The effect of plasticizers on transport and electrochemical properties of PEO-based electrolytes for lithium rechargeable batteries Yong-Tae Kim, Eugene S. Smotkin * Department of Chemical Engineering, Illinois Institute of Technology, Perlstein Hall, Suite 127, 10 West 33rd Street, Chicago, IL 60616, USA Received 20 July 2001; received in revised form 19 February 2002; accepted 28 February 2002 Abstract The effect of plasticizers (or solvent additives) on the transport and electrochemical properties of polymer electrolytes has been examined. First, the mobility of cations and anions of LiN(CF 3 SO 2 ) 2 (LiTFSI) was increased by doping the PEO-based electrolytes with ethylene carbonate and poly(ethylene glycol dimethyl) ether. However, the cation mobility in PEO-based electrolytes doped with propylene carbonate was suppressed resulting in a lower cation transference number. Nevertheless, all the plasticized electrolytes have higher bulk conductivity than the non-plasticized, even in the case the cation transference number decreased. This is due to increased ion mobilities and/or higher degree of ion dissociation upon the introduction of plasticizers. A study of the electrochemical stability of the electrolytes, by linear sweep voltammetry, shows that the plasticizers have a negligible effect on the electrolyte stability windows. Finally, the electrolyte/lithium metal electrode interfacial impedance was monitored by A.C. impedance as a function of storage time. The formation and growth rate of the passivation layer was diminished in the presence of the plasticizers. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasticizer; Lithium; Cation transference number; Polyethylene oxide; A.C. impedance spectroscopy 1. Introduction Rechargeable lithium batteries are promising energy sources for consumer, industrial, and military applica- tions. This battery offers many advantages over con- ventional batteries, such as higher energy density, benign environmental impact, and long cycling life. The overall performance of a lithium rechargeable battery depends on the choice of cathode, anode, electrolyte, and the electrode–electrolyte interfacial properties. One approach to realize high energy-density lithium batteries is to employ polymer electrolytes. Fast alkali ion transport was first observed in complexes formed by alkali metal salts and poly(ethylene oxide) (PEO) at around 100 jC by Fenton et al. [1]. This report of ionic conductivity of PEO/LiClO 4 spawned intense efforts towards the development of all solid-state sec- ondary lithium batteries [2–9]. Polymer materials as battery electrolytes are also of interest because of advantageous mechanical properties, ease of fabrica- tion as thin films and their ability to form a good contact with electrode materials. 0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-2738(02)00130-3 * Corresponding author. Tel.: +1-312-567-3453; fax: +1-312- 567-5921. E-mail address: smotkin@iit.edu (E.S. Smotkin). www.elsevier.com/locate/ssi Solid State Ionics 149 (2002) 29 – 37