Nano-composite solid polymer electrolytes

Abstract

Plasticizer free complexes of lithium salts (LiX) with a high molecular weight polymer such as polyethylene oxide (PEO) are emerging as potential candidates for replacing the liquid electrolytes currently used in lithium batteries. One of the major drawbacks of PEO-LiX electrolytes, however, is their low ionic conductivity at ambient temperatures. Much of the recent research efforts to improve the ambient temperature conductivity while retaining the mechanical properties and the stability towards metallic lithium anode have been directed towards the addition of ultra-fine nano-sized particles of ceramic fillers such as Al2O3, gamma-LiAlO2, SiO2 and TiO2 into PEO based polymer electrolytes. In these PEO based nano-composite polymer electrolytes, conductivity enhancements of up to two orders of magnitude have been achieved. Thermal, electrical conductivity and dielectric relaxation measurements performed on several nano-composite polymer electrolyte systems have shown that the degree of enhancement depends on the grain size and the nature of the surface group (acidic, weakly acidic, neutral or basic) on the grains. It is suggested that the conductivity enhancement is due to the creation of additional sites and favourable conduction pathways for ionic transport through Lewis acid-base type interactions between the filler surface groups (H/OH) and the ionic species. In this paper, the two nanocomposite polymer electrolyte systems PEO:LiTFSI: Al2O3 and PEO:LiTf Al2O3 are discussed as representative examples.