WP 2. Surface Engineering

Objectives: (i) to develop a method for surface deposition of “oxygen-storage” materials; (ii) to single out the surface reactivity from the electrochemical redox activity of constituent anion.

Tasks and justification: The tasks include the application of surface deposition techniques that allows to cover homogeneously the surface of the intercalation compounds with CeO₂ and/or Ce-doped ZrO₂ oxides having unique redox properties of oxygen storage and release.

Task 2.1. Chemical deposition of oxygen storage materials on intercalation oxides. The task will start with a deposition of CeO₂ on intercalation oxides obtained in the framework of WP1. We plan to apply a chemical wet deposition in non-aqueous media. The choice of the reactive media is of importance in order to avoid the side reactions of layered sodium transition metal oxides with water. The deposition method will be optimized so that to reach a good dispergation and homogenisation of dispersed systems comprising CeO₂ and P3-Na_xNi_1/2-yM_yMn_1/2O₂ (or Na_xFe_1/3-yM_yMn_1/3O₂). Depending on the results, the task will be extended to other oxygen-storage materials such as Ce-doped ZrO₂, etc.

Task 2.2. Determination of the surface state of metal ions in deposited oxygen-storage materials. The combination of surface sensitive techniques will be applied.

2.2.1. X-ray photoelectron spectroscopy for evaluation at the top surface (up to 5 nm) the valence states and the amounts of deposited elements. This technique will also provide unique information about the processes occurring at the interfaces between CeO₂ and P3-Na_xNi_1/2-yM_yMn_1/2O₂ (or P3-Na_xFe_1/3-yM_yMn_2/3O₂).

2.2.2. Assessing the coordination state of cerium ions by IR spectroscopy of CO molecular probes.

2.2.3. Probing the oxidation states of cerium ions in CeO₂-covered P3-Na_xNi_1/2-yM_yMn_1/2O₂ by EPR.

Task 2.3. Electrochemical evaluation of intercalation capacity of surface modified oxides: The intercalation capacity of surface modified oxides, as well as the reversibility of the intercalation reaction, will be determined from galvanostatic charge-discharge and cyclic voltammetry experiments. The surface reaction of oxygen will be controlled by varying the electrolyte composition from conventional organic electrolytes (i.e. LiPF₆ or NaPF₆ in EC/DMC/PC) to innovative ionic liquids (i.e. LiTFSI or NaTFSI salts dissolved in Pyr₁₄FSI, etc.).

Milestones for WP2: (i) selection of the most suitable synthetic methods for surface deposition; (ii) choice of appropriate chemical and phase composition among “oxygen-storage” materials.

Main result of WP2: (i) new surface deposition method that allows to ensure a good contact between oxygen storage materials and the intercalation compounds; (ii) new data on the surface chemistry of intercalation materials; (iii) new strategy to reduce the surface reactivity of oxygen.