Effect of SiO 2 coating on polyethylene separator with different stretching ratios for application in lithium ion batteries K. Prasanna a , Chang-Soo Kim b , Chang Woo Lee a, * a Department of Chemical Engineering, College of Engineering, Kyung Hee University,1732 Deogyeong-daero, Gihung, Yongin, Gyeonggi 446-701, South Korea b Department of Materials Science and Engineering, University of WisconsineMilwaukee, Milwaukee, WI 53211, USA highlights  Thickness is maintained around 25 mm after coating.  The stretched, coated separator shows reduced interfacial resistance.  The SiO 2 coated separator possesses higher thermal properties.  The stretched, SiO 2 coated separator shows improved Li-ion cell performance. article info Article history: Received 15 September 2013 Received in revised form 8 February 2014 Accepted 5 April 2014 Keywords: Ceramics Coatings Electrochemical properties Mechanical properties abstract To enhance the properties of polyethylene separators in lithium ion batteries, we tested separators with uni-axial stretching ratios of 180% and 300%. We also tested stretched separators coated with SiO 2 ceramic substance to increase ionic conductivity and thermal stability without sacrificing mechanical properties. To test the thermal and tensile properties, thermomechanical analyzer (TMA) is employed. CR 2032-type coin cells are prepared by sandwiching pristine and coated stretched separators, respectively, between the Li anode and Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 cathode to evaluate the AC impedance and cycling performance. The coated separators are observed with superior ionic conductivity, thermal and tensile properties. The cells prepared with coated separator have slightly higher discharge capacity and a better capacity retention ratio than the cells with pristine separators. These results suggest that the coated separator is a better option for lithium ion batteries. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Although high power-density and high energy-density batteries are desired, there are safety issues with these batteries to be addressed. Internal short-circuits can occur due to direct contact between the anode and cathode, which can cause explosions [1,2]. The thermal and mechanical properties of the separators play a key role in avoiding internal short circuits. The most common separa- tors in lithium ion batteries are polyolefin separators, namely polyethylene (PE) and polypropylene (PP) [3]. In general separators should have high ionic conductivity, good mechanical and dimen- sional stability, high electrolyte wettability, sufficient physical strength to allow easy handling during battery assembly, good thermal properties, and uniform thickness [4]. Polyolefin separators used in lithium ion batteries have several positive and negative characteristics. Recent research has tried to reduce the negative characteristics of polyolefin separators. The most important drawbacks of polyolefin separators are the thermal insta- bility and mechanical strength, which can result in deformation leading to fire or explosion [3,4]. Research has also focused on improving ionic conductivity. To enhance separator properties, re- searchers have developed self-standing inorganic separators [3], stretched separators [5], nanocomposite-coated PE separators [6], non-woven-ceramic composite separators [7], and nanofiber-based separators prepared by electrospining [8]. Among these, coating separators with ceramic composite has drawn considerable attention because it improves thermal and mechanical properties at low cost. The disadvantage of ceramic coating is its increased thickness. Increasing separator thickness reduces the active material weight [4] during battery assembly. We compared the ionic conductivity and tensile strength of PE separators uni-axially stretched to ratios of 180% and 300%. We also * Corresponding author. Tel.: þ82 31 201 3825; fax: þ82 31 204 8114. E-mail address: cwlee@khu.ac.kr (C.W. Lee). Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys http://dx.doi.org/10.1016/j.matchemphys.2014.04.014 0254-0584/Ó 2014 Elsevier B.V. All rights reserved. Materials Chemistry and Physics 146 (2014) 545e550