Elsevier

Polymer

Volume 107, 19 December 2016, Pages 54-60
Polymer

PVDF/PAN blend separators via thermally induced phase separation for lithium ion batteries

https://doi.org/10.1016/j.polymer.2016.11.008Get rights and content

Highlights

  • PVDF/PAN blend separators are prepared via thermally induced phase separation method.

  • PVDF/PAN blend separators show enhanced tensile strength and thermal stability with the introduction of PAN.

  • The lithium ion batteries with PVDF/PAN blend separators exhibit high C-rate performance.

  • The lithium ion batteries with PVDF/PAN blend separators present good reversible charge/discharge cycle stability.

Abstract

PVDF/PAN blend porous membranes were prepared via thermally induced phase separation (TIPS) and used as separators for lithium ion batteries. TIPS behavior was investigated in detail to control the morphology, pore size, porosity, and mechanical properties of the blend separators as a function of PAN content. Rod-like pores are the typical structure resulted from solid−solid phase separation, while the pore size and porosity decrease with an increase of PAN in the blend. The introduction of PAN enhances the tensile strength and the thermal stability of the blend separator. The electrolyte uptake and the ionic conductivity reduce correspondingly with the decrease of the pore size and the porosity. However, the graphite/polymer electrolyte/LiFePO4 batteries with the blend separators exhibit higher C-rate performance, and better reversible charge/discharge cycle stability than those with PVDF separators and the commercial Celgard 2400.

Introduction

Lithium ion batteries (LIBs) have been used as the most employed power source and they have grown tremendously to keep pace with consumer electronics in the past decade [1]. At the same time, the safety and high efficiency of LIBs become the key points, which are closely related to the properties and structures of the separators [2], [3], [4]. That is because the separators play important roles to prevent internal short circuiting between the cathode and the anode, and meanwhile allows rapid transport of ionic charge carriers in the liquid electrolyte through the interconnected pores [5]. Therefore, it is mandatory to develop highly safe separators with good electrolyte wettability, superior thermal properties, and high mechanical strength for advanced LIBs.

Porous separators have been widely reported for LIBs, such as polyethylene, polypropylene [6], [7], poly(vinylidene fluoride) (PVDF) [8], [9], [10], [11], polyacrylonitrile (PAN) [12], [13], [14], [15], poly(vinyl alcohol) (PVA) [16], poly(ethylene oxide) (PEO) [17], and poly(methyl methacrylate) (PMMA) [8]. Among these, PVDF has attracted significant attention, and has been considered as the next candidate for high performance LIBs [5]. This is mainly due to its high polarity, high dielectric constant, and excellent anodic stability, which result in good affinity with polar electrolytes and dissociating the lithium salt to lithium ions [3]. Nevertheless, PVDF is soluble in liquid electrolytes, which will lead to internal short-circuits and a loss of mechanical strength. Also, the crystalline phase of PVDF even restricts the migration of Li ions because the stable LiF and C=CF unsaturated bonds can be formed by interactions of the fluorine atoms in PVDF with lithium or lithiated graphite [18]. Hence, LIBs with PVDF-based electrolytes show low charge/discharge capacities and poor C-rate performance.

In contrast, PAN-based separators are known to be attractive due to their ionic conductivity, thermal stability, mechanical strength, electrolyte uptake and compatibility with lithium electrodes [19], [20]. Moreover, PAN can hinder the lithium dendrite formation during the charging/discharging process of LIBs [21]. The PAN-based separators have been reported to show rigid matrix and their corresponding polymer electrolytes provided augmented Li ion conduction. It is thus clear that PAN has beneficial characteristics, which can compensate for the weakness of PVDF-based polymer electrolytes [22], [23], [24]. Subramania et al. [23] and Oh et al. [24] found that blending PAN with PVDF-HFP presents synergistic advantages of both PVDF and PAN. The blend polymer matrix shows enhanced mechanical stability, thermal stability and structural rigidity, and their polymer electrolytes possess good ionic conductivity. However, the preparation of PVDF/PAN blend porous matrixes by far has been still relied on non-solvent induced phase separation (NIPS) [25], [26] or electrospinning method [22], [27].

Recently, a few studies were reported on the porous separators via thermally induced phase separation (TIPS) for LIBs [9], [28]. In principle, TIPS is based on a rule that a polymer is miscible with a diluent at high temperature, but demixes at low temperature. A typical TIPS process begins by dissolving a polymer in a diluent to form a homogeneous solution at an elevated temperature, which is cast or extruded into a desired shape. Then, a cooling bath is employed to induce a phase separation (e.g. liquid−liquid, solid−liquid, liquid−solid, or solid−solid demixing) [29], [30]. The diluent will be extracted to yield a porous membrane. In contrast to NIPS or electrospinning, TIPS is reliable to obtain diverse pore structures with ease, such as cellular, spherulitic, lacy, sponge-, needle- or sheet-like pores, depending on the composition, thermal driving mode, and polymer−diluent interaction [31], [32], [33]. Furthermore, the fabricated porous matrixes possess high mechanical strength, high porosity, and more uniform porous structures, which are beneficial to obtain high electrolyte uptake and mechanical stability. For example, Cheng et al. [28] employed TIPS to fabricate PVDF/polysulfone blend separators that showed the maximum electrolyte uptake of 129.76% and the maximum ionic conductivity of 2.03 × 10−3 S/cm at 20 °C. It is promising to develop new kinds of PVDF/PAN blend porous separators via TIPS for LIBs.

We aim to combine the advantages of TIPS method with PVDF/PAN blend, and report a new kind of blend separators with high performance. PVDF/PAN blend separators were prepared by TIPS, in which dimethyl sulfone was chose as a crystallizable diluent and polyethylene glycol was used as a compatilizer. The blend separators were up-taken with liquid electrolyte and activated to form polymer electrolytes for LIBs. The blend separators have the synergistic advantages of PVDF and PAN for LIBs with high performance.

Section snippets

Materials

Poly(vinylidene fluoride) (PVDF, Mn = 110,000, Solef 6010) was purchased from Solvay Solexis, Belgium, and polyacrylonitrile (PAN, Mη = 50,000) was kindly supplied by Anqing Petroleum Chemicals Co., China. The polymer powders were dried to constant weight before use. Dimethyl sulfone (DMSO2, 99% purity) was a commercial product of Dakang Chemicals Co., China. Polyethylene glycol (PEG400, MW = 380–430), ethanol and hexane in AR grade were all supplied by Sinopharm Chemical Reagent Co. Ltd, and

Morphology of the blend separators

Fig. 1 shows SEM images of the cross-section and surfaces of the obtained separators. All the separators comprise irregularly distributed rod-like pores, and their shape almost cannot be affected by adding PAN. Actually, these rod-like pores are typical results from solid−solid (S−S) phase separation of PVDF/DMSO2/PEG400 and PVDF/PAN/DMSO2/PEG400 systems, which were detailedly investigated by polarized optical microscopy and DSC (See Fig. S1∼S3 in Supporting Information). The phenomena were

Conclusions

A series of PVDF/PAN blend separators were prepared by TIPS method, in which DMSO2 was used as a crystallizable diluent and PEG400 was chose as a compatilizer. These separators show rod-like pores as the typical structure via solid−solid phase separation. The pore size and porosity decrease with the increase of PAN content, which affect the electrolyte uptake of the blend separators. The introduction of PAN enhances the tensile strength and raises the melting points of the blend separators,

Acknowledgements

This research is financially supported by the National Natural Science Foundation of China (Grant no. 51403107), Natural Science Foundation of Ningbo (Grant no. 2015A610014), Open Project of Key Laboratory of Novel Adsorption and Separation Materials and Application Technology of Zhejiang (Grant no. 512301-I21502), and K.C. Wong Magna Fund in Ningbo University.

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