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Gel AM Stephan, Central Electrochemical Research Institute, Karaikudi, India S Thomas, Mahatma Gandhi University, Kottayam, India & 2009 Elsevier B.V. All rights reserved. Introduction The rapid growth of the miniature electronic and com- puter-related industries has led to great demand for smaller and lighter batteries with improved safety, en- ergy, and power characteristics. Lithium polymer bat- teries are expected to meet the above requirements and are thus considered as next-generation rechargeable batteries. The conventional lithium-ion batteries, which contain a large amount of liquid electrolyte, emit an appreciable amount of gas and this is attributed to the decomposition of a protective layer at the carbon surface. This phenomenon eventually leads the battery system to safety hazards. By virtue of their advantages such as high theoretical capacity, improved safety, lower material costs, ease of f abrication into flexible geometries, and the absence of electrolyte leakage, lithium polymer batteries have placed an unprecedented demand for battery researchers. Polymer electrolyte can be defined as a membrane that possesses transport properties comparable with that of common liquid ionic solutions. In principle, a polymer electrolyte battery can be formed by sandwiching the electrolyte between a lithium metal (or a composite carbon) anode and a composite cathode as depicted in Figure 1. Because of its rigid structure, the electrolyte can also serve as a separator. The prerequisites for a polymer electrolyte to be used in any battery system are (1) high ionic conductivity at ambient and subambient temperatures, (2) good mechanical strength, (3) appre- ciable transference number, (4) good thermal and elec- trochemical stabilities, and (5) better compatibility with electrodes. Although the ionic conduction in polymer electrolyte was discovered by D. E. Fenton and coworkers in 1973, its technological importance was recognized only after a decade. Polymer electrolytes used in lithium batteries can be classified into three categories: (1) dry polymer electrolytes, (2) gel polymer electrolytes, and (3) com- posite polymer electrolytes. The physical properties of some of the polymer hosts are displayed in Table 1. Dry Polymer Electrolytes The dry polymer electrolyte generally contains an alkali metal salt complexed with the polymer matrix. The very first example of ‘dry solid’ polymer electrolyte is the poly(ethylene oxide) (PEO)-based system that showed very low ambient temperature conductivities on the order of 10 À8 Scm À1 . This system does not possess any organic liquid and thus the polymer host is used as a solid solvent. However, the cycling performance of this dry solid polymer electrolyte with lithium metal electrodes was not satisfactory as the usage was as low as 200–300 cycles. In a deviation from conventional thinking, where conductivity was believed to be confined to amorphous polymer electrolytes, and all crystalline polymer elec- trolytes were thought to be insulators, P. G. Bruce and coworkers demonstrated conductivity in crystalline polymer electrolytes, although the levels at room tem- perature were very low for ambient temperature appli- cations. For the first time, the ionic conductivity was enhanced significantly by 1.5 orders of magnitude by aliovalent doping. The replacement of o5 mol% of SbF 6 À with SiF 6 2À and incorporation of an equivalent amount of additional Li þ ions within the structure of the 6:1 polymer–salt complex PEO 6 :LiSbF 6 demonstrated a substantial increase in conductivity and thus established a new route for enhancing the conductivity of crystalline polymer electrolytes. Figure 2 displays the substitution Anode typically lithium Solid polymeric electrolyte Current collector Intercalation cathode Figure 1 Schematic diagram of basic construction of polymer- based batteries. The thin membrane composites are flexible and can be arranged into several desired geometries. Reproduced with permission from Song JJ, Wang YY, and Wan CC (1999) Journal of Power Sources 77:183. 140 . Gel AM Stephan, Central Electrochemical Research Institute, Karaikudi, India S Thomas, Mahatma Gandhi. in lithium batteries can be classified into three categories: (1) dry polymer electrolytes, (2) gel polymer electrolytes, and (3) com- posite polymer electrolytes. The physical properties of some