Exploring High-Entropy Molybdate as a Highly Stable Anode for Li-Ion Batteries

Entropically stabilized compositionally complex materials have recently emerged as a class of materials aimed at addressing the limited cycle stability of conventional conversion-based anodes in Li-ion batteries (LiB). Herein, we inaugurally represent first-row transition-metal-based high-entropy molybdates (HEMo) as a high-performance conversion-based LiB anode. The HEMo was synthesized by using a one-pot coprecipitation approach followed by annealing, resulting in a material with a layered morphology consisting of close-packed two-dimensional (2D) sheets. In a half-cell configuration, the HEMo demonstrated a high initial capacity of 1315 mAh/g at 0.1 A/g (second discharge) with an approximately 89% Coulombic efficiency. The HEMo exhibited excellent cycle stability, maintaining a specific capacity of 426 mAh/g over 1500 cycles at 0.5 A/g and a specific capacity of 206 mAh/g after 1800 cycles at 1 A/g. The charge storage mechanism was elucidated with an ex situ X-ray diffraction (XRD) study, showing a phase transition to the constituent metals during initial discharge and reoxidation to metal oxides during subsequent charging. A charge storage kinetics study showed a diffusion-dominated charge storage mechanism, the contribution of which further increased in the cycled cell, which is a possible reason behind the excellent long-term capacity retention of the HEMo. Additionally, the lithium-ion diffusion characteristics were examined across various states of discharge (SOD), revealing approximately a 103-fold enhancement in lithium diffusivity from open circuit voltage (OCV) to an SOD of 0.5 V. Moreover, this trend exhibited high reversibility upon charging. Further, the practical utility of the HEMo as a LiB anode was validated in a full cell with a LiCoO2 (LCO) cathode, exhibiting a stable capacity of 45 mAh/g after 200 cycles at 2C. Our initial findings should encourage further exploration and engineering of high-entropy molybdates as potential LiB anodes.