Sb–Si Alloys and Multilayers for Sodium-Ion Battery Anodes

Silicon has a theoretical sodium-storage capacity of 954 mAh/g, which even exceeds that of tin (847 mAh/g). However, this capacity has never been reached in practice. Antimony is one of the best-performing Na-storage materials in terms of both capacity and cycling stability. By combining silicon and antimony, either by cosputtering or depositing multilayers with bilayer thickness down to 2 nm, we can achieve capacities exceeding even the theoretical capacity of Sb (660 mAh/g). Minor addition of silicon, 7 at. % or 7 wt % (25 at. %), increases the measured reversible capacity from 625 mAh/g for pure Sb to 663 and 680 mAh/g, respectively. All Sb-rich (>50 at. %) compositions show improved cycling stability over elemental Sb. Si0.07Sb0.93 reached a maximum capacity of 663 mAh/g after 140 cycles and showed negligible capacity degradation up to 200 cycles. The fully sodiated state in cosputtered films evolves from single-phase amorphous to a mixture of a Sb-rich and Si-rich sodiated phases as cycling progresses, when the Si content is between 75 and 50 at. %. The typical desodiation signature of c-Na3Sb is observed only after 100 cycles or more. Careful examination of the voltage profiles of multilayers shows that they initially tend toward intermixing between the Si and Sb layers, contrary to expectations based on the phase diagram. When the Si and Sb layer thickness is decreased to 2 nm, the multilayer and cosputtered film behave almost identically. A general direction for finding promising multicomponent sodium-ion battery (SIB) alloy anodes is proposed.