WP 1. Structural Engineering.

Objectives: (i) to modulate electronic structure of the oxide material by metal substitution and vacancy stabilization; (ii) to elaborate synthetic methods enabling the preparation of oxide materials with desired cationic distribution; (iii) to evaluate the oxidation state of the transition metal ions and their local cation distribution in multicomponent systems; (iv) to quantify the relationships between metal substituents, local ion distribution and intercalation capacity. 

Tasks and justification: classified with respect to research target and method. Each method is selected in a way guaranteeing interrelated information about the different structural aspects of multicomponent systems. 

Task 1.1. Tailored synthesis of metal substituted oxide materials. Starting from P3-Na_xNi_1/2Mn_1/2O₂ with three layered stacking, a series of metal substituted compounds will be obtained: Na_xNi_1/2-yM_yMn_1/2O₂, where M denotes the intercalation inactive ions Mg²⁺, Ti⁴⁺ and Ge⁴⁺. The next series of materials includes monoclinic distorted iron-manganese oxides, P‘3-Na_xFe_1/3Mn_2/3O₂, in which the iron ions will be selectively replaced with Mg²⁺, Ti⁴⁺ and Ge⁴⁺: Na_xFe_1/3-yM_yMn_2/3O₂. All oxide materials will be obtained by the same method consisting in thermal decomposition of homogeneous precursors containing all metal ions mixed at an atomic level. The precursors will be prepared through freeze-drying methods of mixed aqueous solutions of metal salts. To ensure a homogeneous distribution of all metal ions in aqueous solutions, citric acid will be utilized as complexation agents. Thermal treatment of precursors leads to the formation of target compositions. Furthermore, the synthetic procedure of target oxides will be optimized in respect of the oxidation states of transition metal ions by varying the reaction atmosphere (Ar or O₂), heating temperature and reaction time.  Finally, this method will guarantee the formation of all series of metal substituted oxides P3-Na_xNi_1/2-yM_yMn_1/2O₂ and P’3-Na_xFe_1/3-yM_yMn_2/3O₂, which will be studied further in next WPs. 

Task 1.2. Stabilization of metal vacancies in alkali transition metal oxides. The creation of metal vacancies in metal substituted oxides Na_xNi_1/2-yM_yMn_1/2O₂ and Na_xFe_1/3-yM_yMn_2/3O₂ will be controlled by the rate of cooling from the temperature of preparation. In the Proposal we will use two modes of cooling: slow cooling rate (i.e. 5^o/min) and fast cooling rate (i.e. quenching from the annealing temperature). 

Task 1.3. Structural and spectroscopic analysis of metal substituted and vacancy-containing oxides. The structure of all samples will be determined and refined by combined utilization of X-ray analysis applying the Rietveld refinement method, selected area electron diffraction (SAED) and high-resolution TEM. The iodometric titration against a standardized sodium thiosulfate solution will be performed to determine the total oxidation state of metal ions. The oxidation states of Mn⁴⁺, Fe³⁺, Ni³⁺ and Ni²⁺ will be quantified by EPR. This method is also effective to analyse the distribution of metal ions at a local-range. The results from EPR will be completed with solid state ²³Na NMR, infrared and Raman spectroscopy.      

Task 1.4. Electrochemical evaluation of intercalation capacity of modified oxides. The intercalation of Li⁺ and Na⁺ ions into metal substituted P3-Na_xNi_1/2-yM_yMn_1/2O₂ and P‘3-Na_xFe_1/3Mn_2/3O₂ (M=Mg²⁺, Ti⁴⁺, Ge⁴⁺) will be measured by galvanostatic charge-discharge and cyclic voltammetry. To steer the intercalation reaction in the desired direction, the electrolyte contents will be varied as well: 1М LiPF₆ in EC/DMC solution, 1М NaPF₆ in PC solution, as well as 1М Li(Na)TFSI in diglyme solution. 

Milestones for WP1: (i) selection of the most suitable synthetic methods; (ii) harmonizing of the information obtained from application of the physicochemical methods to each individual compound; (iii) construction of the electrochemical cells.

Main result of WP1: (i) new method for the preparation of metal substituted layered oxides with a pre-set defect structure; (ii) new correlations between the geometry and the energetics of the cationic and anionic entities based on data from spectroscopic and diffraction studies; (iii) clarification of the impact of the synthetic method on the structural features of the compounds; (iv) new materials with colossal intercalation capacity.