Proton intercalation into different CoO2 layer matrices

Highlights: • Li_xCoO₂ cathode and Na_xCoO₂ thermoelectrics are based on hexagonal CoO₂ layer. • These A_xCoO₂ materials readily release A⁺ cations and incorporate protons. • Crystal structure of the resultant H_yCoO₂ phase depends on the starting material. • Also the thermoelectric misfit compound [CoCa₂O₃]_0.62CoO₂ can be protonated. • It is also possible to prepare CoO₂-layer stacks with mixtures of A⁺ and H⁺ ions. -- Abstract: The hexagonal CoO₂ layer is an integral part of many important functional materials such as the Li_xCoO₂ battery electrode, the Na_xCoO₂ and [CoCa₂O₃]_0.62CoO₂ thermoelectrics, and the Na_0.3CoO₂(H₂O)_1.3 superconductor. However, the CoO₂-layer piling is prone to incorporate not only positively-charged metal and metal oxide layers but also protons between successive CoO₂ layers. Here we systematically investigate the de-intercalation of the intervening alkali metal and metal oxide layers, and the subsequent intercalation of protons for an extensive series of LiCoO₂, Na_xCoO₂ (x = 0.6–0.9) and [CoCa₂O₃]_0.62CoO₂ samples. First, we demonstrate the partial extraction of sodium from Na_xCoO₂ through immersion in an aqueous solution of either n-butylamine or NH₄NO₃. Such a treatment results in an appreciably low Na content of x = 0.37 or 0.10, respectively, but a simultaneous incorporation of certain amounts of charge balancing protons into the crystal lattice. Then, using a newly developed hydrothermal route we demonstrate the complete de-intercalation of the alkali and alkaline earth metal species from LiCoO₂, Na_xCoO₂ and [CoCa₂O₃]_0.62CoO₂, and the intercalation of protons between the CoO₂ layers. Finally we synthesize and characterize a water-containing Na_xCoO₂(H₂O)_z sample for comparison. As the protonation is found to be a common tendency of all the studied multilayered functional materials based on the CoO₂ layer we believe that our work is not only of fundamental scientific interest but also of more practical relevance.