Ab initio studies on the stability and electronic structure of ${\mathrm{LiCoO}}_2$ (003) surfaces

The electronic structures of $2\times 2$ polar (003) surfaces of ${\mathrm{LiCoO}}_2$ have been studied using the first-principles projector augmented-wave (PAW) method within the generalized gradient approximation (GGA), based on the density functional theory (DFT). Several geometry structure models were considered with different terminations in order to obtain the most stable structure. From cleavage energies of separation it reveals that, for ${\mathrm{LiCoO}}_2$ (003), the configuration with O termination on one side and Li termination on the other side was the most stable one. Because the exposed Li-O groups easily react with the electrolytes so that the structure is destroyed, this conclusion can indirectly explain the phenomenon that ${\mathrm{LiCoO}}_2$ coated by MgO possesses the more stable structure [J. Electrochem. Soc. 149, A466 (2002)]. It is also found that structure relaxation is considerably large, and the electronic density of states (DOS) shows large difference between (003) surfaces and bulk, with, for example, a decrease of the band gap between the valance band and the empty band in (003) surfaces region compared to the bulk. It is possible to reconstruct for the polar (003) surfaces of ${\mathrm{LiCoO}}_2$ crystallite.

Figure 1

(Color online.) Crystal structure of the layered ${\mathrm{LiCoO}}_2$ with $R\overline{3}m$ symmetry.

Figure 2

(Color online.) Two clean and unreconstructed slab models with 12 atom layers obtained by cleaving the crystal perpendicular to the $c$ axis. $A$ type with O- and Li-terminated polar surface and $B$ type with O- and Co-terminated polar surfaces.

Figure 3

(Color online.) Lithium terminations model used in the calculations of the (003) ${\mathrm{LiCoO}}_2$ nonpolar surface. Lithium atoms are removed in a checkerboard style on both surfaces. (b) and (c) are view from the top of the surface. Only surface Li atoms are appeared in (c) and the bottom layer Li atoms are in gray.

Figure 4

DOS of bulk ${\mathrm{LiCoO}}_2$ (bottom panel) together with local DOS on Co, O, and Li sites. In the case of total DOS one should read (formula unit)-1 instead of (atom)-1 in the density units description. The Fermi level is set to zero.

Figure 5

Partial local DOS of (a) Li, (b) Co, and (c) O in ${\mathrm{LiCoO}}_2$. Fermi level is set to zero. When comparing contributions of states with different character note the different scale of some panels.

Figure 6

DOS of bulk and both clean, and unreconstructed (003) surfaces of ${\mathrm{LiCoO}}_2$. (a) the DOS of $A$-type termination, (b) the density of states of $B$-type termination, and (c) the density of states of both Li terminations. Fermi level is set to zero.

Figure 7

Partial DOS of each atom from top layer to bottom layer for $A$ and $B$ types. $s,p$, and $d$ states are presented with straight, dashed, and dotted lines, respectively. Fermi level is set to zero. When comparing contributions of states different character note the different scale of some panels.

Figure 8

Similar to Fig. 7 but for nonpolar surfaces.