Solid Polymer Electrolytes from Polyanionic Lithium Salts Based on the LiTFSI Anion Structure

Effects of anion size on ionic conductivity were studied for a series of solid polymer electrolytes prepared from lithium polyanionic salts based on a series of lithium bis[(perfluoromethyl)sulfonyl]imide (LiTFSI) units connected together by perfluoroalkane linkers to form oligomeric anionic chains of variable length. Solid polymer electrolytes were prepared from the salts using polyethylene oxide as the host and characterized using X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy. Ionic conductivities were measured over a temperature range between 120°C and ambient for electrolytes with ethylene oxide (EO)/Li ratios of 30:1 and 10:1. Solid polymer electrolytes prepared from the lithium polyanionic salts exhibited ionic conductivities that were consistently lower (by factors of between 2 and 10) relative to those of monomeric LiTFSI-based electrolytes over the entire temperature and salt concentration ranges. This finding probably reflects a diminished contribution of anions to the overall conductivity for salts with large, polymeric anions. Trends in ionic conductivity with respect to anion chain length and EO/Li ratio were studied. The existence of an optimal anion chain length that is different for solid polymer electrolytes of differing EO/Li ratio was noted and is rationalized in terms of the cumulative effects of anion mobility, ion-pairing, variations in host chain dynamics in the vicinity of ions as a function of anion structure, and salt-phase segregation on the conductivity. © 2004 The Electrochemical Society. All rights reserved.