A new crystalline LiPON electrolyte: Synthesis, properties, and electronic structure

The new crystalline compound, Li2PO2N, was synthesized using high temperature solid state methods starting with a stoichiometric mixture of Li2O, P2O5, and P3N5. Its crystal structure was determined ab initio from powder X-ray diffraction. The compound crystallizes in the orthorhombic space group Cmc21 (# 36) with lattice constants a = 9.0692(4) Å, b = 5.3999(2) Å, and c = 4.6856(2) Å. The crystal structure of SD-Li2PO2N consists of parallel arrangements of anionic chains formed of corner sharing (PO2N2) tetrahedra. The chains are held together by Li+ cations. The structure of the synthesized material is similar to that predicted by Du and Holzwarth on the basis of first principles calculations (Phys. Rev. B81, 184106 (2010)). The compound is chemically and structurally stable in air up to 600 °C and in vacuum up to 1050 °C. The Arrhenius activation energy of SD-Li2PO2N in pressed pellet form was determined from electrochemical impedance spectroscopy measurements to be 0.6 eV, comparable to that of the glassy electrolyte LiPON developed at Oak Ridge National Laboratory. The minimum activation energies for Li ion vacancy and interstitial migrations are computed to be 0.4 eV and 0.8 eV, respectively. First principles calculations estimate the band gap of SD-Li2PO2N to be larger than 6 eV.

► High temperature solid state methods were used to synthesize the new crystalline compound Li2PO2N.
► X-ray analysis shows the synthesized compound to have a structure similar to first-principles predictions.
► The structure is characterized by parallel chains of corner sharing PO2N2 tetrahedra with planar PNPN backbones.
► Li2PO2N is chemically and structurally stable in air up to 600 °C and in vacuum up to 1050 °C.
► The measured Arrhenius activation energy for ionic conductivity of Li2PO2N in pressed pellet form is 0.6 eV.