Phase Relationships and Conductivity of the Polymer Electrolytes Poly (ethylene oxide)/Lithium Tetrafluoroborate and Polyethylene oxide)/Lithium Trifluoromethanesulfonate

Solid polymer electrolytes are potentially useful in all solid-state rechargeable batteries because of their elastic properties that allow for reduced interfacial contact resistance between the electrolyte and electrodes as well as for thin-film configurations. In this work a study of the complexes of poly(ethylene oxide) with two different salts, LiBF₄and LiCF₃SO₃, is presented. As a result pseudobinary pseudoequilibrium phase diagrams (PPD) have been constructed that provide an improved basis for understanding the mechanism of ionic transport. Differential scanning calorimetry and X-ray diffraction have been used to collect data necessary for the PPD. A single-phase crystalline complex over the stoichiometrics between 4:1 (monomer/salt) and 3:1 has been established in the case of the BF4 salt. The PEO₈(LiBF₄) sample exhibited the highest conductivity (9 × 10^-4 S/cm) with an activation energy of 0.27 eV. The LiCF₃SO₃complex was unusual in that evidence for a second complex (7:1) was obtained in addition to the more commonly observed 3.5:1 phase. These results were compared to phase diagrams published by others for the same and similar systems.