Delivered by Publishing Technology to: McMaster University IP: 206.214.6.136 On: Mon, 04 Jan 2016 11:30:12 Copyright: American Scientific Publishers Copyright © 2016 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 16, 956–961, 2016 www.aspbs.com/jnn Electro-Spun Poly(vinylidene fluoride) Nanofiber Web as Separator for Lithium Ion Batteries: Effect of Pore Structure and Thickness Seung-Gyu Lim 1 , Hye-Dam Jo 1 , Chan Kim 2 , Hee-Tak Kim 3 ∗ , and Duck-Rye Chang 1 ∗ 1 Gwangju Research Center, Korea Institute of Industrial Technology, Gwangju 500-480, Republic of Korea 2 AMOMEDI Co. Ltd., New Materials Research Center, Kimpo 415-887, Republic of Korea 3 Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-701, Republic of Korea Electro-spun nanofiber web is highly attractive as a separator for lithium ion batteries because of its high electrical properties. In moving toward wider battery applications of the nanofiber sep- arators, a deeper understanding on the structure and property relationship is highly meaningful. In this regard, we prepared electro-spun poly(vinylidene fluoride) (PVdF) webs with various thick- nesses (10.5∼100 m) and investigated their structures and electrochemical performances. As the thickness of the web is decreased, a decrease of porosity and an increase of pore size are resulted in. For the 10.5 m-thick separator, a minor short-circuit was detected, stressing the impor- tance of reducing pore-size on prevention of short-circuit. However, above the thickness of 21 m, well-connected, submicron-sized pores are generated, and, with lowering the separator thickness, discharge capacity and rate capability are enhanced owing to the lowered area-specific resistance. Keywords: Lithium Ion Battery, Separator, PVdF, Nanofiber, Electrolyte Wetting, Area Specific Resistance. 1. INTRODUCTION Lithium ion batteries have been expanding their appli- cation even to vehicle electrification and energy storage systems, however, for meeting needs from these energy applications, they should evolve further in terms of energy density, durability, safety, and cost-competitiveness. In this regard, new active material and separator with improved performances are highly demanding. The separator of the lithium ion battery is an important component that deter- mines battery performance and safety. It prevents elec- tric shorting between cathode and anode, and it provides lithium ion transfer when it absorbs electrolyte. Polyolefin porous separators are typically used in lithium ion batteries because of their high tensile strength, appropriate poros- ity, and shutdown function. However, low thermal stabil- ity and low electrolyte wetting are the major drawbacks of polyolefin separator. In battery design for automotive applications, fast electrolyte wetting is required to ensure uniform electrolyte distribution in large sized-electrode ∗ Authors to whom correspondence should be addressed. and separator, and higher level of safety to prevent any disaster from unexpected release of huge battery energy. Among various approaches to overcoming the drawbacks of polyolefin-based separators, the use of nonwovens comprising multi-fibrous layers has drawn considerable attention due to their excellent thermal prop- erties, high porosity, and cost competiveness. 1–8 However, the excessively large pore size and broad pore size dis- tribution of conventional nonwovens, which may provoke self-discharge and internal short-circuit of cells, often hinder their successful application to lithium-ion batter- ies. There have been extensive efforts to resolve these limitations of the nonwovens, including the coating of ceramic powders/binders to nonwovens, 5 impregnation of gel-polymer electrolytes into nonwovens, 6–8 and use of nanofiber nonwovens. 9–16 Among these approaches, electro-spinning method is highly effective in fabricating nanofibers and obtaining high porous nanofiber nonwoven separa- tor. Poly(vinylidiene fluoride) 9 (PVdF), Poly(vinyl- idiene fluoride-hexafluoropropylene) (PVdF-HFP), 10 956 J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 1 1533-4880/2016/16/956/006 doi:10.1166/jnn.2016.11599