Solid Solution Strengthening of Bismuth Antimonide as a Sodium Storage Material

A relationship between the mechanical property and charge–discharge performance of bismuth antimonide (Bi–Sb) solid solution electrodes was experimentally investigated for the first time as Na-ion battery anodes. The nanoindentation measurements revealed that three kinds of electrodes of Bi–Sb solid solution (Bi0.8Sb0.2, Bi0.5Sb0.5, and Bi0.2Sb0.8) showed 100 times higher indentation elastic modulus compared to the Bi electrode by the effect of solid solution strengthening and that Bi0.2Sb0.8 has the most elastic property among the solid solutions. The charge–discharge cycling tests confirmed that the reversible capacities of a Bi electrode and Sb electrode rapidly decayed, owing to pulverization of active materials induced by their large volumetric changes during sodiation–desodiation reactions. The Bi-rich solid solution (Bi0.8Sb0.2) also showed poor cyclability. It is suggested that the pulverization of Bi0.8Sb0.2 could not be avoided because of its less elastic property. The cycling performance of Bi–Sb electrodes was remarkably improved with increasing the ratio of Sb. The Sb-rich solid solution (Bi0.2Sb0.8) electrode exhibited the best performance with the reversible capacity of 520 mA h g–1, even at the 200th cycle. We consider that the pulverization of Bi0.2Sb0.8 was effectively suppressed because it has reasonably elastic property in addition to a high indentation elastic modulus as a result of the solid solution strengthening. This knowledge would be very meaningful in the development of the next-generation alloying-type anode materials with high capacities.