Cation and vacancy ordering inLixCoO2

Abstract

Using a combination of first-principles total energies, a cluster expansion technique, and Monte Carlo simulations, we have studied the Li/Co ordering in ${\mathrm{LiCoO}}_2$ and Li-vacancy/Co ordering in the $\square {\mathrm{CoO}}_2$. We find: (i) A ground-state search of the space of substitutional cation configurations yields the CuPt structure as the lowest-energy state in the octahedral system ${\mathrm{LiCoO}}_2$ (and $\square {\mathrm{CoO}}_2)$, in agreement with the experimentally observed phase. (ii) Finite-temperature calculations predict that the solid-state order-disorder transitions for ${\mathrm{LiCoO}}_2$ and $\square {\mathrm{CoO}}_2$ occur at temperatures $(\sim 5100\hspace{0.5em}\mathrm{K}$ and $\sim 4400\hspace{0.5em}\mathrm{K}$, respectively) much higher than melting, thus making these transitions experimentally inaccessible. (iii) The energy of the reaction $E_{\mathrm{tot}}(\sigma ,\mathrm{Li}\mathrm{Co}{\mathrm{O}}_2)-E_{\mathrm{tot}}(\sigma ,\square \mathrm{Co}{\mathrm{O}}_2)-E_{\mathrm{tot}}(\mathrm{L}\mathrm{i},\mathrm{}\mathrm{bcc})$ gives the average battery voltage $\overline{V}$ of a ${\mathrm{Li}}_x{\mathrm{CoO}}_2/\mathrm{Li}$ cell for the cathode in the structure $\sigma$. Searching the space of configurations $\sigma$ for large average voltages, we find that $\sigma =\mathrm{CuPt}$ [a monolayer $〈111〉$ superlattice] has a high voltage $(\overline{V}=3.78\mathrm{}\hspace{0.5em}\mathrm{V})$, but that this could be increased by cation randomization $(\overline{V}=3.99\mathrm{}\hspace{0.5em}\mathrm{V})$, by partial disordering $(\overline{V}=3.86\mathrm{}\hspace{0.5em}\mathrm{V})$, or by forming a two-layer ${\mathrm{Li}}_2{\mathrm{Co}}_2{\mathrm{O}}_4$ superlattice along $〈111〉$ $(\overline{V}=4.90\mathrm{}\hspace{0.5em}\mathrm{V})$.