Lithium-Metal Anode Instability of the Superionic Halide Solid Electrolytes and the Implications for Solid-State Batteries.

Owing to high ionic conductivity and good oxidation stability, halide-based solid electrolytes regain interest for application in solid-state batteries. While stability at the cathode interface seems to be given, the stability against the lithium metal anode has not been explored yet. Herein, the formation of a reaction layer between Li3 InCl6 (Li3 YCl6 ) and lithium is studied by sputter deposition of lithium metal and subsequent in situ X-ray photoelectron spectroscopy as well as by impedance spectroscopy. The interface is thermodynamically unstable and results in a continuously growing interphase resistance. Additionally, the interface between Li3 InCl6 and Li6 PS5 Cl is characterized by impedance spectroscopy to discern whether a combined use as cathode electrolyte and separator electrolyte, respectively, might enable long-term stable and low impedance operation. In fact, oxidation stable halide-based lithium superionic conductors cannot be used against Li, but may be promising candidates as cathode electrolytes.

Local Charge Inhomogeneity and Lithium Distribution in the Superionic Argyrodites Li6PS5X (X = Cl, Br, I).

The lithium argyrodites Li6PS5X (X = Cl, Br, I) exhibit high lithium-ion conductivities, making them promising candidates for use in solid-state batteries. These solid electrolytes can show considerable substitutional X-/S2- anion disorder, typically correlated with higher lithium-ion conductivities. The atomic-scale effects of this anion site disorder within the host lattice-in particular how lattice disorder modulates the lithium substructure-are not well understood. Here, we characterize the lithium substructure in Li6PS5X as a function of temperature and anion site disorder, using Rietveld refinements against temperature-dependent neutron diffraction data. Analysis of these high-resolution diffraction data reveals an additional lithium position previously unreported for Li6PS5X argyrodites, suggesting that the lithium conduction pathway in these materials differs from the most common model proposed in earlier studies. An analysis of the Li+ positions and their radial distributions reveals that greater inhomogeneity of the local anionic charge, due to X-/S2- site disorder, is associated with more spatially diffuse lithium distributions. This observed coupling of site disorder and lithium distribution provides a possible explanation for the enhanced lithium transport in anion-disordered lithium argyrodites and highlights the complex interplay between the anion configuration and lithium substructure in this family of superionic conductors.

High-Throughput Screening of Solid-State Li-Ion Conductors Using Lattice-Dynamics Descriptors.

Low lithium-ion migration barriers have recently been associated with low average vibrational frequencies or phonon band centers, further helping identify descriptors for superionic conduction. To further explore this correlation, here we present the computational screening of ∼14,000 Li-containing compounds in the Materials Project database using a descriptor based on lattice dynamics reported recently to identify new promising Li-ion conductors. An efficient computational approach was optimized to compute the average vibrational frequency or phonon band center of ∼1,200 compounds obtained after pre-screening based on structural stability, band gap, and their composition. Combining a low computed Li phonon band center with large computed electrochemical stability window and structural stability, 18 compounds were predicted to be promising Li-ion conductors, one of which, Li3ErCl6, has been synthesized and exhibits a reasonably high room-temperature conductivity of 0.05-0.3 mS/cm, which shows the promise of Li-ion conductor discovery based on lattice dynamics.

Ionic Conductivity of the NASICON-Related Thiophosphate Na1+x Ti2-x Gax (PS4 )3.

Inspired by the recent interest in fast ionic conducting solids for electrolytes, the ionic conductivity of a novel ionic conductor Na1+x Ti2-x Gax (PS4 )3 has been investigated. Using X-ray diffraction and impedance spectroscopy the sodium ionic conductivity in this compound was demonstrated, in which bond valence sum analysis suggests a tunnel diffusion for Na+ . Substitution with Ga3+ leads to an increasing Na+ content, an expansion of the lattice and an increasing conductivity with increasing x in Na1+x Ti2-x Gax (PS4 )3 . Given the relation to the NASICON family, upon replacement of the phosphate by a thiophosphate group, a rich structural chemistry can be expected in this class of materials. This work demonstrates the potential for making NaTi2 (PS4 )3 an ideal system to study structure-property relationships in ionic conductors.