Visualization of Crystallization and Transformation Pathway of Li₃PS₄ in Multidimensional Diagrams

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The synthesis protocols critically influence the polymorph selection and performance of solid-state electrolytes, but understanding of the thermodynamics and kinetics remains incomplete even in traditional systems. Recent findings revealed that α-Li₃PS₄, a high-conductivity phase stable at high temperatures but metastable at intermediate ones, can be stabilized via rapid heating of amorphous Li₃PS₄ followed by quenching [Kimura et al., JACS, 14466, 2023]. This contrasts with β-Li₃PS₄, the thermodynamically stable middle-temperature phase formed via slow heating, and the unquenchable α-Li₃PS₄ formed at high temperatures. This study investigates the crystallization and phase transformation of amorphous Li₃PS₄ using in situ synchrotron X-ray diffraction (XRD) with sub-millisecond resolution and computational calculations. In situ synchrotron XRD with sub-millisecond resolution was employed to investigate the crystallization behavior of Li₃PS₄, revealing the competing thermodynamic stability of α- and β-phases across various temperatures and constructing an experimental Time-Temperature-Transformation (TTT) diagram. The α-phase was found to form rapidly at intermediate temperatures (~350 °C), transforming into the β-phase over time, indicating its metastable nature. On the other hand, it became stable with higher crystallinity at elevated temperatures (~500 °C). The TTT diagram identified 350 °C with several seconds of heating as an optimal condition for synthesizing α-phase Li₃PS₄. Guided by the TTT diagram, we demonstrated the synthesis of nano-sized α-Li₃PS₄ by heating amorphous Li₃PS₄ at 350 °C for 10 seconds, achieving an ionic conductivity of 5.8 × 10⁻⁴ S cm⁻¹, significantly higher than that of the β-phase. A computational phase diagram was used to describe how the nucleation and transformation processes of α- and β-phases depend on particle size and temperature, as represented by temperature-size-energy phase diagrams. The α-phase was found to be stabilized in nanoparticle form due to its low surface energy, favoring its nucleation during rapid heating at intermediate temperatures and remaining stable as a nanoparticle during cooling. In contrast, bulk α-phase was stabilized at high temperatures, preventing transformation into the γ-phase, which is stable as a nanoparticle at low temperatures in bulk. In summary, we demonstrated the utility of multi-dimensional phase diagrams, such as TTT and temperature-size-energy phase diagrams, in understanding phase (meta)stability and designing efficient synthesis protocols for solid electrolytes.

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