Synthesis and Structure of the Sodium Phosphidosilicate Na2SiP2

Ion conductors of light alkaline metals based on earth-abundant elements are important components for all-solid-state batteries. The new sodium-rich phosphidosilicate Na2SiP2 was synthesized by solid state reaction of stoichiometric amounts of the elements at 973 K and characterized by single-crystal X-ray diffraction (space group Pccn (no. 56), a = 12.7929(5) angstrom, b = 22.3109(9) angstrom, c = 6.0522(2) angstrom and Z = 16) and solid-state NMR under MAS conditions. The compound forms dark-red twinned crystals, and its crystal structure contains edge-sharing SiP4 tetrahedra connected to infinite (1)(infinity)[SiP4/2] chains. The sodium ions between the chains are fairly mobile. Electrochemical impedance spectroscopy shows a total ionic conductivity of sigma(Na+, 373 K) = 2.3 ? 10(-6) Scm(-1) with an activation energy of E-a = 0.43 eV, and the galvanostatic polarization reveals mixed conduction behavior with a transference number of 0.8.