Calcium Phosphate Incorporated Poly(ethylene oxide)- Based Nanocomposite Electrolytes for Lithium Batteries. I. Ionic Conductivity and Positron Annihilation Lifetime Spectroscopy Studies A. Manuel Stephan, 1 T. Prem Kumar, 1 Sabu Thomas, 2 P. Selvin Thomas, 2 Roberta Bongiovanni, 3 Jijeesh R. Nair, 3 N. Angulakshmi 1 1 Electrochemical Power Systems Division, Central Electrochemical Research Institute, Karaikudi 630 006, India 2 School of Chemical Sciences, Mahatma Gandhi University, Kottayam 68 65 60, India 3 Department of Materials Science and Chemical Engineering, Politecnico di Torino c. Duca degil Abruzzi 24 10129, Torino, Italy Received 22 November 2010; accepted 17 June 2011 DOI 10.1002/app.35219 Published online 18 November 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Nanocomposite polymer electrolytes (NCPEs) composed of poly(ethylene oxide), calcium phos- phate [Ca 3 (PO 4 ) 2 ], and lithium perchlorate (LiClO 4 )/lith- ium bis(trifluoromethane sulfonyl)imide [LiN(CF 3 SO 2 ) 2 or LiTFSI] in various proportions were prepared by a hot- press method. The membranes were characterized by scan- ning electron microscopy, differential scanning calorime- try, thermogravimetry–differential thermal analysis, ionic conductivity testing, and transference number studies. The free volume of the membranes was probed by positron annihilation lifetime spectroscopy at 30  C, and the results supported the ionic conductivity data. The NCPEs with LiClO 4 exhibited higher ionic conductivities than the NCPE with LiTFSI as a salt. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 124: 3245–3254, 2012 Key words: composites; FT-IR; thermogravimetric analysis (TGA) INTRODUCTION The development of polymer electrolytes has long been the subject of research interest because of their potential applications, not only in the area of rechargeable lithium batteries but also in other elec- trochemical devices. 1–3 Commercially available lith- ium-ion cells possess a lithium-intercalating poly- crystalline oxide as the cathode and a carbon anode with a nonaqueous liquid electrolyte. The develop- ment of polymer electrolytes for lithium batteries has led to an unprecedented interest in the last 2 decades because of their potential advantages, such as nonleakage of electrolyte, high energy density, flexible geometry, and better safety. 4,5 The ionic conductivity of dry polymer electrolytes has been found to be on the order of 10 4 S/cm at temperatures above 90  C, which excludes them from practical applications. 5 In the last 3 decades, numer- ous attempts have been made to improve the ionic conductivity of polymeric membranes at ambient and subambient temperatures through the addition of low-molecular-weight plasticizers, such as ethyl- ene carbonate and propylene carbonate. However, the incorporation of plasticizers adversely affects the mechanical robustness of membranes and the inter- facial properties with lithium metal anodes and eventually deteriorates the system. 6,7 Recent studies have revealed that only ceramic/ inert filler (e.g., ZrO 2 , SiO 2 , Al 2 O 3 ) incorporated composite polymer electrolytes can offer safe and reliable batteries. 8–10 The concept of incorporating inert fillers into polymer–LiX salt complexes is not new. This procedure has already been adopted suc- cessfully to enhance the mechanical stability (brought about by a network of filler particles in the polymer matrix), improve the compatibility of solid polymer electrolytes with lithium metal anodes, and achieve a high ionic conductivity. 10 A variety of polymer hosts have been studied. Among the polymer hosts explored so far, poly(eth- ylene oxide) (PEO) has been the most extensively studied system because of its ability to form com- plexes with a wide variety of lithium salts for bat- tery applications. PEO chains adopt a helical confor- mation with all the CAO bonds in trans configuration and the CAC bonds in either the gauche or gauche minus configuration. 11 In this Correspondence to: A. M. Stephan (arulmanuel@gmail. com). Contract grant sponsor: Department of Science and Technology, New Delhi, through an SERC scheme. Journal of Applied Polymer Science, Vol. 124, 3245–3254 (2012) V C 2011 Wiley Periodicals, Inc.