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.