LI9SIALO8 - A LITHIUM ION ELECTROLYTE FOR VOLTAGES ABOVE 5.4V

The solid solution compound Li9SiAlO8 represents a single-phase Li4SiO 4 derivative formally composed of 50 atom percent (a/o) Li4SiO4 and 50 a/o Li5AlO4. It exhibits an ionic conductivity sigma(Li+) = 2.3 x 10( -1) S cm(-1) at 25 degrees C. The total electronic conductivity in con tact with elemental lithium or highly positively polarized electrodes is found to be around four orders of magnitude smaller. Detailed studi es under different conditions and in different cell types are presente d. In the asymmetric cell type (-) LilLi(9)SiAlO(8)lTa (+), the tantal um electrode served as an arbitrary positive electrode simulating diff erent lithium activities due to the applied external voltage. Upon ext reme polarization (E vs. Li/Li+ less than or equal to 6.5 V) below 100 degrees C, stable tarnishing layers were formed at the Li9SiAlO8-elec trode interfaces, thus resisting further decomposition, which makes Li 9SiAlO8 a suitable electrolyte for investigating cathodes which exhibi t very high positive voltages vs. lithium. Moreover, chemical stabilit y experiments show this electrolyte to be inert against elemental lith ium and Li0.9Mn2O4 up to at least 100 and 500 degrees C, respectively. Li9SiAlO8 crystallizes in a monoclinic unit cell having the parameter s a = 5.108(4) Angstrom, b = 5.388(4) Angstrom, c = 6.205(6) Angstrom, and beta = 90.40(9)degrees.