Tunable electronic properties of silicon nanowires as sodium-battery anodes

Although materials for lithium-ion batteries have been extensively studied, alternatives such as sodium-ion batteries have acquired a renewed interest due to the abundance of Na compared to Li. However, the investigation of new materials for Na battery anodes is still in progress. In this work, a density functional study of the electronic properties of hydrogen passivated silicon nanowires (H-SiNWs) with interstitial Na atoms is presented. The studied H-SiNWs are grown along the [001] crystallographic direction and have a diameter close to 2.5 nm. Moreover, from 1 to 12 interstitial Na atoms per H-SiNW unit cell were considered. The results reveal that the former semiconducting nanowires become metallic for all the Na concentrations, even for the case of a single Na atom. The formation energy diminishes as a function of the concentration of Na atoms, revealing a loss of energetic stability since the size of the Na atoms strongly modify the Si-Si bonds. Moreover, when the Na atoms are removed from the metallic sodiated H-SiNW and relaxed again, for concentrations between 1 and 8 Na atoms, the resulting structure corresponds to the original H-SiNW one, indicating that the Na insertion/extraction process is a reversible one. In contrast, for concentrations between 10 and 12 Na atoms, the structure that results from removing of these Na atoms has a different atomic arrangement, in comparison with the initial H-SiNW, and also smaller band gap. These results open the possibility to consider the H-SiNWs as potential anodic materials in sodium rechargeable batteries.

Automated summary

This study investigates the electronic properties of hydrogen passivated silicon nanowires with interstitial sodium atoms, finding that semiconducting nanowires become metallic with the insertion of sodium atoms. The reversible nature of the sodium insertion/extraction process is observed for concentrations between 1 and 8 sodium atoms, while concentrations of 10 to 12 sodium atoms result in a different atomic arrangement and a smaller band gap.