Tuning the Composition and Structure of Bi–Sb Alloy@C Composites for Superior Potassium-Ion Storage

Potassium-ion batteries (PIBs) are considered promising alternatives or supplements to lithium-ion batteries due to the abundant reserve of potassium and its redox potential being close to that of lithium. Among the candidate anode materials for PIBs, high-capacity alloy-type anodes such as Sb and Bi are usually confronted with the critical challenge of poor cycling stability caused by significant volume expansion. To address this issue, this work designed and synthesized a series of Bi_<i>x</i>Sb_1-<i>x</i>@C (BSC, 0 ≤ <i>x</i> ≤ 1) composite materials with different Bi/Sb ratios and microstructures, using metal-organic frameworks (MOFs) as precursors. Owing to fine particle size and suitable composition of the Bi_0.5Sb_0.5 alloy that promotes the antipulverization property, the favorable small-sized micronribbons carbon matrix that effectively inhibits particle agglomeration and buffers volume change, and the unique radially assembled structure that facilitates stress relief and enhances electron and ion transport, the preferred composite (BS23C) exhibits outstanding electrochemical performance for K⁺ storage. It demonstrates a long cycle life (capacity retention rate of 94.1% after 1500 cycles at 2 A g^-1) and outstanding rate capability (275.2 mAh g^-1 at 5.0 A g^-1). In situ XRD analysis reveals unique two-step phase evolution process of Bi_0.5Sb_0.5: Bi_0.5Sb_0.5 ↔ K(Bi_0.5Sb_0.5) ↔ K₃(Bi_0.5Sb_0.5). The discharge end-state in the first cycle is the hexagonal phase K₃(Bi_0.5Sb_0.5), while it becomes the cubic phase K₃(Bi_0.5Sb_0.5) in the subsequent cycles. The formation of cubic phase K₃(Bi_0.5Sb_0.5) is associated with a lower volume expansion rate, contributing to enhanced cycling stability. This study provides insightful guidance for the design of high-performance alloy-type anode materials for PIBs.