Phase formation through synthetic control: polymorphism in the sodium-ion solid electrolyte Na4P2S6

Abstract

The development of all-solid-state sodium batteries for scalable energy storage solutions requires fast sodium conducting solid electrolytes. To fast-track their discovery, candidate materials need to be identified that are synthesized from abundant resources via cheap and green synthesis routes. Their ion conducting mechanism has to be understood and adapted to meet the stringent requirements for long-term operation in all-solid-state batteries. Here, structure and properties of the sodium hexathiohypodiphosphate Na₄P₂S₆ obtained by two different synthesis methods are compared: a solid-state reaction and a precipitation route from aqueous solution. Combined investigations using powder X-ray diffraction (PXRD), precession electron diffraction (PED), differential scanning calorimetry (DSC), solid-state nuclear magnetic resonance spectroscopy (ssNMR), and Raman spectroscopy reveal that the solid-state synthesized material is characterized by a Na⁺ and vacancy disorder-driven enantiotropic phase transition at 160 °C (α- to β-Na₄P₂S₆), which is accompanied by a symmetry change of the P₂S₆⁴⁻ anion. Precipitated Na₄P₂S₆ already crystallizes in a β-like polymorph at room temperature, likely assisted by inter- and intralayer defects. Bond-valence and nudged elastic band (NEB) calculations were employed to identify a low energy, 2D conduction network in β-Na₄P₂S₆, suggesting facile 2D long-range Na⁺ diffusion. Electrochemical impedance spectroscopy reveals a higher ionic conductivity at room temperature in precipitated β-like Na₄P₂S₆ (2 × 10⁻⁶ S cm⁻¹) compared to the solid-state α polymorph (7 × 10⁻⁷ S cm⁻¹). The activation energy is around 0.4 eV for both materials. The findings highlight that even subtle structural changes can significantly impact the sodium-ion diffusion in solid electrolytes and at the same time reveal an intricate interplay between phase formation and synthetic control.