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

Abstract

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 (Bi_<i>x</i>Sb_1-<i>x</i>@P, (0 ≤ <i>x</i> ≤ 1)) for KIB anodes <i>via</i> a facile solution precipitation at room temperature. Bi_<i>x</i>Sb_1-<i>x</i>@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 Bi_<i>x</i>Sb_1-<i>x</i> nanocrystals can be systematically tuned for the best KIB anode performance. Second, extremely small (diameter ≈ 3 nm) Bi_<i>x</i>Sb_1-<i>x</i> 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 Bi_<i>x</i>Sb_1-<i>x</i>@P electrodes resulted in low resistance of the overall battery electrode. Lastly, <i>in situ</i> X-ray diffraction analysis of K⁺ ion insertion/extraction into/from the Bi_<i>x</i>Sb_1-<i>x</i>@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>i.e.</i>, Bi_0.5Sb_0.5@P, exhibited excellent electrochemical performance (in terms of specific capacity, rate capacities, and cycling stability) as KIB anodes. Bi_0.5Sb_0.5@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 Bi_0.5Sb_0.5@P as an anode and PTCDA (perylenetetracarboxylic dianhydride) as a cathode were used to demonstrate the applicability of Bi_0.5Sb_0.5@P electrodes to full cells. This study shows that Bi_<i>x</i>Sb_1-<i>x</i>@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.