Nano-encapsulation of LiCoO 2 cathodes by a novel polymer electrolyte and its influence on thermal safeties of Li-ion batteries Sang-Young Lee * , Seok Koo Kim, Soonho Ahn Batteries R&D, LG Chem, 104-1, Moonji-dong, Yusong-gu, Daejeon 305-380, Republic of Korea Received 19 October 2007; received in revised form 10 November 2007; accepted 13 November 2007 Available online 21 November 2007 Abstract A new approach enabling the target control of exothermic reaction between delithiated LiCoO 2 and liquid electrolytes has been pre- sented, which is based on the nano-encapsulation of LiCoO 2 by cPVA (cyanoethyl polyvinylalcohol)-based gel polymer electrolytes. This novel morphology and the possible formation of coordinated complexes between the cyano (–CN) groups of cPVA and the cobalt cat- ions of LiCoO 2 are considered as key factors to significantly suppress the exothermic reaction in the delithiated LiCoO 2 . Such an improved thermal stability of the cPVA-modified LiCoO 2 has led to a noticeable achievement in the hot-oven safety behavior of cells. Meanwhile, it was observed that both the excellent ionic conductivity of cPVA-based gel polymer electrolytes and the well-preserved porous structure of modified cathodes contribute to the satisfactory C-rate capability and the cyclability of cells. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Lithium; Batteries; LiCoO 2 ; Polymer electrolytes; Thermal safeties; Nano-encapsulation 1. Introduction As energy densities and performances of lithium ion bat- teries are continually improving, the accompanying safety issues are considered to be a major concern. More notably, the Dell notebook PC’s battery recall in 2006 has brought great attention to the safety failures. It has been believed that the vigorous exothermic reaction between delithiated cathode active materials and liquid electrolytes is a main cause for triggering the battery failures [1,2]. A lot of attempts to suppress the exothermic reaction have been carried out, which have focused mainly on the synthesis of electrochemically stable structured-cathode active mate- rials [3,4] or the modification of cathode active materials by inorganic coating [5,6] or the addition of functional addi- tives into electrolytes [7,8]. However, it is a pity that most of these approaches have suffered from the loss of other electrochemical performances such as the poor C-rate (charge/discharge) capability and the deteriorated cyclabil- ity. Such technical challenges have motivated us to develop a completely new solution that makes it possible to achieve the safety enhancement, while the electrochemical perfor- mances are little influenced. Gel polymer electrolytes consisting generally of polar polymers and liquid electrolytes have drawn great interest due to their promising safety characteristics as well as excellent ionic conductivity [9,10]. Up to now, most appli- cations of gel polymer electrolytes have been focusing on a function as a kind of separator locating between an anode and a cathode, however, unfortunately which has not given satisfactory results for mitigating the safety concerns, because the gel polymer electrolytes have acted only as an ion-conducting separator, not as an effective protecting layer to control the interfacial reaction between cathode active materials and liquid electrolytes [9–12]. In this study, a new gel polymer electrolyte with well-designed functional groups, cPVA (cyanoethyl polyvinylalcohol) is employed as a coating material for LiCoO 2 for the purpose of con- trolling the interfacial reaction with liquid electrolytes, 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.11.011 * Corresponding author. Tel.: +82 42 866 5851; fax: +82 42 863 2934. E-mail address: syleeq@lgchem.com (S.-Y. Lee). www.elsevier.com/locate/elecom Available online at www.sciencedirect.com Electrochemistry Communications 10 (2008) 113–117