CONDUCTIVITY ENHANCEMENT MECHANISM OF INTERSTITIAL-TYPE LI+ CONDUCTOR, LI4+XBXSI1-XO4 (0 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 0.7)

An electrical conductivity of sigma-100-degrees-C = 4.5 x 10(-6) S cm-1 in Li4.5B0.5Si0.5O4 was observed, which is more than three orders of magnitude higher than that of Li4SiO4. This enhancement is due to an increase of both the ionic hopping rate and the mobile-ion concentration. An expansion of the crystallographic unit-cell volume of Li4+xBxSi1-xO4 (0 less-than-or-equal-to x less-than-or-equal-to 0.5) was observed with increasing x. A simple point-charge model calculation shows that the nearest-neighbor Li+-Li+ repulsive force mainly contributes to the expansion of each axis in the unit cell. The conductivity and only the b-axis length among the crystal lattice parameter decreases with x beyond x = 0.5. This result may be interpreted by the application of an BO45- + Li+ reversible BO33- + LiO- equilibrium model.