Li7GeS5Br-An Argyrodite Li-Ion Conductor Prepared by Mechanochemical Synthesis

Synthetic Tailoring of Ionic Conductivity in Multicationic Substituted, High-Entropy Lithium Argyrodite Solid Electrolytes

Superionic conductors are key components of solid-state batteries (SSBs). Multicomponent or high-entropy materials, offering a vast compositional space for tailoring properties, have recently attracted attention as novel solid electrolytes (SEs). However, the influence of synthetic parameters on ionic conductivity in compositionally complex SEs has not yet been investigated. Herein, the effect of cooling rate after high-temperature annealing on charge transport in the multicationic substituted lithium argyrodite Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I is reported. It is demonstrated that a room-temperature ionic conductivity of ∼12 mS cm-1 can be achieved upon cooling at a moderate rate, superior to that of fast- and slow-cooled samples. To rationalize the findings, the material is probed using powder diffraction, nuclear magnetic resonance and X-ray photoelectron spectroscopy combined with electrochemical methods. In the case of moderate cooling rate, favorable structural (bulk) and compositional (surface) characteristics for lithium diffusion evolve. Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I is also electrochemically tested in pellet-type SSBs with a layered Ni-rich oxide cathode. Although delivering larger specific capacities than Li6PS5Cl-based cells at high current rates, the lower (electro)chemical stability of the high-entropy Li-ion conductor led to pronounced capacity fading. The research data indicate that subtle changes in bulk structure and surface composition strongly affect the electrical conductivity of high-entropy lithium argyrodites.

Probing the Lithium Substructure and Ionic Conductivity of the Solid Electrolyte Li4PS4I

In search of high-performance solid electrolytes, various materials have been discovered in the past, approaching or even exceeding the ionic conductivity of conventional liquid electrolytes. Among the reported classes of superionic electrolytes for solid-state battery applications, lithium thiophosphates appear to be the most promising owing to their high ionic conductivity and mechanical softness. A recent example is the Li₄PS₄I phase (P4/nmm). Surprisingly, this material shows a comparatively low ionic conductivity at room temperature ranging from 10^-4 to 10^-5 S cm^-1 despite having favorable structural characteristics. Because of discrepancies between experiment and theory regarding the Li-ion conductivity and polymorphism in Li₄PS₄I, we herein examine the crystal structure over a broad temperature range using ex situ and in situ X-ray and neutron powder diffraction techniques. We demonstrate the absence of polymorphic transitions, with a lithium redistribution at low temperatures though, and confirm the relatively poor room-temperature ionic conductivity despite the presence of a three-dimensional (3D) percolation network for facile charge transport.

Ionic and Electronic Conductivities of Lithium Argyrodite Li6PS5Cl Electrolytes Prepared via Wet Milling and Post-Annealing

Lithium argyrodite Li₆PS₅Cl powders are synthesized from Li₂S, P₂S₅, and LiCl via wet milling and post-annealing at 500°C for 4 h. Organic solvents such as hexane, heptane, toluene, and xylene are used during the wet milling process. The phase evolution, powder morphology, and electrochemical properties of the wet-milled Li₆PS₅Cl powders and electrolytes are studied. Compared to dry milling, the processing time is significantly reduced via wet milling. The nature of the solvent does not affect the ionic conductivity significantly; however, the electronic conductivity changes noticeably. The study indicates that xylene and toluene can be used for the wet milling to synthesize Li₆PS₅Cl electrolyte powder with low electronic and comparable ionic conductivities. The all-solid-state cell with the xylene-processed Li₆PS₅Cl electrolyte exhibits the highest discharge capacity of 192.4 mAh·g⁻¹ and a Coulombic efficiency of 81.3% for the first discharge cycle.

Sn Substitution in the Lithium Superionic Argyrodite Li6PCh5I (Ch = S and Se)

The lithium argyrodites Li₆PS₅X (X = Cl, Br, and I) have attracted interest as fast solid ionic conductors for solid-state batteries. Within this class of materials, it has been previously suggested that more polarizable anions and larger substituents should influence the ionic conductivity (e.g., substituting S by Se). Building upon this work, we explore the influence of Sn substitution in lithium argyrodites Li_6+xSn_xP_1-xSe₅I in direct comparison to the previously reported Li_6+xSn_xP_1-xS₅I series. The (P⁵⁺/Sn⁴⁺)Se₄^3/4- polyhedral volume, unit cell volume, and lithium coordination tetrahedra Li(48h)-(S/Se)₃-I increase with Sn substitution in this new selenide series. Impedance spectroscopy reveals that increasing Sn⁴⁺ substitution results in a fivefold improvement in the ionic conductivity when compared to Li₆PSe₅I. This work provides further understanding of compositional influences for optimizing the ionic conductivity of solid electrolytes.