Effects of heat setting on the morphology and performances of

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Materials and Interfaces

Effects of heat setting on the morphology and performances of polypropylene separator for lithium ion batteries Fangxinyu Zeng, Ruizhang Xu, Lei Ye, Bijin Xiong, Jian Kang, Ming Xiang, Lu Li, Xingyue Sheng, and Zengheng Hao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05782 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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Industrial & Engineering Chemistry Research

Effects of heat setting on the morphology and performances of polypropylene separator for lithium ion batteries

Fangxinyu Zeng 1, Ruizhang Xu1, Lei Ye 1, Bijin Xiong 2, Jian Kang 1,*, Ming Xiang 1, Lu Li 3, Xingyue Sheng 3, Zengheng Hao 3

1

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan

University, Chengdu 610065, China 2

School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology

(HUST), Wuhan 430074, China 3

Chongqing Zhixiang Paving Technology Engineering Co., Ltd., Chongqing, 401336, China

* Corresponding

author. Jian Kang, e-mail: [email protected]

Abstract: In this study, effects of heat setting treatment (pre-set heat setting ratio λ and heat setting temperature Tsetting) on the crystalline structures, orientation status, microporous structures, gas permeability and electrochemical performances of the separators were studied in detail by means of differential scanning calorimetry (DSC), infrared dichroism, scanning electron microscope (SEM), ultra-small angle X-ray scattering (U-SAXS), Gurley value test and electrochemical measurement. It was found that with the decrease of λ or the increase of Tsetting, the melting temperature and degree of crystallinity changes regularly, the orientation degree decreases gradually, resulting in small sized micropores, lowered porosity and S/V ratio of the separator. Finally, the gas permeability and ionic conductivity of the separator decreased while the dimensional stability increased. By tuning λ and Tsetting, the microstructures, morphology and performances of the separators can be efficiently controlled.

Keywords: isotactic polypropylene; separator; heat setting; lithium ion batteries

1. Introduction 1

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Lithium-ion batteries (LIBs) are widely adopted as one of the most popular energy storage system for portable electronic devices1, 2. Commonly, a lithium-ion battery consists of five functional components, i.e., the anode3, cathode4, separator5, electrolyte6 and the covering cases7. The separator is a critical component in LIBs to ensure battery safety, and is placed between the positive electrode and the negative electrode to physically and electronically isolate the electrodes while permitting free ionic transport within its porous network5. It is mostly a microporous layer consisting of polymeric separator, such as polypropylene (PP)8, 9 and polyethylene (PE)10-12. The melt-stretching method is usually used for the fabrication of PP separators where pores are formed by controlling crystallization of melt-extruded films (hard elastic films) and then uniaxial stretching, which generally consists of heating, extruding, annealing, uniaxially stretching and heat setting steps (as described in Scheme 1)10-12. After stretching at high temperature, actually, there is a principal process, so-called heatsetting, to enhance the dimensional stability and to eliminate evident shrinkage or the occurrence of wrinkles upon heating, which is of great importance for the enhancement of the safety property and controlling of the final performances of all the LIBs separators.

Scheme 1

Manufacturing process of the PP separator via uniaxially drawing process.

Hitherto, much works had been done about heat setting of films. Nohtomi et al.13 focused on the means of obtaining polyethylene terephthalate tubular films with uniform physical properties by adjusting the process and apparatus for heat setting. Crass et al.14 studied specific parameters in heat post-treatment of the biaxially oriented polypropylene film for the purpose of dimensional stabilization. Also, one invention15, related to processing of asymmetrically and biaxially oriented polyester film and more particularly to the improved heat setting of such film, found that heat setting can provide a substantial improvement of dimensional stability in the transverse direction without a 2


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substantial loss in tensile strength at 5% elongation in the machine direction. Hale et al.16 investigated the role of heat-setting temperature on the moisture vapor transmission rate (MVTR) of linear low-density polyethylene/CaCO3 film, and claimed that as the heat-setting temperature increased, the MVTR of the film decreased and storage modulus increased; Greener et al.17 studied the effects of heat setting treatment on the properties and microstructure for biaxially stretched polyester films, and found that as the heat setting temperature increased, the crystallinity and dimensional stability of the film increased; Bicakci et al.18 investigated the structural hierarchy evolution during heat setting of PEEK/PEI films with varied composition and preorientation status; Lei et al. contributed elegant works concerning the impact of heat setting temperature on the morphology and mechanical properties of PP membrane 19 by means of universal tensile testing machine equipped with a heating chamber, and elucidated the roles of heat setting temperature on the microstructure of the membrane 20. In fact, for microporous PP separators manufactured by melt-stretching for LIBs, long-time dimensional stability is very important for the safety of the LIBs. Therefore, it is necessary to elucidate the influences caused by heat setting in detail. However, few works focused on the impacts of the heat setting in terms of morphology and performance; what’s more, no reports concerned the combination effects of two important parameters, heat setting temperature Tsetting and pre-set heat setting ratio λ, on the microstructure and morphology evolution of the separator. In this manuscript, the effects of heat setting temperature Tsetting and pre-set heat setting ratio λ on the microstructure, morphology, orientation status, permeability and electrochemical performances of PP separators were investigated. To ensure the accuracy and reliability of the results, a series of rolls from the industrialized production line was used to prepare the separators, which is far different from conventional lab-scale production reported in previous few studies and can provide valuable and interesting results from both scientific and practical aspects.

2. Experimental section 3



2.1 Materials and sample preparation Semi-manufactured PP separators for Li-ion battery were produced according to a standard manufacturing process as shown in Scheme 1a, with the help of Shenzhen Senior Technology Material Corp., China. The heat setting process was performed directly after low/high temp stretching, using a series of rolls from the industrialized production line as described in Scheme 2. The involved parameters are: diameter of the rolls, roll speed, roll temperature and the tensile force. In this paper, the diameter of the rolls was 1000 mm, the roll temperature (heat setting temperature, Tsetting) was 135-150oC, and the tensile force was set as constant value of 10 N. Using the differences of roll speed between the rolls, the total pre-set heat setting ratio, λ, was applied as 0.85-1.0. To be more exactly, λ=1 means the roll speed of each roll was the same value of 5 m/min; λ=0.9 means the roll speeds of the six rolls were 5.0, 4.9, 4.8, 4.7, 4.6 and 4.5 m/min from Roll 1# to Roll 6#, respectively.

Scheme 2

Illustration of the heat setting process of the PP separator using a series of rolls from the industrialized production line.

2.2 Characterization 2.2.1

Differential scanning calorimeter (DSC) Calorimetric experiments were carried out on differential scanning calorimeter

(Mettler Toledo DSC1, Mettler Toledo Corp., Switzerland), under the protection of nitrogen atmosphere (50 mL/min). To ensure reliability of the results, the temperature scale was calibrated using indium as standard21-23. The heating rate is 10℃/min. The 4


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relative degree of crystallinity (Xc) of samples was calculated using Equation (1)24-26: ∆𝐻

(1)

𝑋𝑐 = ∆𝐻𝑜

where ΔH represents the value of fusion enthalpy measured by DSC, while ΔHo is the completely crystalline heat of fusion. The values used for ΔHo for 100 % crystalline iPP was taken as 209 J / g according to previous study 27.

2.2.2

Scanning electron microscope (SEM) To directly study the microporous morphology of the separators, SEM observation

was performed on a FEI Inspect F environmental scanning electron microscope at an accelerating voltage of 5 kV. All the samples were precoated with a thin layer of gold by ion sputtering before observation in order to prevent charge accumulation28-30.

2.2.3

Porosity measurement The porosity was tested by means of liquid absorption methods through their

density according to ASTM D-2873, using Equation (2) 31-34: 𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦 =

𝜌𝑐 ― 𝜌𝑒

(2)

𝜌𝑐

where 𝜌𝑐 is the calculated density for the absorbed films, and 𝜌𝑒 is the density for original separator. 2.2.4

Infrared dichroism Infrared dichroism was carried out to detect the orientation status of amorphous

phase and crystalline phase of the separators using NICOLET 6700 FT-IR instrument (Thermo Fisher Scientific Comp., USA). Using a zinc selenide wire grid polarizer, the beam was polarized into two orthogonal directions, parallel and perpendicular to a reference axis. The range of scanning is 4000-400 cm-1, the resolution is 1 cm-1 and it was scanned 32 times. The ratio of these two absorption values is defined as the dichroic ratio, D. According to the method proposed by Tabatabaei et al.10, orientation function of the vibration was obtainí:

5


35,

the Herman


𝐷―1

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(3)

𝑓𝑖,𝑀𝐷 = 𝐷 + 2

where D is the ratio of the absorption parallel and absorption perpendicular to the machine direction. For PP, the absorptions at the wave number of 998 cm−1 and

972

cm−1 are attributed to the crystalline phase (c axis), and both crystalline and amorphous phases, respectively. The orientation of crystalline phase (fc) and average orientation function (fav, including contributions of both crystalline and amorphous phases) could be calculated by Equation (3).

Air permeability

2.2.5

To characterize the ionic conductivity of separators, the air permeability, represented by Gurley value, was measured in this study using a Gurley Densometer (4110N, Gurley). The Gurley value of the separators was determined by measuring the time required for 100 mL air to pass through a determined volume under fixed pressure of 20 kgfcm-2. Low Gurley value indicates high air permeability.

2.2.6

Dimensional stability The dimensional stability, represented by thermal shrinkage, was determined by

exposing the separators in an oven at 110oC for 1 h. After that, dimensional variations of the samples were carefully calculated by Equation (4) 36: ∆𝑙

𝑆ℎ𝑟𝑖𝑛𝑘𝑎𝑔𝑒(%) = 𝑙0

(4)

where ∆𝑙 represents the dimensional change in length in the stretching direction, and 𝑙0 is the original length. 2.2.7

Ultra-small angle X-ray scattering (U-SAXS)

Ultra-small angle X-ray scattering (U-SAXS) measurement was carried on a modified Xeuss system of Xenocs France equipped with the same detector and X-ray source to NanoinXider, to explore the effects of heat setting on the microporous structures of the separators. The sample-to-detector distance was set at 6510 mm. The effective range of the scattering vector q (q=4πsinθ/λ, where 2θ is the scattering angle 6


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and λ is the wavelength) is 0.017 - 0.27 nm-1 in horizontal direction and 0.017 - 0.11 nm-1 in vertical direction. Each U-SAXS pattern was recorded within 60 s including exposure time 59 s and recording time 1 s. The obtained scattering patterns were firstly corrected by background subtraction, and then they were calibrated to an absolute intensity according to previous reported methods36. The porosity and specific interface area (surface / volume ratio, S/V) were calculated according to the method reported in our previous studies36, 37.

2.2.8

Electrochemical performances

In order to study the electrochemical performances of the separators, liquid electrolyte of 1 M LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) (1/1, v/v, Tinci Materials Technology Co., Ltd., China) was used. Two important parameters including ionic conductivity and bulk resistance of the liquid electrolyte-soaked separators (diameter 20 mm) between two stainless-steel plate electrodes were evaluated by an AC impedance analysis using electrochemical working station (Chenhua, CHI660E, China) over a frequency range of 101-106 Hz at room temperature. The ionic conductivity of separator was calculated by Equation (5)38-40:

=

d RA

(5)

where R was the bulk resistance of the liquid electrolyte-soaked separator, d and A were the thickness and the area of the separator respectively.

3. Results and discussions 3.1 Melting behavior The melting curves of the separators at the heating rate of 10°C/min are shown in Fig. 1. From Fig. 1, the melting parameters, including the peak melting temperature (Tm), melting peak width and degree of crystallinity (Xc), are presented in Fig. 2.

7



Figure 1 DSC melting curves of the separators for various heat-setting ratios.

Figure 2 Variations of (a) peak melting temperature (Tm), (b) melting peak width and (c) degree of crystallinity (Xc) of separators with different heat setting ratio λ as a function of heat setting temperature Tsetting.

Figs. 1 and 2 reveal that both λ and Tsetting greatly influence of the melting curves of the separators. Tm increases gradually with the increase of Tsetting and λ, reflecting the annealing effect of heat setting treatment. Meanwhile, the degree of crystallinity Xc decreases gradually with the increase of Tsetting and the decrease of λ, indicating that lower Tsetting and λ encourage higher Xc and lower Tm. Moreover, the width of melting peak increases gradually with the increase of λ, but exhibits different dependencies on the Tsetting: when λ=0.95 and 1, it increases gradually with the increase of Tsetting, indicating that higher Tsetting broadens the lamellar thickness distribution of the separator when λ is high; when λ=0.85 and 0.9, it decreases gradually with the increase of Tsetting, reflecting the at low λ, high Tsetting tends to narrow the lamellar thickness distribution of the separator. Moreover, it can be seen from Fig. 1 that besides the main melting peak, there is a shoulder peak located at high temperature direction (Fig. 1 (a)-(d)) and one plateau at low temperature direction (only in Fig. 1(a)). According to references, the shoulder peak corresponds to the connecting bridges between separated lamellae19,20, which is also supported by the study of Sadeghi et al. 8. Compared with the original separators, 8


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the shoulder peaks of the separators after heat setting become more pronounced, indicating that the structure of connecting bridges comes to sound during heat setting and crystal structure is more stable. As to the appearance of the plateau at low temperature direction in Fig. 1(a), it becomes more evident when the heat setting temperature Tsetting increases to 145 oC and 150 oC due to the crystallization of tie chains on both sides of lamellae, suggesting that the occurrence of crystallization is more pronounced at the Tsetting of 145 oC and 150 oC. Previous works41,

42

reported the

existence of endotherm plateau located at lower temperature range before the appearance of the main melting peak on DSC melting profiles of annealed film, which is quite similar to the DSC melting profiles of the heat-set separators in Fig. 1 Therefore, it is believed that the occurrence of endotherm plateau for heat-set separators comes from the crystallization of some chains around initial row-nucleated lamellar structures19,20,43. To be more exactly, heat setting treatment applies an annealing effect on the separators, probably making the molecular segments of some tie chains evolve into crystallites19.

3.2 Orientation status measurement Using infrared dichroism, the orientation functions of global and crystalline (fav and fc) of separators after heat setting are determined as shown in Fig. 3. The original infrared dichroism spectra are provided in the supporting information (Fig. S1).

Figure 3

Variation of orientation function of (a) average (fav) and (b) crystalline phase (fc) as a function of heat setting ratio λ.

Fig. 3 shows that the orientation functions fav and fc of the separators decrease with 9



raising Tsetting, indicating that higher heat setting temperature can melt larger amount of crystalline phase of the separator during heat setting process, and provide higher possibility for the molecular thermal movement of both crystalline region and amorphous region, thus lead to more reduction of orientation degree; On the other hand, the lower the heat setting ratio λ, the lower the fav and fc, suggesting that besides heat setting temperature Tsetting, the heat setting ratio is also an important factor determining the orientation degree of the separator. By tuning both Tsetting and λ, the orientation function of both crystalline phase and amorphous phase of the separator can be efficiently controlled. Moreover, Fig. 3(b) also reveals that if the Tsetting is too low (135oC in this study), the orientation degree of the crystalline phase (fc) remains almost unchanged with the variation of λ, indicating that Tsetting is the first determining factor of fc. Meanwhile, the fav decreases gradually with the decrease of λ when Tsetting =135oC, exhibiting less dependency of the orientation degree of amorphous phase on Tsetting.

3.3 Morphology study To explore the impact of Tsetting and λ on microporous morphology of the separators, SEM observation of the separators after heat setting treatment at Tsetting of 140oC but different pre-set heat setting ratios (λ=0.85-1.0) are performed. Moreover, the statistical data concerning the lengths of the pores in machine direction (MD) and transverse direction (TD) are calculated. The results are shown in Fig. 4.

10


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Figure 4 SEM images of separators after treated at Tsetting=140oC, (a) λ=0.85, (b) λ=0.9, (c) λ=0.95, (d) λ=1.0; Lengths of the pores along (e) machine direction (MD) and (f) transverse direction (TD) of separators after heat setting treatment at Tsetting =140oC and λ=0.85-1. (g) Averaged lengths of the pores in MD and TD as a function of heat setting ratio λ.

Interestingly, it can be clearly seen from Fig. 4 that, when Tsetting is fixed, the shape of the pores is greatly influenced by the heat setting ratio λ. With the decrease of heat setting ratio λ, the number of the pores decreases evidently, indicating that many pores disappear due to the heat setting treatment, which is in accord with the decrease of orientation degree in Fig. 3; with the decrease of λ, the length of the pores along MD decreases gradually, while the length of pores in TD increases gradually. In other words, the pores become more round with the decrease of λ. This interesting phenomenon reflects the feature that the variation of λ applies different influences on the pores along MD and TD. Fig. 5 is the SEM images of the separators at λ=0.9 under different Tsetting and the results of pore lengths along MD and TD.

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Figure 5 SEM images of separators at λ=0.9, (a) Tsetting=135oC; (b) Tsetting=140oC; (c) Tsetting=145oC; (d) Tsetting=150oC. Lengths of the pores along (d) machine direction (MD) and (e) transverse direction (TD) of separators after heat setting treatment at λ=0.9 and Tsetting =135-150oC. (f) Averaged lengths of the pores in MD and TD as a function of heat setting ratio λ.

Fig. 5 reveals that with the increase of heat setting temperature Tsetting, the number of the pores decreases significantly, suggesting that higher heat setting temperature causes higher extent of thermal shrinkage, resulting in less pores in the separator. Moreover, the sizes of pores in both MD and TD decrease gradually with increasing Tsetting, indicating that the pores are destructed, even disappeared at higher temperature. It should be noted that the heat setting ratio λ and heat setting temperature Tsetting have different influences on the shape of the pores as reported above. A possible explanation is given below. Tuning the pre-set heat setting ratio λ mainly influences the molecular movement of the separators along MD, while the change of heat setting temperature provides impact in all directions on the separators. Therefore, different shapes of the micropores can be observed after heat setting.

3.4 Microporous structure study of the separators The porosity of the separators was examined by density method and U-SAXS. Moreover, the surface / volume ratio (S/V, also called specific interface area), which reflects information about the inner area of the micropores, was calculated via U-SAXS measurement according to the manner reported in the previous study36. The higher the S/V value, the larger the interfacial area of the micropores within the separator. The results are shown in Fig. 6.

12


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Figure 6 Variations of porosity, specific interface area (S/V), bulk resistance and ionic conductivity with the change of the pre-set heat setting ratio λ and heat setting temperature Tsetting of the separators.

Fig. 6 reveals that with the decrease of heat setting ratio λ and the increase of heat setting temperature Tsetting, the porosity of the separator decreases evidently, indicating that higher Tsetting and lower λ prefer the occurrence of higher shrinkage during the heat setting process, which is in accord with the results of orientation status study (FT-IR); Moreover, the porosity results obtained by density method and U-SAXS are quite similar from each other, indicating that both of density method and U-SAXS can well characterize the porosity of the separator. On the other hand, with the decrease of heat setting ratio λ and the increase of heat setting temperature Tsetting, the S/V value decreases gradually, suggesting that the heat setting process decreases not only the porosity but also the interfacial area within the separator. Moreover, electrochemical performances of the separators were studied, the results of ionic conductivity and bulk resistance are shown in Figs. 6(e) and 6(f). Evidently, the bulk resistance of the separator increases gradually with the decrease of λ and the increase of Tsetting, while the ionic conductivity decreases at the same time, indicating that the heat setting treatment decreases the ionic conductivity and increases the bulk resistance of the separator, which should be attributed to the variation of microporous structures. It should also be noted that a more evident change of electrochemical performances of the separator with the change of λ can be seen, indicating that compared with Tsetting, λ is a more important factor determining the electrochemical performances.

13



3.5 Gas permeability and dimensional stability during heating To understand both porosity and tortuosity of the separator, the gas permeability of the separators are studied by Gurley value measurement. The higher the Gurley value, the longer time for gas to pass through the separator44. Moreover, the heat shrinkage property of the separators was measured. The results of gas permeability and heat shrinkage are shown in Fig. 7.

Figure 7

Relationship between the Gurley value and (a) λ; (b) heat setting temperature. (c) Variation of heat shrinkage properties of the separators.

Fig. 7(a) reveals that Gurley value exhibits significant dependence on the heat setting ratio λ. As λ increases, the Gurley value of the separator decreases gradually, suggesting that the permeability of the separator increases with the increase of λ; On the other hand, Fig. 7(b) reveals that the Gurley value is also slightly dependent on the heat setting temperature Tsetting. When λ=0.85 and 0.9, the Gurley value decreases gradually with the increase of Tsetting, while the Gurley value stays almost unchanged when λ=0.95 and 1.0, indicating that on the one hand, the microstructure might only slightly change as the Tsetting varies at the high λ of 0.95 and 1.0, and on the other hand, λ is the first determining factor of the permeability of the separator compared with Tsetting. The dimensional stabilities of separators under heating was conducted by measuring the heat shrinkage of the separator under thermal treatment in machine direction, and the results are shown in Fig. 7(c). It should be noted that for the separators manufactured by uniaxially stretching process, its dimensional stability in transverse direction (TD) is very high, and the thermal shrinkage under heating is almost zero. Therefore, the dimensional stability of the separators in TD is not reported in this study. 14


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Since the manufacturing process of separators includes a stretching step, these separators shrink easily by the heat treatment due to internal stress. Clearly, the heat shrinkage decreases evidently with the decrease of λ and the increase of Tsetting. It should be noted that when λ = 1.0, the heat shrinkage value is very high, which stays unchanged with the variation of Tsetting, exhibiting the worst dimensional stability. It indicates that for dimensional stabilities, λ is the first determining factor of the dimensional stabilities during heating of the separators compared with Tsetting. In general, since the experiments have been carried out using a series of rolls from the industrialized production line, which is far different from conventional lab-scale production reported in previous few studies, this study provides valuable results concerning the mechanism of heat setting on the microstructure and performances of the separators. Results indicate that the heat setting treatment plays important role in determining the crystalline structures, orientation status, microporous structures (including pore sizes and distribution, porosity and S/V ratio), gas permeability and electrochemical performances. To be more exactly, in the aspect of microstructure evolution, during the heat setting process, the connecting bridges come to sound due to the crystallization of tie chains on both sides of lamellae, while the crystal structure becomes more stable; on the other hand, higher heat setting temperature can melt larger amount of crystalline phase of the separator during heat setting process, and provide higher possibility for the molecular thermal movement in both crystalline region and amorphous region, thus lead to more reduction of orientation degree. The reduction of orientation degree is more easily observed in amorphous phase compared with crystalline phase, determined by the heat setting temperature Tsetting. More evidently, the pore sizes and distribution, porosity and S/V ratio change significantly. In the aspect of performances, the permeability, electrochemical performances and dimensional stability are found to be dependent on the heat setting parameters λ and Tsetting. Therefore, by carefully choosing pre-set heat setting ratio λ and heat setting temperature Tsetting, the microstructure and performances of the separator could be well 15



controlled. The results and mechanism proposed in this study could be useful for both theoretical aspect and industrial production.

4. Conclusions In this study, the influence of heat setting treatment (pre-set heat setting ratio λ and heat setting temperature Tsetting) on the crystalline structures, orientation status, microporous structures (including pore sizes and distribution, porosity and S/V ratio), gas permeability and electrochemical performances of the separators were studied in detail by means of DSC, infrared dichroism, SEM, U-SAXS, Gurley value test, density method and electrochemical measurement. The combination effects of λ and Tsetting were also discussed. It was found that with the decrease of λ or the increase of Tsetting, the melting temperature and degree of crystallinity changes regularly, the orientation degree decreases gradually, resulting in small sized micropores, lowered porosity and S/V ratio of the separator. Finally, the gas permeability and ionic conductivity of the separator decreased while the dimensional stability increased. By tuning λ and Tsetting, the microstructures, morphology and performances of the separators can be efficiently controlled.

Supporting Information Figure S1 Infrared dichroism spectra of the separators after heat setting treatment at the indicated temperature (Tsetting) and heat setting ratio (λ). The solid lines correspond the spectra obtained parallel to the machine direction, while the dashed lines correspond to the spectra obtained perpendicular to the machine direction.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC 21604088, 51503134, 51721091, 51702282), China Postdoctoral Science Foundation (Grant No. 2015LH0050), State Key 16


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Laboratory of Polymer Materials Engineering (Grant No. SKLPME 2017-3-02) and the Key Industry Technology Innovation Projects of Chongqing (CSTC2017zdcyzdyf0297).

References (1) Lin, D.; Liu, Y.; Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat. Nanotechnol. 2017, 12, 194. (2) Li, Y.; Yang, J.; Song, J. Design principles and energy system scale analysis technologies of new lithium-ion and aluminum-ion batteries for sustainable energy electric vehicles. Renew. Sust. Energ. Rev. 2017, 71, 645-651. (3) Cheng, X. B.; Zhang, R.; Zhao, C. Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117, 10403-10473. (4) Wu, F.; Yushin, G. Conversion cathodes for rechargeable lithium and lithium-ion batteries. Energ. Environ. Sci. 2017, 10, 435-459. (5) Lagadec, M. F.; Zahn, R.; Wood, V. Designing Polyolefin Separators to Minimize the Impact of Local Compressive Stresses on Lithium Ion Battery Performance. J. Electrochem. Soc. 2018, 165, A1829-A1836. (6) Manthiram, A.; Yu, X.; Wang, S. Lithium battery chemistries enabled by solid-state electrolytes. Nat. Rev. Mater. 2017, 2, 16103. (7) Zeng, F.; Chen, J.; Yang, F.; Kang, J.; Cao, Y.; Xiang, M. Effects of Polypropylene Orientation on Mechanical and Heat Seal Properties of Polymer-Aluminum-Polymer Composite Films for Pouch Lithium-Ion Batteries. Materials 2018, 11, 144. (8) Wu, T.;

Xiang, M.;

Cao, Y.;

Kang, J.; Yang, F. Influence of lamellar structure on double yield

behavior and pore size distribution in β nucleated polypropylene stretched membranes. RSC Adv. 2014, 81, 43012-43023. (9) Wu, T.; Xiang, M.; Cao, Y.; Kang, J.; Yang, F. Pore formation mechanism of β nucleated polypropylene stretched membranes. RSC Adv. 2014, 4, 36689-36701. (10) Tabatabaei, S. H.; Carreau, P. J.; Ajji, A. Effect of processing on the crystalline orientation, morphology, and mechanical properties of polypropylene cast films and microporous membrane formation. Polymer 2009, 50, 4228-4240. (11) Tabatabaei, S. H.; Carreau, P. J.; Ajji, A. Microporous membranes obtained from PP/HDPE multilayer films by stretching. J. Membrane Sci. 2009, 345, 148-159. (12) Tabatabaei, S. H.; Carreau, P. J.; Ajji, A. Structure and properties of MDO stretched polypropylene. Polymer 2009, 50, 3981-3989. (13) Ryota, N.; Masayoshi, S.; Tuyoshi, S. Process for biaxially producing oriented tubular polyethylene terephthalate films by simultaneous stretching. U. S. Patent 4,061,707A, October 8, 1975. (14) Chandavasu, C.; Xanthos, M.;

Sirkar, K. K.; Gogos, C. G. Preparation of Microporous Films from

Immiscible Blends via Melt Processing. J. Plast. Film Sheet. 2000, 16, 288-300. (15) Katsuya, T.; Satoru. N.; Toshiya, N. Process for producing a biaxially oriented polyester film and biaxially oriented polyester film. U. S. Patent 6,685,865B2, Febrary 2, 2002. (16) Hale, W. R.; McGuire, J.; Sand, I. D.; Dohrer, K. K. Heat setting of stretched and microvoided 17



PE/CaCO3 films. J. Appl. Polym. Sci. 2001, 82, 2454-2471. (17) Greener, J.; Tsou, A. H.; Blanton, T. N. Physical and microstructural effects of heat setting in polyester films. Polym. Eng. Sci. 1999, 39, 2403-2418. (18) Bicakci, S.; Cakmak, M. Kinetics of rapid structural changes during heat setting of preoriented PEEK/PEI blend films as followed by spectral birefringence technique. Polymer 2002, 43, 2737-2746. (19) Lei, C.; Wu, S.; Cai, Q.; Xu, R.; Hu, B.; Shi, W. Influence of heat-setting temperature on the properties of a stretched polypropylene microporous membrane. Polym. Int. 2014, 63, 584-588. (20) Cai, Q.; Xu, R.; Wu, S.; Chen, C.; Mo, H.; Lei, C.; Li, L.; Li, Z. Influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane. Polym. Int. 2015, 64, 446-452. (21) Yu, Y.; Zeng, F.; Chen, J.; Kang, J.; Yang, F.; Cao, Y.; Xiang, M. Regulating polycrystalline behavior of the β‐nucleated isotactic polypropylene/graphene oxide composites by melt memory effect. Polym. Composite. 2018, doi.org/10.1002/pc.24745. (22) Yu, Y.; Zeng, F.; Chen, J.; Kang, J.; Yang, F.; Cao, Y.; Xiang, M. Isothermal Crystallization Kinetics and Subsequent Melting Behavior of β‐Nucleated Isotactic Polypropylene / Graphene Oxide Composites with Different Ordered Structure. Polym. Int. 2018, 67, 1212-1220. (23) Qin, Y.; Xu, Y.; Zhang, L.; Zheng, G.; Yan, X.; Dai, K.; Liu, C.; Shen, C.; Guo, Z. Interfacial interaction enhancement by shear-induced β-cylindrite in isotactic polypropylene/glass fiber composites. Polymer 2016, 100, 111-118. (24) Hao, Z.; Li, L.; Liao, X.; Sheng, X.; Zhang, Y. Preparation and toughening performance investigation of epoxy resins containing carbon nanotubes modified with hyperbranched polyester. Polym. Bull. 2018, 75, 1013-1026. (25) Han, W.; Liao, X.; Yang, Q.; Li, G.; He, B.; Zhu, W.; Hao, Z. Crystallization and morphological transition of poly(l-lactide)–poly(ε-caprolactone) diblock copolymers with different block length ratios. RSC Adv. 2017, 7, 22515-22523. (26) Zhang, C.; Wang, B.; Yang, J.; Ding, D.; Yan, X.; Zheng, G.; Dai, K.; Liu, C.; Guo, Z. Synergies among the self-assembled β-nucleating agent and the sheared isotactic polypropylene matrix. Polymer 2015, 60, 40-49. (27) Kang, J.; Chen, J.; Cao, Y.; Li, H. Effects of ultrasound on the conformation and crystallization behavior of isotactic polypropylene and β-isotactic polypropylene. Polymer 2010, 51, 249-256. (28) Hao, Z.; Tan, Y.; Zhang, X.; Zhang, F. Epithermal ageing mechanism of gussasphalt. J. Wuhan Univ. Technol. 2009, 24, 466-470. (29) Liu, Z.; Liu, X.; Zheng, G.; Dai, K.; Liu, C.; Shen, C.; Yin, R.; Guo, Z. Mechanical enhancement of melt-stretched β-nucleated isotactic polypropylene: The role of lamellar branching of β-crystal. Polym. Test. 2017, 58, 227-235. (30) Zhang, L.; Qin, Y.; Zheng, G.; Dai, K.; Liu, C.; Yan, X.; Guo, J.; Shen, C.; Guo, Z. Interfacial crystallization and mechanical property of isotactic polypropylene based single-polymer composites. Polymer 2016, 90, 18-25. (31) Xu, R.; Xu, G.; Wang, J.; Chen, J.; Yang, F.; Kang, J.; Xiang, M. Influence of l-lysine on the permeation and antifouling performance of polyamide thin film composite reverse osmosis membranes. RSC Adv. 2018, 8, 25236-25247. (32) Wang, J.; Xu, R.; Yang, F.; Kang, J.; Cao, Y.; Xiang, M. Probing influences of support layer on the morphology of polyamide selective layer of thin film composite membrane. J. Membrane Sci. 2018, 556, 374-383. 18


Page 18 of 20

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Yu, Y.; Xiong, B.; Zeng, F.; Xu, R.; Yang, F.; Kang, J.; Xiang, M.; Li, L.; Sheng, X.; Hao, Z. Influences of Compression on the Mechanical Behavior and Electrochemical Performances of Separators for Lithium Ion Batteries. Ind. Eng. Chem. Res. 2018, 57, 17142-17151. (34) Xu, R.; Wang, J.; Chen, D.; Yang, F.; Kang, J.; Xiang, M.; Li, L.; Sheng, X. Preparation of pHresponsive asymmetric polysulfone ultrafiltration membranes with enhanced anti-fouling properties and performance by incorporating poly(2-ethyl-2-oxazoline) additive. RSC Adv. 2018, 8, 41270-41279. (35) Tabatabaei, S. H.; Carreau, P. J.; Ajji, A. Microporous membranes obtained from polypropylene blend films by stretching. J. Membrane Sci. 2008, 325, 772-782. (36) Xiong, B.; Chen, R.; Zeng, F.; Kang, J.; Men, Y. Thermal shrinkage and microscopic shutdown mechanism of polypropylene separator for lithium-ion battery: In-situ ultra-small angle X-ray scattering study. J. Membrane Sci. 2018, 545, 213-220. (37) Xiong, B.; Kang, J.; Chen, R.; Men, Y. Initiation of cavitation upon drawing of pre-oriented polypropylene film: In situ SAXS and WAXD studies. Polymer 2017, 128, 57-64. (38) Jung, Y. C.; Kim, S. K.; Kim, M. S.; Lee, J. H.; Han, M. S.; Kim, D. H.; Shin, W. C.; Ue, M.; Kim, D. W. Ceramic separators based on Li + -conducting inorganic electrolyte for high-performance lithiumion batteries with enhanced safety. J. Power Sources 2015, 293, 675-683. (39) Raja, M.; Sanjeev, G.; Prem Kumar, T.; Manuel Stephan, A. Lithium aluminate-based ceramic membranes as separators for lithium-ion batteries. Ceram. Int. 2015, 41, 3045-3050. (40) Zhou, B.; Li, Y.;

Zheng, G.; Dai, K.; Liu, C.; Ma, Y.; Zhang, J.; Wang, N.; Shen, C.; Guo, Z.

Continuously fabricated transparent conductive polycarbonate/carbon nanotube nanocomposite films for switchable thermochromic applications. J. Mater. Chem. C 2018, 6, 8360-8371. (41) Lei, C.; Wu, S.; Xu, R.; Cai, Q.; Hu, B.; Peng, X.; Shi, W. Formation of stable crystalline connecting bridges during the fabrication of polypropylene microporous membrane. Polym. Bull. 2013, 70, 13531366. (42) Lei, C.; Chen, D.; Wu, B.; Xu, Y.; Li, S.; Huang, W. Melt-grafting mechanism study of maleic anhydride onto polypropylene with 1-decene as the second monomer. J. Appl. Polym. Sci. 2011, 121, 3724-3732. (43) Lee, S. Y.; Park, S. Y.; Song, H. S. Effects of melt-extension and annealing on row-nucleated lamellar crystalline structure of HDPE films. J. Appl. Polym. Sci. 2007, 103, 3326-3333. (44) Xie, J.; Xu, R.; Chen, X.; Cai, Q.; Lei, C. Influence of heat-treatment temperature on the structure and properties of polypropylene microporous membrane. Polym. Bull. 2016, 73, 265-277.

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