Ionic conductivity of covalently interconnected polyphosphazene–silicate hybrid networks Harry R. Allcock a, * , Youngkyu Chang b , Daniel T. Welna a a Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, PA 16802, USA b Samsung Cheil Ind., R&D Center, 332-2 Gochun-Dong Euiwang-Shi, Kyoungki-Do 437-010, South Korea Received 12 January 2005; received in revised form 3 November 2005; accepted 25 November 2005 Abstract Silicate sol – gel precursors of poly[bis(methoxyethoxyethoxy)phosphazene] and their corresponding hybrid networks were synthesized by hydrolysis and condensation reactions. Conversion of the precursor polymers to covalently interconnected hybrid networks with controlled morphologies and physical properties was achieved. Thermal analyses showed no melting transitions for the networks and low glass transition temperatures that ranged from approximately À 38 to À 67 -C. Solid solutions with lithium bis(trifluoromethanesulfonyl)amide in the network showed a maximum ionic conductivity value of 7.69 Â 10 À 5 S/cm, making these materials interesting candidates for dimensionally stable solid polymer electrolytes. D 2005 Elsevier B.V. All rights reserved. Keywords: Polyphosphazene; Ionic conductivity; Sol – gel; Solid polymer electrolyte 1. Introduction Solid polymer electrolytes (SPEs) have been studied extensively since the discovery of ionic conduction in complexes of poly(ethylene oxide) (PEO) with alkali metal salts and the subsequent suggestion that such ionic conductors could be used as electrolytes in electrochemical energy storage devices [1–3]. Since then, PEO has become one of the most widely studied polymers for applications in secondary bat- teries. However, due to the crystalline nature of PEO at room temperature, the high ionic conductivities required for com- mercial applications have not yet been achieved [4]. Therefore, alternative polymer systems have been the subject of intense investigation during the last decade in an effort to find a polymer system with a room temperature ionic conductivity suitable for commercial applications [5,6]. Previous research has shown that poly[bis(methoxyethox- yethoxy)phosphazene] (MEEP) has many advantages over PEO [7,8]. MEEP has oligoethyleneoxy side chains, which are similar to the backbone of PEO (Scheme 1). These side chains attached to a flexible polyphosphazene backbone result in a completely amorphous polymer with a low glass transition temperature (T g ) of À 83 -C [9]. These properties allow MEEP to achieve room temperature ionic conductivities of approxi- mately 10 À 4 S/cm, which is much higher than that of PEO (10 À 7 S/cm) [7]. MEEP is also an excellent solvent for species such as lithium trifluoromethanesulfonylimide (LiTFSI), which is a common salt used in SPEs. Each MEEP repeating unit has six oxygen atoms which can coordinate to metal cations and facilitate ion-pair separation. However, MEEP is a visco-elastic gum at room temperature and can flow like a viscous liquid when subjected to an external force [9]. Therefore, various attempts have been made to improve the dimensional stability by radiation or other crosslinking methods [10,11]. The crosslinking of MEEP by g-ray and ultraviolet irradiation has been investigated earlier in our program. However, this approach does not provide a sufficiently high degree of control over the crosslinking density. Recently, sol– gel techniques were developed in order to confer dimensional stability to MEEP. The first approach combined a silicate network with a polyphosphazene. Kim et al. prepared a polyphosphazene/silicon oxide partial IPN-type composite as a solid electrolyte from a mixture of MEEP and tetraethy- 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.11.017 * Corresponding author. Tel.: +1 814 865 3527; fax: +1 814 865 3314. E-mail address: hra@chem.psu.edu (H.R. Allcock). Solid State Ionics 177 (2006) 569 – 572 www.elsevier.com/locate/ssi