From partial to complete neutralization of 2,5-dihydroxyterephthalic acid in the Li–Na system: crystal chemistry and electrochemical behavior of Na2Li2C8H2O6vs. Li†
The 2,5-dihydroxyterephthalic acid (H4-p-DHT) is of special interest in the field of materials science because of the two symmetric sets of oxygen donor functional groups (i.e., β-hydroxy acid moieties). In its partially (–2H) or totally (–4H) deprotonated form, it is an efficient organic ligand making the synthesis of numerous coordination polymers possible. In addition, it exhibits dual redox-active properties, which make it very interesting as electrode material when combined with alkali and alkaline-earth elements. Herein, we report on a specific study on alkali salts obtained from partial to complete neutralization of 2,5-dihydroxyterephthalic acid in the Li–Na system, which notably enable us to prepare the mixed phase Na2(Li2)-p-DHT (Na2Li2C8H2O6) and to test it electrochemically vs. Li. Depending on the experimental conditions, two Li-polymorphs and one Na compound, containing H2-p-DHT2− ligands with deprotonated carboxylic groups, were obtained, namely, M2(H2)-p-DHT(H2O)4 M = Li (1, 2) and Na (3) as well as the first mixed alkali metal compound, i.e. Na2(Li2)-p-DHT(H2O)8 (4). The single crystal structure analyses showed that all compounds display various inorganic motifs with discrete LiO4 tetrahedra (1, 2), Li2O6 dimers of edge-sharing LiO4 tetrahedra (2), 1∞[NaO4]−7 1-D chains of edge-sharing NaO6 octahedra (3) and [Li2Na2O14]24− clusters of LiO4 tetrahedra and NaO6 octahedra (4). The electrochemical assessment of the anhydrous Na2(Li2)-p-DHT compound measured in Li half-cell revealed however poor performances compared to Li4-p-DHT (Li4C8H2O6) due to cationic disorder of the two alkali ions in the crystal structure giving rise to the progressive conversion of the pristine phase to Li4-p-DHT upon cycling.