on cross-section, (f) pore size along ND on cross-section; (g) the mechanism diagram of PP-DU during compression process.
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evolution during compression can be well demonstrated. The compres sion process of all kinds of separators mainly consisted of two parts. The dramatic decline of porosity but slightly change of connectivity along thickness direction under lower strain firstly. And secondly, the con nectivity along thickness direction decreases sharply at higher strain. However, the reasons for the sharp decline of connectivity of different kinds of separators were also different due to two completely different microporous structures. For PP-D-U, the vertical lamellae along thick ness direction bended and folded violently and a characteristic multi layered structure with serious shear and glide between layers formed under higher stress. Therefore, the connectivity of PP-D-U decreased rapidly by further shearing and gliding of multilayered structure at higher strain. But for the other two separators, expect for the shrinkage of micropores itself, the dense area of the next layer would severely block the microporous structure of the previous layer at higher strain because the micropores did not corresponded to each other between layers.
micropores on the surface layer. Another and perhaps the most impor tant reason was that the dense area of the next layer would block the microporous structure of the previous layer during compression because the micropores did not corresponded to each other between layers, causing the rapid increase of Gurley value at this stage. Therefore, the residual trace of micropores can be observed although the micropores closed, which can be clearly observed on the separator surface at 100 MPa, especially on the surface of PE-W-B due to softer material for PE, accompanied by the much more violent shrinking of pore area at higher stress (60–100 MPa) compared with PP-D-B (Fig. 7f). In addition, a little experiment was designed to further verify the structure evolution during compression process. The separators were cut into 0.5 mm*50 mm along MD direction and then compressed to specific stress. Then the samples were vertically suspended in the different beakers with 150 mL mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 1:1 (small amount of red pigment was added to enhance contrast) and a vertical scale (The schematic diagrams were shown in Fig. 8). Finally, recording the electrolyte level elevation of samples after 60 min. When the samples were placed in a sealed beaker with the bottom immersed in the electrolyte (Fig. 8a), the elec trolyte level elevation of three kinds of separators rose gradually with the increase of stress or strain, following by a maximum and finally decline rapidly. This is a very interesting phenomenon. The previous literature verified that the suction height of the separators to electrolyte would decrease in the open system at the early stage of compression, while the upward and downward trend of the height during compression had not been discovered and demonstrated. The above data showed that the violent decrease of porosity inside the separators mainly occurred at the initial stage of compression, while the micropores connectivity changed slightly. Later the connectivity has plummeted at the end of compression process. Therefore, the sequential change inside the sepa rators was probably the main reason and further experiments were carried out to verify this phenomenon. In the unsealed beaker, the suction height of the separators with bottom immersed in electrolyte (Fig. 8b) showed a monotonous down ward trend with the increase of stress or strain, meaning that the ab sorption or diffusion rate of separators perpendicular to thickness direction decreased strictly due to the compaction along thickness di rection. In addition, the suction height of separators hanged over the electrolyte in sealed beaker (Fig. 8c) could indicate the adsorption ca pacity of the electrolyte vapor in the sealed beaker parallel to thickness direction t. Clearly, Fig. 8c exhibited the exactly the same variation trend compared with Fig. 8a, the electrolyte level elevation increased initially and decrease rapidly afterwards. By adding the electrolyte level elevation of Fig. 8b and 8c numerically (shown in Fig. d), it can be found that the sum showed an amazing consistency with Fig. 8a, which also indicated that the suction capacity of the separators with the bottom immersed in electrolyte in the sealed environment was composed of two parts, namely the adsorption capacity for electrolyte liquid perpendic ular to thickness direction (Fig. 8b) and the ability to absorb electrolyte vapor along thickness direction (Fig. 8c). According to the previous data, it is not difficult to understand that the surface micropore structure hardly changed at the initial stage of compression, and the Gurley value of the membrane also changed slightly, which indicated that the connectivity of micropores along thickness direction did not change significantly, therefore, the absorp tion ability of the separators to electrolyte vapor actually does not change much. But due to the significant decrease of porosity, the sepa rator only needed to adsorb lesser electrolyte to fill the micropores, therefore, the separators showed faster apparent absorption capacity and higher suction height at the early stage of compression process. Subsequently, the micropores on surface closed or blocked drastically at the later stage of compression and the separator cannot absorb the electrolyte vapor, causing a significant decline of suction height at the end of compression process. It can be seen from this simple experiment that the mode of structure
4. Conclusions In this article, the deformation mode, microstructure evolution and performances variation of three kinds of commercial polyolefin sepa rator under compression load were systematically analyzed, which was important to explore the long-term behavior of batteries. The compression results indicated that three kinds of separator presented two entirely different deformation modes due to the different microporous structure. There were abundant parallel lamellae aligned along thickness direction on cross-section for PP-D-U, leading to excel lent ability to resist compression loads. Therefore, PP-D-U presented the unique compression behavior, which had a striking yield behavior firstly. Then the compression plateau can be observed, accompanied by compaction along thickness and formation of multilayered structure due to the bending or folding of parallel lamellae aligned along thickness direction on cross-section. Finally the multilayered structure further sheared or flowed violently, causing the strain hardening in compression curve and rapid increase of Gurley value. More interestingly, all prop erties variation with stress displayed striking uniformities with compression curves. On the other hand, the separator made by biaxial stretching, both dry and wet process, had numerous stacked-layer structure with poor supporting structure along thickness direction, therefore, there was no obvious yield behavior on compression curve. And all properties showed a gentle trend for biaxial stretching separator, which did not exhibit sudden changes like PP-D-U. However, three kinds of separator also shared some common char acteristics, which the compression process mainly consisted of two similar parts. The dramatic decline of porosity but slightly change of connectivity along thickness direction under lower strain firstly. And secondly, the connectivity along thickness direction decreases sharply at higher strain. However, the reasons for the sharp decline of connectivity of different kinds of separators were also different due to two completely different microporous structures. For PP-D-U, the vertical lamellae along thickness direction bended and folded violently, but the microporous connectivity in the thickness direction was affected slightly under lower stress. While a characteristic multilayered structure with serious shear and glide between layers formed under higher stress. Therefore, the connectivity of PP-D-U decreased rapidly by further shearing and gliding of multilayered structure at higher strain. But for the other two sepa rators, expect for the shrinkage of micropores itself, the dense area of the next layer would severely block the microporous structure of the pre vious layer at higher strain because the micropores did not corresponded to each other between layers, causing the sudden change of Gurley value. Declaration of competing interest The authors declare that they have no known competing financial 9
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Fig. 7. The SEM images of PP-D-B and PE-W-B under specific pressure stress: (a) surface morphology of PP-D-B, (b) cross-section morphology of PP-D-B, (c) surface morphology of PE-W-B, (d) cross-section morphology of PE-W-B; the area variation of pores on surface: (e) PP-D-B, (f) PE-W-B; (e) the mechanism diagram of two kinds of separator during compression process.
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Fig. 8. The electrolyte level elevation of three kinds of separators at different cases after 60min: (a) in sealed beaker and bottom of samples (10 mm) were immersed in electrolyte; (b) in non-sealed beaker and bottom of samples were immersed in electrolyte; (c) in sealed beaker and suspended samples over electrolyte level; (d) the sum of electrolyte level elevation of (b) and (c).
interests or personal relationships that could have appeared to influence the work reported in this paper.
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