New Lithium Ion Conductors

Abstract

Conductivity data for several new lithium conductors are presented. The materials discussed fall into three groups. The first class are solid solutions of lithium orthosilicate, Li₄SiO₄, and lithium phosphate, Li₃PO₄. The unit cell of Li₄SiO₄ contains two SiO₄ ⁻⁴ tetrahedra linked by 8 lithium ions, which are distributed over 18 possible sites. Although the conductivity of pure Li₄SiO₄ is rather low, an improvement of 3 to 4 orders of magnitude has been obtained at the maximum dopant level studied, which corresponds to 30 mole percent Li₃PO₄.

The second materials group are based upon the anti-fluorite structure. One family of compounds studied includes Li₅A1O₄, Li₅GaO₄, and Li₆ZnO₄, which have a partially occupied cation sublattice. Like many known anionic conductors having the fluorite structure, these materials exhibit a transition to a more highly conducting state--at about 380°C. D.T.A. experiments indicate a large endothermic effect associated with the onset of this transition.

The third, Li₃N, has a unique structure with large intersecting tunnels in 2 dimensions. High ionic conductivity and negligible electronic conductivity are found at relatively low temperatures. Data on both pure and impure poly-crystalline samples indicate that the conductivity parameters can be improved by suitable doping.

Ionic conductivities of some of these materials at several temperatures are tabulated below.

This work was supported by ARPA through ONR.