Structure transitions of lithium ionic conductor Li<sub>3</sub>PS<sub>4</sub>

Abstract

<p indent="0mm">Traditional lithium-ion batteries have a low energy density and pose safety risks due to the use of graphite-based anode materials and combustible organic liquid electrolytes. Advanced next-generation battery devices are urgently needed to store more energy and be used more safely in emerging and existing technologies such as electric vehicles and distributed energy storage. All-solid-state batteries made up of high-voltage cathodes and lithium metal anodes are promising candidates for achieving high safety and competitive energy power density. The nonflammable solid electrolytes (SEs) are key components that separate cathode and anode, migrate lithium ions, and support the cell framework. Among different types of SEs, thiophosphates Li₃PS₄, as a thio-LISICON (lithium superionic conductor), have attracted considerable interest of researchers as inorganic SEs due to their superior room-temperature ionic conductivities, desirable mechanical properties, and compliant chemical compatibility. According to the arrangement mode of PS₄³⁻ tetrahedron, Li₃PS₄ possesses different polymorph structures (glass-, α-, β-, and γ-Li₃PS₄), which play a critical role in ionic conductivity because of the strong dependence on ionic conductivity and crystalline phases. Under heat treatment conditions, polymorphs can transform into each other. Thus, investigating the synthesis conditions and phase transitions between different crystal structures is of great significance for the application of SEs. Ball-milling combined with a heat-sintering process is used to create different-structured Li₃PS₄. <italic>In situ</italic> variable temperature Raman and room-temperature X-ray diffraction are used to delineate the transitions between different phases, focusing on investigating the transition conditions among different crystal structures and the relationship between the structures and ionic conductivity. Glass-Li₃PS₄ can be easily obtained by ball-milling Li₂S and P₂S₅ powders and preferentially transformed into the metastable β-phase when the sintering temperature is raised to 240°C, which can retain the β-Li₃PS₄ structure and show high ionic conductivity (0.65 mS cm⁻¹) after cooling to room temperature. Once at a higher sintering temperature (&gt;480°C), the β-phase will then transform into the super-higher ionic conductivity but thermodynamically unstable α-phase. More interesting, the α-phase would directly transform into a thermodynamically stable γ-phase without the appearance of β-phase during the subsequent cooling process, while the γ-phase is not a favorable structure for Li-ions migration due to its low ionic conductivity (0.004 mS cm⁻¹). Furthermore, we discuss some potential strategies for lowering the phase transition temperature to obtain α-Li₃PS₄ superconductors at room temperature (e.g., generation of pinning effect from element doping to restrict the movement and rearrangement of PS₄³⁻ tetrahedron, use of the high surface-area framework to stabilize the high-temperature structure, and the quenching process). Moreover, γ-phase and β-phase Li₃PS₄ show a certain fabric memory effect, which enables them to retain their inherent structure even after resintering at 240°C. The results reveal that the thermodynamically metastable β-Li₃PS₄ can be easily obtained at room temperature by precisely controlling the sintering temperature; there is no need for an assisted route such as forming nanoporous using solvent-assisted route or substituting cationic to obtain “β-like” phase. Finally, the Li−Li symmetric batteries assembled from different structural Li₃PS₄ phases show that the β-Li₃PS₄ provides better interfacial stability, which may meet the demands of SEs in solid-state batteries.