Sodium effects on the electronic and structural properties of porous silicon for energy storage

Porous silicon is a promising anode material in Na-ion batteries, however, there are still no theoretical studies describing the Na storage mechanism within this material. In this work, we present a density functional theory study on the effects of interstitial and substitutional Na atoms on the electronic and structural properties of hydrogen-passivated porous silicon (pSi_H). The results show that the substitutional Na reduces the band gap, while the interstitial Na induces metallic properties on the pSi_H. The diffusion analysis by the nudged elastic band scheme, reveals that the interstitial Na atoms migrate from the silicon lattice to the pore surface, while the pSi_H energy barrier decreases by 20.42% relative to the bulk silicon energy barrier value. Finally, the hydrogenated surface proves to be beneficial for both Na adsorption and diffusion. These results could be important for understanding the storage and diffusion mechanism of Na on pSi_H.