Lithium ion transport in micro- and nanocrystalline lithium sulphide Li2S

Ion dynamics in binary Li-bearing compounds such as LiF, Li2O and Li2S is rather poor. These compounds do, however, form as decomposition products at the interface between the electrolyte and the electrode materials in lithium-based batteries. They are expected to severely influence the charge transport across this electrode-electrolyte interface and, thus, the overall performance of such systems. Yet, ion dynamics in the nanostructured forms of these binary compounds has scarcely been investigated. Here, we prepared bulk nanostructured Li2S through high-energy ball milling and studied its temperature-dependent ionic conductivity by means of broadband impedance spectroscopy. It turned out that, compared to the unmilled form, Li+ ion conductivity in ball-milled Li2S increased by approximately 3 orders or magnitude. This striking increase is accompanied by a decrease of the average activation energy from ca. 0.9 eV to approximately 0.7 eV. Structural disorder, stress and local distortions are assumed to be responsible for this clear change in macroscopic transport parameters. Fast ion dynamics in or near the interfacial space charge zones might contribute to enhanced dynamics, too. We conclude that Li ion transport in interfacial Li2S, if present in a disordered nanostructured form in lithium-ion batteries, is much faster than originally thought for its ordered counterpart.