First-principles study on the mechanism of lithium intercalation in cubic CoN

Intercalation mechanism of Li into cubic Co₄N₄ has been investigated by the first-principles calculations. Lattice constants, ratio of volume expansion, and formation energies of LixCo₄N₄ (x = 0, 1, 2, 3, 4) were calculated. Results indicate that Li prefers to fill the octahedral interstitial site (Oh) rather than the tetrahedral interstitial site (Th). With the increase in intercalation Li, the ratio of volume expansion increases from 8.29% (x = 1) to 31.58% (x = 4). Ternary phase Li₄Co₄N₄ has the most stability with the negative intercalation energy, and the corresponding theoretical specific capacity reaches 367 mA/g. Furthermore, the analysis of density of states, valence electron density distribution maps, and electron localization function (ELF) of Co₄N₄ and Li₄Co₄N₄ indicates that Li intercalation enhances the electrical conductivity of Co₄N₄ and weakens the bonding of Co and N. Finally, Li-ion migration dynamics in the Co₄N₄ bulk were investigated with nudged elastic band (NEB) methods. Results show that the migration path of Li-ion is along Oh → Th → Oh with the energy barrier of 0.44 eV.