Crystal structure and ionic conductivity of Li14Zn(GeO4)4 and other new Li+ superionic conductors

This paper reports the synthesis and characterization of a number of new Li+ superionic conductors with the type formula Li16−2xDx(TO4)4, where D is a divalent cation (Mg2+ or Zn2+), T is a tetravalent cation (Si4+ or Ge4+), and 0 < x < 4. One of these materials, Li14Zn(GeO4)4, has a resistivity of 8 ω-cm at 300°C, lower than that of any Li+-ion conductor so far reported. The structure of this compound, which we have named LISICON (for Li superionic conductor), has been determined by single-crystal x-ray analysis. The space group is Pnma, with cell parameters a=10.828 Å, b=6.251 Å, c=5.140 Å, and z=1. The structure has a rigid three-dimensional network of Li11Zn(GeO4)4. The three remaining Li+ ions have occupancies of 55 and 16%, respectively, at the 4c and 4a interstitial positions. Each 4c position is connected to two 4a positions and vice versa. The bottlenecks betweenthese positions have an average diameter that is larger than twice the sum of the Li+ and O2− ionic radii, thus satisfying thegeometrical condition for fast Li+ -ion transport. Moreover, all four sp3 orbitals of the O2− ion are shared by strong tetrahedral covalent bonds with the network cations. Therefore, the anion charge is polarized away from the interstitial Li+ ions, weakening the Li1bO bond and increasing the Li+-ion mobility.