A Nanoscale Design Approach for Enhancing the Li-Ion Conductivity of the Li₁₀GeP₂S₁₂ Solid Electrolyte

Abstract

The discovery of the lithium superionic conductor Li 10 GeP 2 S 12 (LGPS) has led to significant research activity on solid electrolytes for high-performance solid-state batteries. Despite LGPS exhibiting a remarkably high room-temperature Li-ion conductivity, comparable to that of the liquid electrolytes used in current Li-ion batteries, nanoscale effects in this material have not been fully explored. Here, we predict that nanosizing of LGPS can be used to further enhance its Li-ion conductivity. By utilizing state-of-the-art nanoscale modeling techniques, our results reveal significant nanosizing effects with the Li-ion conductivity of LGPS increasing with decreasing particle volume. These features are due to a fundamental change from a primarily one-dimensional Li-ion conduction mechanism to a three-dimensional mechanism and major changes in the local structure. For the smallest nanometric particle size, the Li-ion conductivity at room temperature is three times higher than that of the bulk system. These findings reveal that nanosizing LGPS and related solid electrolytes could be an effective design approach to enhance their Li-ion conductivity.

Ionic Conductivity of Nanocrystalline and Amorphous Li10GeP2S12: The Detrimental Impact of Local Disorder on Ion Transport

“…The activation energy follows this trend and slightly increases from 0.31 to 0.34 eV. This behavior does not support the proposed increase in ionic conductivity, as suggested by Dawson and Islam for nanocrystalline LGPS benefiting from local disorder, changes in local ion coordination or even a change in the dimensionality of the dynamic process . The simulations suggest that diffusion along the ab -plane, that is, in-plane diffusion, is facilitated when going from the bulk to nanocrystals with a grain volume of 10 nm 3 ; hence, they postulate a change from 1D to 3D diffusion.…”

“…According to the simulations, the ionic transport in LGPS shifts from a preferential diffusion along the c -direction to quasi 3D for the nanosized structure. Additionally, it is reported that for a structure made of grains with a volume of 10 nm 3 the diffusion length for Li-ion transport is significantly reduced, which results in increased intergranular diffusion …”

“…Based on their computational results, Dawson and Islam proposed 42 that the already high ionic conductivity of LGPS might be further increased by a factor of 3 when reducing the crystallite size from the conventional micrometer range down to a grain volume of 10 nm 3 . 42 They explained the slight enhancement seen through changes of the local Li + ion coordination, that is, local disorder. These structural changes are assumed to facilitate the slower diffusion process in the abplane of LGPS.…”

“…Additionally, it is reported that for a structure made of grains with a volume of 10 nm 3 the diffusion length for Li-ion transport is significantly reduced, which results in increased intergranular diffusion. 42 A relatively simple and established way to decrease the average crystal size and to introduce structural (point) disorder is given by high-energy ball-milling, which is a top-down approach to prepare nm-sized crystallites. 43,44 If starting with rather poorly conducting coarse-grained materials, many studies reported on enhanced ionic conductivities seen for the nanocrystalline single phase counterparts such as γ-LiAlO 2 , 45 β-spodumene LiAlSi 2 O 6 , 46,47 the glass former Li 2 B 4 O 7 , 48 Li 2 TiO 3 , 49 LiTaO 3 , 50 LiNbO 3 , 51 Li 2 S, 52 and also thiophosphates such as argyrodite-type Li 6 PS 5 I.…”

Design Principles for Grain Boundaries in Solid-State Lithium-Ion Conductors

“…Some progress has been made regarding ion transport at solid electrolytes GBs using molecular dynamics studies with classical potentials. 10,[16][17][18][19][20] For example, it has been established why oxidebased solid electrolytes generally exhibit higher GB resistance 16 than sulfides and how GBs provide an explanation for the often observed differences in calculated and experimental Li-ion conductivities in some solid electrolyte families. 10 Although classical models are valuable as their low computational expense enables the investigation of large grain boundaries and nanostructures containing many tens or even hundreds of thousands of atoms over long timescales, their utility is limited by the fact that they lack access to electronic properties.…”