Design and synthesis of a crystalline LiPON electrolyte*

In the course of a computation study of the broad class of lithium phosphorus oxy-nitride materials of interest for solid electrolyte applications, Du and Holzwarth,{\footnote{Y. A. Du and N. A. W. Holzwarth, {\em{Phys. Rev. B}} {\bf{81}} 184106 (2010)}} recently predicted a stable crystalline material with the stoichiometry Li$_2$PO$_2$N. The present paper reports the experimental preparation of the material using high temperature solid state synthesis and reports the results of experimental and calculational characterization studies. The so-named $SD$-Li$_2$PO$_2$N crystal structure has the orthorhombic space group $Cmc2_1$ with lattice constants a=9.0692(4) \AA, b=5.3999(2) \AA, and c=4.6856(2) \AA. The structure is similar but not identical to the predicted structure, characterized by parallel arrangements of anionic phosphorus oxy-nitride chains having planar P$-$N$-$P$-$N backbones. Nitrogen 2p$\pi$ states contribute to the strong bonding and to the chemical and thermal stablility of the material in air up to 600$^{\circ}$ C and in vacuum up to 1050$^{\circ}$ C. The measured Arrhenius activation energy for ionic conductivity is 0.6 eV which is comparable to computed vacancy migration energies in the presence of a significant population of Li$^{+}$ ion vacancies.