Acoustic Shock Wave-Induced Amorphous to Crystalline Phase Transitions of Li2SO4─Raman Spectroscopic and Thermal Calorimetric Approach

In this context, we have reexamined the acoustical shock wave-induced amorphous-glassy-crystalline-amorphous phase transitions in the Li2SO4 sample under 0, 1, 2, and 3 shocked conditions by implementing the detailed Raman spectroscopic approach. The recorded Raman spectroscopic data clearly reveal that the transition from the amorphous-glassy-crystalline state occurs because of a significant reduction of the translational disorder of lithium cations, particularly [Li (2)] ions wherein a slight reduction of the librational disorder of SO4 anions takes place, whereas the crystalline to amorphous transition occurs only at the third shocked condition because of the librational disorder of SO4 anions. The double degenerate υ2 and υ4 Raman modes provide a clear indication of the occurrence of the librational disorder of SO4 anions at the third shocked condition. Followed by the internal Raman modes, a detailed discussion is provided on the external Raman modes of the Li ions and SO4 ions with respect to the observed phase transitions, wherein it is found that the regions of lattice modes are significantly altered at each and every point of phase transition. Furthermore, the thermal and magnetic measurements have been performed for the above-mentioned state of Li2SO4 samples, whereby the obtained results of the magnetic loops and the thermal property resemble the observed structural transitions with respect to the number of shock pulses such that the inter-relationship of the structure-electrical-magnetic-thermal properties of Li2SO4 could be explored.

Superionic Conduction in the Plastic Crystal Polymorph of Na4P2S6

Sodium thiophosphates are promising materials for large-scale energy storage applications benefiting from high ionic conductivities and the geopolitical abundance of the elements. A representative of this class is Na₄P₂S₆, which currently shows two known polymorphs-α and β. This work describes a third polymorph of Na₄P₂S₆, γ, that forms above 580 °C, exhibits fast-ion conduction with low activation energy, and is mechanically soft. Based on high-temperature diffraction, pair distribution function analysis, thermal analysis, impedance spectroscopy, and ab initio molecular dynamics calculations, the γ-Na₄P₂S₆ phase is identified to be a plastic crystal characterized by dynamic orientational disorder of the P₂S₆ ^4- anions translationally fixed on a body-centered cubic lattice. The prospect of stabilizing plastic crystals at operating temperatures of solid-state batteries, with benefits from their high ionic conductivities and mechanical properties, could have a strong impact in the field of solid-state battery research.

Structural Disorder in Li6PS5I Speeds 7Li Nuclear Spin Recovery and Slows Down 31P Relaxation-Implications for Translational and Rotational Jumps as Seen by Nuclear Magnetic Resonance

Lithium-thiophosphates have attracted great attention as they offer a rich playground to develop tailor-made solid electrolytes for clean energy storage systems. Here, we used poorly conducting Li6PS5I, which can be converted into a fast ion conductor by high-energy ball-milling to understand the fundamental guidelines that enable the Li+ ions to quickly diffuse through a polarizable but distorted matrix. In stark contrast to well-crystalline Li6PS5I (10-6 S cm-1), the ionic conductivity of its defect-rich nanostructured analog touches almost the mS cm-1 regime. Most likely, this immense enhancement originates from site disorder and polyhedral distortions introduced during mechanical treatment. We used the spin probes 7Li and 31P to monitor nuclear spin relaxation that is directly induced by Li+ translational and/or PS4 3- rotational motions. Compared to the ordered form, 7Li spin-lattice relaxation (SLR) in nano-Li6PS5I reveals an additional ultrafast process that is governed by activation energy as low as 160 meV. Presumably, this new relaxation peak, appearing at T max = 281 K, reflects extremely rapid Li hopping processes with a jump rate in the order of 109 s-1 at T max. Thus, the thiophosphate transforms from a poor electrolyte with island-like local diffusivity to a fast ion conductor with 3D cross-linked diffusion routes enabling long-range transport. On the other hand, the original 31P nuclear magnetic resonance (NMR) SLR rate peak, pointing to an effective 31P-31P spin relaxation source in ordered Li6PS5I, is either absent for the distorted form or shifts toward much higher temperatures. Assuming the 31P NMR peak as being a result of PS4 3- rotational jump processes, NMR unveils that disorder significantly slows down anion dynamics. The latter finding might also have broader implications and sheds light on the vital question how rotational dynamics are to be manipulated to effectively enhance Li+ cation transport.

Low-temperature paddlewheel effect in glassy solid electrolytes

Glasses are promising electrolytes for use in solid-state batteries. Nevertheless, due to their amorphous structure, the mechanisms that underlie their ionic conductivity remain poorly understood. Here, ab initio molecular dynamics is used to characterize migration processes in the prototype glass, 75Li₂S-25P₂S₅. Lithium migration occurs via a mechanism that combines concerted motion of lithium ions with large, quasi-permanent reorientations of PS₄^3- anions. This latter effect, known as the 'paddlewheel' mechanism, is typically observed in high-temperature crystalline polymorphs. In contrast to the behavior of crystalline materials, in the glass paddlewheel dynamics contribute to Lithium-ion mobility at room temperature. Paddlewheel contributions are confirmed by characterizing spatial, temporal, vibrational, and energetic correlations with Lithium motion. Furthermore, the dynamics in the glass differ from those in the stable crystalline analogue, γ-Li₃PS₄, where anion reorientations are negligible and ion mobility is reduced. These data imply that glasses containing complex anions, and in which covalent network formation is minimized, may exhibit paddlewheel dynamics at low temperature. Consequently, these systems may be fertile ground in the search for new solid electrolytes.