Alloying of Alkali Metals with Tellurene

Graphite is ubiquitous as the anode material in lithium-ion batteries, but offers relatively low volumetric capacity (330 to 430 mAh cm-3). By contrast, Tellurene (Te) is expected to alloy with alkali metals with high volumetric capacity (~2620 mAh cm-3), but to date there is no detailed study on its alloying behavior. In this work, we have investigated the alloying response of a range of alkali metals (A = Li, Na or K) with few-layer Te. In-situ transmission electron microscopy and density functional theory both indicate that Te alloys with alkali metals forming A2Te. However, the crystalline order of alloyed products varied significantly from single-crystal (for Li2Te) to polycrystalline (for Na2Te and K2Te). It is well established that typical alloying materials (e.g., silicon, tin, black phosphorous) lose their crystallinity when reacted with Li. The ability of Te to retain its crystallinity is therefore surprising. Nudged elastic band calculations and ab-initio molecular dynamics simulations reveal that compared to Na or K, the migration of Li is highly “isotropic” in Te, enabling its crystallinity to be preserved. Such isotropic Li transport is made possible by Te’s peculiar structure comprised of chiral chains bound by van der Waals forces. To evaluate the electrochemical performance of Te, we tested Te electrodes in half-cells vs Li/Na/K metal. While alloying with Na and K showed poor performance, with Li, the Te electrode exhibited a volumetric capacity of ~700 mAh cm-3, which is about two-times the practical capacity of commercial graphite. Such Te based batteries could play an important role in applications where high volumetric energy and power density are of paramount importance.