Bi–Sb Nanocrystals Embedded in Phosphorus as High-Performance Potassium Ion Battery Electrodes

The development of high-performance potassium ion battery (KIB) electrodes requires a nanoengineering design aimed at optimizing the construction of active material/buffer material nanocomposites. These nanocomposites will alleviate the stress resulting from large volume changes induced by K+ ion insertion/extraction and enhance the electrical and ion conductivity. We report the synthesis of phosphorus-embedded ultrasmall bismuth–antimony nanocrystals (BixSb1–x@P, (0 ≤ x ≤ 1)) for KIB anodes via a facile solution precipitation at room temperature. BixSb1–x@P nanocomposites can enhance potassiation–depotassiation reactions with K+ ions, owing to several attributes. First, by adjusting the feed ratios of the Bi/Sb reactants, the composition of BixSb1–x nanocrystals can be systematically tuned for the best KIB anode performance. Second, extremely small (diameter ≈ 3 nm) BixSb1–x nanocrystals were obtained after cycling and were fixed firmly inside the P matrix. These nanocrystals were effective in buffering the large volume change and preventing the collapse of the electrode. Third, the P matrix served as a good medium for both electron and K+ ion transport to enable rapid charge and discharge processes. Fourth, thin and stable solid electrolyte interface (SEI) layers that formed on the surface of the cycled BixSb1–x@P electrodes resulted in low resistance of the overall battery electrode. Lastly, in situ X-ray diffraction analysis of K+ ion insertion/extraction into/from the BixSb1–x@P electrodes revealed that the potassium storage mechanism involves a simple, direct, and reversible reaction pathway: (Bi, Sb) ↔ K(Bi, Sb) ↔ K3(Bi, Sb). Therefore, electrodes with the optimized composition, i.e., Bi0.5Sb0.5@P, exhibited excellent electrochemical performance (in terms of specific capacity, rate capacities, and cycling stability) as KIB anodes. Bi0.5Sb0.5@P anodes retained specific capacities of 295.4 mA h g–1 at 500 mA g–1 and 339.1 mA h g–1 at 1 A g–1 after 800 and 550 cycles, respectively. Furthermore, a capacity of 258.5 mA h g–1 even at 6.5 A g–1 revealed the outstanding rate capability of the Sb-based KIB anodes. Proof-of-concept KIBs utilizing Bi0.5Sb0.5@P as an anode and PTCDA (perylenetetracarboxylic dianhydride) as a cathode were used to demonstrate the applicability of Bi0.5Sb0.5@P electrodes to full cells. This study shows that BixSb1–x@P nanocomposites are promising carbon-free anode materials for KIB anodes and are readily compatible with the commercial slurry-coating process applied in the battery manufacturing industry.