Novel metastable Bi:Co and Bi:Fe alloys nanodots@carbon as anodes for high rate K-ion batteries

Bi is a promising anode material for potassium-ion batteries (PIBs) due to its high theoretical capacity. However, severe pulverization upon cycling limits its practical applications. In this work, we propose a new approach of using metastable alloys with Bi elements. Metastable Bi:Co and Bi:Fe alloys nanodots@carbon anode materials (Bi:Co and Bi:Fe@C) are synthesized for the first time via simple annealing of their metal-organic frameworks (MOF) precursors. These prepared materials are demonstrated as ideal hosts for high-rate K-ion storage. Bi_0.85Co_0.15@C and Bi_0.83Fe_0.17@C electrodes respectively deliver superior 178 and 253 mAh·g⁻¹ at 20 A·g⁻¹, as well as stable cycling performance at 2 A·g⁻¹. Ex situ scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) studies on Bi:Co@C indicate that the elemental Co separates out during the initial potassiation and stands during the following discharge/charge cycles. In situ formed Co precipitates can act as (1) “conductive binders” as well as (2) “separators” to prevent the severe aggregation of adjacent active elemental Bi nanoparticles and (3) accelerate the potassiation/de-potassiation kinetics in elemental Bi precipitates after initial discharge/charge cycles. This work could inspire the development of metal-type anodes.