Synthesis, structure and diffusion pathways of fast lithium-ion conductors in the polymorphs α- and β-Li8SnP4

Abstract

The increasing demand for a high-performance and low-cost battery technology promotes the search for Li⁺-conducting materials. Recently, phosphidotetrelates and -aluminates were introduced as an innovative class of phosphide-based Li⁺-conducting materials featuring ionic conductivities of up to 3 mS cm⁻¹ at ambient temperature. In order to get a deeper understanding in structure–property relationship of lithium ion conductors closely related structures that differ in their ionic conductivity are of special interest. Here, we report on the two polymorphs α- and β-Li₈SnP₄, which show ionic conductivities of up to 0.7 mS cm⁻¹ and low activation energies E_A of about 28 kJ mol⁻¹ (0.29 eV) at 298 K. The structures of the two phases are determined by single crystal X-ray and powder neutron diffraction experiments at different temperatures, and their significantly different ionic conductivities allow for a detailed insight into the structure–property relationship. The investigations are completed by ⁶Li, ³¹P and ¹¹⁹Sn solid state magic angle spinning NMR, temperature-dependent ⁷Li NMR experiments and electrochemical impedance spectroscopy. Negative nuclear density maps reconstructed from experimental structure factors were analyzed by the maximum entropy method (MEM) and the one-particle-potential (OPP) formalism. Distinct Li⁺ migration pathways including divergent activation barriers have been identified, which allow to interpret the different conductivities of the two modifications. The importance of partially occupied octahedral sites in the β-phase is ascertained to cause considerably lower energy barriers to adjacent tetrahedral voids, which promote the higher conductivity in comparison to the α-phase. The title compounds complete the series of three phosphidotetrelates (α-)Li₈SiP₄, α-Li₈GeP₄ and β-Li₈GeP₄, and allow a detailed investigation of the structure–property relationships for further tailoring of the material properties.