Clay Nanosheets in Skeletons of Controlled Phase Inversion Separators for Thermally Stable Li-Ion Batteriesby Min Kim, Jung Kyu Kim, Jong Hyeok Park

Adv. Funct. Mater.

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ER © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1wileyonlinelibrary.com method has received attention as an effective way to prepare porous membranes, which are based on poly(vinylidene fl uoride- co -hexafl uoropropylene) (PVdF-HFP) polymer dissolved in a controlled mixture of a good solvent and a poor solvent. [ 11–13 ]

Even though reasonable pores have been obtained in PVdF-HFP membranes using the phase inversion method, fi ner control over the pore size and size distribution of the separators are still challenging issues in order to realize cell performance exceeding that of cells using conventional

PE separators. [ 11–20 ]

In our previous report, a novel facial approach was adopted to provide multiscale pore structure in a PVdF-HFP membrane by means of a controlled phase inversion method, including a more fi nely controlled evaporation rate of the good solvent and the poor solvent. [ 21 ] By combining two different PVdF-HFP polymers having different phase inversion behaviors, we introduced a new paradigm for the preparation of separators of Li-ion batteries that can meet standard application requirements for battery separators, such as charge/discharge performance, cycle performance, and dimensional stability.

In the present work, exfoliated clay 2D nanosheets were simply dispersed into PVdF-HFP skeletons, which originally had multiscale pore structures ( Figure 1 A). First, additional pores generated from exfoliated 2D nanosheets in PVdF-HFP skeletons combined with the original multiscale pores generated by controlled phase inversion can generate additional ionic transport pathways, thereby improving separator performance.

Second, PVdF-HFP-incorporating 2D nanosheets exhibited greatly improved thermal stability that decreased the possibility of battery short circuit, thereby enhancing battery safety. 2. Results and Discussion

The use of two different acetone evaporation rates during the drying process leads to two different phase separation paths that produce two different PVdF-HFP polymers (Figure 1 B). In this study, 18-µm-thick wet-laid nonwoven (Figure S1, Supporting

Information) was used as the mother substrate for the PVdFHFP-based separator, and the PVdF-HFP polymer solution with well-dispersed clay nanoparticle was coated onto the nonwoven substrate by a dip-coating method, followed by drying at 25 °C and 40% relative humidity. Because clay nanoparticles are

Clay Nanosheets in Skeletons of Controlled Phase

Inversion Separators for Thermally Stable Li-ion Batteries

Min Kim , Jung Kyu Kim , and Jong Hyeok Park *

Phase inversion is a powerful alternative process for preparing ultra-thin separators for various secondary batteries. Unfortunately, separators prepared from phase inversion generally suffer from uneven pore size and pore size distribution, which frequently results in poor battery performance. Here, a straightforward route is demonstrated to solve the drawbacks of phase-inversion-based separators for Li-ion batteries by means of directly incorporating 2D clay sheets in the skeleton of poly(vinylidene fl uoride- co -hexafl uoropropylene) (PVdF-HFP) with multiscale pore generation from a simple one-step solution coating method. Additionally generated pores by the inclusion of 2D nanosheets in PVdF-HFP skeletons, combined with the multiscale pores (several micrometers + sub-micrometers) originally generated by means of the controlled phase inversion, can generate additional ionic transport pathways, leading to Li-ion battery performances better than those of commercialized polyethylene separators. Moreover, the addition of extremely low contents of 2D clay sheets in PVdF-HFP separators allows thermally stable polymer separators to be realized.

DOI: 10.1002/adfm.201500758

M. Kim, J. K. Kim

School of Chemical Engineering

Sungkyunkwan University

Suwon 440-746 , Republic of Korea

Prof. J. H. Park

Department of Chemical and Biomolecular Engineering

Yonsei University 50 Yonsei-ro , Seodaemun-gu

Seoul 120-749 , Republic of Korea

E-mail: lutts@skku.edu 1. Introduction

Li-ion battery components, including the cathode, anode, electrolyte, and separator, are under development to fulfi ll standard requirements of electric or hybrid-electric vehicles (EVs or

HEVs). [ 1–7 ] Even though many kinds of emerging cathode and anode materials with high specifi c capacities have been reported recently, the separators in commercialized Li-ion batteries are still being made of polyethylene (PE) or polypropylene because of these materials’ uniform pore size, high tensile strength, and chemical stability. [ 8–10 ] However, these materials also have poor thermal and dimensional stability, which has raised serious concerns over the ability to maintain safety in Li-ion batteries under abnormal heating or hard internal shorting, and cell performance also should be improved for next-generation Li-ion batteries. Recently, the phase inversion

Adv. Funct. Mater. 2015,

DOI: 10.1002/adfm.201500758 www.afm-journal.de www.MaterialsViews.com

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ER 2 wileyonlinelibrary.com © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim strongly hydrophilic, the particles were dispersed in nonsolvent phase (water) (Figure S2, Supporting Information), followed by mixing with PVdF-HFP/acetone solution (see the Experimental

Section for more details).

First, the effectiveness of adding clay nanoparticles upon the generation of pores in PVdF-HFP was investigated. The fabricated PVdF-HFP fi lms had both large (∼several micrometers) and small pores (∼sub-micrometers), but the addition of clay nanoparticles to the fi lm seemed not to greatly infl uence its macroscopic morphology ( Figure 2 ). Well-dispersed and nanosized clay nanoparticles were not visible in scanning electron microscope (SEM) observation. To confi rm the existence of clay nanoparticles in the PVdF-HFP skeleton, small-angle X-ray scattering (SAXS) patterns of the PVdF-HFP fi lms with and without clay nanoparticles were investigated ( Figure 3 A). In general, the crystallization of semicrystalline polymers may be disturbed by the addition of inorganic nanoparticles. [ 19,22–37 ] Our SAXS data also provided evidence for this morphology transition of the fi lm because of the clay nanoparticles. The original SAXS peak of the PVDF-HFP fi lm around q = 1 was weakened and almost disappeared in samples to which clay nanoparticles has been added, indicating that the domain structure of the blends became disordered.