Electrochemical investigation of the chemical diffusion, partial ionic conductivities, and other kinetic parameters in Li 3Sb and Li 3Bi

Abstract

The galvanostatic intermittent titration technique (GITT) has been used to electrochemically determine the chemical and component diffusion coefficients, the electrical and general lithium mobilities, the partial lithium ionic conductivity, the parabolic tarnishing rate constant, and the thermodynamic enhancement factor in “Li 3Sb” and “Li 3Bi” as a function of stoichiometry in the temperature range from 360 to 600°C. LiCl, KCl eutectic mixtures were used as molten salt electrolytes and Al, “LiAl” two-phase mixtures as solid reference and counterelectrodes. The stoichiometric range of the antimony compound is rather small, 7 × 10 −3 at 360°C, whereas the bismuth compound has a range of 0.22 (380°C), mostly on the lithium deficit side of the ideal composition. The thermodynamic enhancement factor in “Li 3Sb” depends strongly on the stoichiometry, and has a peak value of nearly 70 000; for “Li 3Bi” it rises more smoothly to a maximum of 360. The chemical diffusion coefficient for “Li 3Sb” is 2 × 10 −5 cm 2 sec −1 at negative deviations from the ideal stoichiometry and increases by about an order of magnitude in the presence of excess lithium at 360°C. The corresponding value for “Li 3Bi” is 10 −4 cm 2 sec −1 with high lithium deficit, and increases markedly when approaching ideal stoichiometry. The activation energies are small, 0.1–0.3 eV, depending on the stoichiometry, in both phases. The mobility of lithium in “Li 3Bi” is about 500 times greater than in “Li 3Sb” with a lithium deficit. The ionic conductivity in “Li 3Sb” increases from about 10 −4 Ω −1 cm −1 in the vacancy transport region to about 2 × 10 −3 where transport is probably by interstial motion at 360°C. For “Li 3Bi” a practically constant value of nearly 10 −1 Ω −1 cm −1 is found at 380°C. The parabolic tarnishing rate constant shows a sharp increase at higher lithium activities in “Li 3Sb” whereas in “Li 3Bi” it has a roughly linear dependence upon the logarithm of the lithium activity. The tarnishing process is about 2 orders of magnitude slower for “Li 3Sb” than for “Li 3Bi.” Because of the fast ionic transport in these mixed conducting materials, “Li 3Sb” and “Li 3Bi” may be called “fast electrodes.”