PADDLE WHEEL MECHANISM IN LITHIUM SULFATES - ARGUMENTS IN DEFENSE AND EVIDENCE AGAINST

The "paddle wheel" mechanism proposed by Lundén et al. to account for fast cation conductivity in "rotator" phase Li2SO4-based compositions involves SO4 rotation/reorientation where SO4 propels passage of the cation. Existing literature results are collated and interpreted to show that these results cannot be reconciled to the paddle wheel mechanism. A number of factors are cited herein that can contribute to facilitating intersite mobility of cations in the enhancement of ionic conductivity. The experimental results suport ionic transport in Li2SO4-based compositions, and other sulfatee-based compositions (viz., Na2SO4, Ag2SO4) by a "percolation-type" mechanism where "free volume" in the lattice framework plays a predominant role. The rotating sulfate anions in Li2SO4-based compositions may contribute to the conductivity, not by direct coupling with the Li+ cation but indirectly by increasing the probability of a successful translational jump when favorable instantaneous orientations of the SO4 oxygens occur at the transport "bottleneck", i.e., acting as a gate. That is, the gate action of the percolation-type mechanism assigns a passive role for SO4 rotation/reorientation in simply allowing passage of the cation with no transfer of momentum. The facility of Li+ passage through a larbe bottleneck size of the highly supple lattice of Li2SO4 is also considered a contributing factor. © 1992 Academic Press, Inc.