Thermal shrinkage and microscopic shutdown mechanism of polypropylene separator for lithium-ion battery: In-situ ultra-small angle X-ray scattering study
Graphical abstract
Introduction
Separators in lithium-ion batteries are porous polymeric membranes that electronically isolate positive and negative electrodes while allow ion transporting between them [1], [2], [3]. Most of the commercial separators are based on semi-crystalline polyolefin materials including high density polyethylene (HDPE) [4], [5], [6], [7], [8] and polypropylene (PP) [9], [10], [11], [12], [13], [14] due to their comprehensive advantages of performance, safety and cost.
Manufacturing of porous polyolefin membranes can be divided into ‘Dry’ process and ‘Wet’ process [3], [15], [16]. ‘Dry’ process of thermoplastic polyolefin utilizes extrusion to cast the polymer into desired shape above its melting temperature. Subsequent annealing and stretching in one or two direction are performed to create pores. Uniaxial stretching causes slit-like pores containing highly oriented fibrils by separating stacked lamellae in machine direction. The orientation can be dictated by annealing and extrusion at high speed. The highly oriented molecular chains cause mechanical properties along the extrusion direction (longitudinal) being superior to the lateral (transverse) direction. Biaxial stretching is another process to create pores with wider size distribution and less molecular orientation comparing to uniaxial stretching [17], [18], [19], [20]. ‘Wet’ processing of the polyolefin separators is produced with the aid of a hydrocarbon liquid mixed with the polyolefin resin in melt phase. The melt mixture is extruded and cast to a sheet, then the oil is extracted leaving behind a solid polymer membrane with random network of pores.
The separator is required to be capable of the shutdown at high temperature for a Lithium-ion battery. This is mainly caused by the pore shutdown above the melting temperature [21], [22]. The shutdown should not result in loss of mechanical integrity, otherwise the electrodes could directly contact and the resulting chemical reactions cause thermal runaway. The separator shutdown is a useful mechanism for limiting temperature and preventing venting in short-circuited cells [3]. Thus, the pore shutdown during heating is important to the thermal safety of the battery.
In previous work, HDPE membrane melting at 135 °C and PP membrane at 165 °C have been made with no mechanical integrity above the melting temperature [16]. The positive dependent of the thermal shutdown on the melting temperature of polymers have been observed. However, in a recent study [23], the onset of the separator shutdown occurring below the melting temperature of crystals have been observed by thermo-mechanical tests.
The molecular orientation contributes to the shrinking behavior in overstretched polymers which have the similar loading history to the porous membranes [24]. Although the highly stretched amorphous phase has been proven to be the response for the shrinkage in stretching direction, there is still considerably less understanding on the relationship between the shrinkage force and the variation of the microscopic structure of the pores. Fortunately, recent ultra-small angle X-ray scattering (USAXS) technique allows dynamic measurements of structural changes from several to hundreds nanometers [25], [26], [27], [28]. This technique is powerful to study porous structures. Time resolved USAXS experiments have been largely used to study the structural variation of pores during the deformation of semi-crystallization polymers [29], [30], [31], [32], [33].
In this work, porous membrane prepared by uniaxial stretching has been investigated by in-situ USAXS tests. As will be reported in following sections, the microstructural variations of the membrane during the thermal shrinkage are probed. Meanwhile, the mechanism of pore shutdown upon heating is elucidated based on the microstructural variation of the porous membrane. All these observations are helpful to understand the shutdown process of separators. (Table 1)
Section snippets
Materials and sample preparation
Commercial polyolefin separator, produced by uniaxial stretching of pre-oriented polypropylene (PP) film, was kindly offered by Shenzhen Senior Technologies Co., Ltd, China. The thickness of the separator was ca. 20 µm. The separator was used as received.
Differential scanning calorimetry (DSC)
A DSC1 Stare System (Mettler Toledo Instruments, Swiss) was used for thermal analysis. The temperature and heat flow scales were calibrated using high purity indium. The heating rate was set at 1 °C/min. The onset temperature of melting,
Microstructure characterization
Fig. 1a shows SEM micrographs of the membrane. Clearly, the image shows oriented porous structure with the heights parallel to the stretching direction. Besides, highly oriented fibrils can be observed in the micrograph indicating a craze-like microstructure. Apparently, the heights of the pores exhibit wide distribution which is from 10 nm to 365 nm according to the statistic results of the heights shown in Fig. S1. However it should be mentioned that SEM micrograph can only give the pore size
Discussion
During the manufacturing processes of the membrane, it was uniaxially stretched inducing massive oriented chains or taut tie chains in the system. Although the samples were annealed below melting temperature in order to remove the residual internal-stress, but the orientation of the chains could not be relaxed without crystal melting. Anisotropic WAXD pattern (Fig. 7) shows clear orientation of the crystalline structure. Thermodynamically, the oriented chains prefer to go back to the
Conclusion
In this work we have investigated the structural variation and shutdown mechanism of polypropylene separator upon heating through in-situ ultra-small angle X-ray scattering (USAXS) tests. SEM micrographs show that the polypropylene separator prepared by uniaxial stretching exhibits high molecular orientation and craze-like pores. Structural parameters including porosity, specific interface and pore size have been successfully estimated by fitting the scattering intensity with craze model. The
Acknowledgements
This work was support by National Natural Science Foundation of China (NSFC, Grant Nos. 21604088, 51503134, 51421061), China Postdoctoral Science Foundation (Grant No. 2015LH0050), International PostDoc Initiative project of State Key Laboratory of Polymer Physics and Chemistry (Grant No. IPI2015) (IPI SKLPPC) and State Key Laboratory of Polymer Materials Engineering (Grant No. SKLPME 2017-3-02).
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