Systematic calculations of On (n = 1 to 6) polytypes of LiCoO₂

Six different polytypes On (n = 1 to 6) of the conventional lithium-ion battery cathode material LiCoO₂ are examined systematically using first-principles calculations within the framework of density functional theory. The calculations correctly predict the most stable form to be O3-LiCoO₂, and indicate that the recently synthesized O4 polytype is slightly more stable than O2-LiCoO₂, with a lattice energy per formula unit intermediate between O2- and O3-LiCoO₂. Extending the calculations to the as-yet unobserved O5 polytype suggests that this is also slightly more stable than the O2 form. The potential differences of each polytype relative to lithium metal are found to decrease as the concentration of stacking faults (and volumetric mass density) increases, with a maximum of 3.89 eV for the polytype with optimum crystal packing (cubic-close-packing, ccp), namely O3-ABCABC, and a minimum for the highest concentration of stacking faults, corresponding to a hexagonal close-packed structure, i.e., O1. Other polytypes can be considered as intergrowths of these two configurations, so that for n beyond 6 the most stable form of each polytype is expected to display properties increasingly similar to those of O3 as the proportion of ccp sequences will increase.