Adsorption and Deposition of Li2O2 on the Pristine and Oxidized TiC Surface by First-principles Calculation

We investigate Li2O2 adsorption and deposition on the low index surfaces of pristine and oxidized TiC, which has been demonstrated recently to be an excellent cathode for Li-air batteries. We found that the pristine TiC surface is not stable toward interaction with Li2O2 and that surface stability can be enhanced greatly by surface oxidation as shown by surface energy reduction from 102 to 23 meV/angstrom(2) for the TiC{100} surface and from 208 to 39 meV/angstrom(2) for the TiC{111} surface after oxidation. Adsorption of two Li2O2 clusters on the 2 x 2 Ti-terminated TiC{111} surface (TiC{111}_Ti) resulted in spontaneous destruction of Li2O2 clusters and formation of a saturated periodic two atomic layer coating in which each O atom is bonded to three Ti atoms to form an O layer equivalent to the layer formed by O-2 surface oxidation, and Li atoms sit on the top. The atomic arrangement of O and Li layers resembles that of Li1O(2) layers normal to the [0001] direction in the Li2O2 crystal structure. Interface models constructed based on this lead showed that the growth of Li2O2 can be continued on oxidized TiC{111}_Ti through a surface conduction mechanism to form Li2O2 coating with lattice parameters almost identical to those of the standard Li2O2 unit cell. On the oxidized TiC{100} surface (TiC{100}_TiO), two Li2O2 clusters adsorbed on two adjacent Ti sites by TiO bonding with the O-O axis in Li2O2 perpendicular to the surface resulted in a Li and O atom configuration which is similar to O2Li3O4 layers in the Li2O2 crystal structure, indicating a potential path for Li2O2 nucleation on the TiC{100}_TiO surface. Interface models by following this path suggested that Li2O2 coating may be grown on the TiC{100}_TiO surface with a dihedral angle between 11.4 degrees and 22.4 degrees, and strains inside the Li2O2 could induce conductivity. These atomistic insights are in good agreement with the experimental findings.