Layered oxides as cathode materials for beyond-Li batteries: A computational study of Ca and Al intercalation in bulk V2O5 and MoO3

Materials for post Li-ion batteries are attracting an increasing interest. Here we have studied the fundamental properties of multi-valent Al and Ca ions intercalated into V2O5 and MoO3 layered oxides. To this end we performed Density Functional Theory calculations using a Hubbard U correction for the electronic structure of the oxides and including dispersion forces. We simulated very low loadings of the intercalated Al and Ca atoms, and we studied both stoichiometric and reduced oxides. Intercalation of the Al and Ca atoms within the oxide layers (intra-layer) and in the spaces between the layers (inter-layer) has been compared. In V2O5 inter-layer intercalation is preferred, and gives rise to theoretical voltages of 1.54 V for Al and of 3.08 V for Ca; for comparison, the theoretical voltage for a Li-V2O5 battery is 3.51 V. Al can also be incorporated within the V2O5 layers, but with a smaller binding energy, while Ca, bigger, is only stable in the inter-layer sites. The situation is partly different on MoO3. Here in fact Ca is stable only in inter-layer sites, and gives rise to theoretical voltages of 1.65 V. Al, on the contrary, prefers to be incorporated into the MoO3 layers, giving rise to a similar theoretical voltage of 1.60 V. For comparison, a Li- MoO3 battery has a computed voltage of 2.16 V. The reduced forms of V2O5 and MoO3 have also been considered. Here the O atoms that are more easily removed are those of the vanadyl, V=O, and molibdyl, Mo=O, groups. The only case where there an increase of the voltage is for Al and Ca intercalation into reduced V2O5, provided that the Al and Ca ions are near an oxygen vacancy, but not in the vacancy. In MoO3 the formation of oxygen vacancies is detrimental for the intercalation process.