Niobium Bronzoids as Negative Electrodes: Synthesis, Structure, and Electrochemical Properties of Li₂Nb₄P₂O₁₆ and Na₂Nb₄P₂O₁₆

In this study, sodium- and lithium-based phosphate niobium bronzes and bronzoids with the general formula <i>A</i>_<i>x</i>(PO₂)₂(NbO₃)_<i>m</i> (where <i>A</i> = Na/Li and <i>m</i> = 4), specifically Na₂Nb₄P₂O₁₆ and Li₂Nb₄P₂O₁₆, are investigated. The crystal structure of Na₂Nb₄P₂O₁₆ is revisited using a combination of laboratory and synchrotron powder X-ray powder diffraction. It is found to crystallize in the <i>P</i>2₁/<i>a</i> space group (different from the previously reported <i>P</i>2₁ space group), with lattice parameters <i>a</i> = 13.2503 (6) Å, <i>b</i> = 5.3498 (2) Å, <i>c</i> = 19.0807 (7) Å, β = 109.9574 ° (3) and <i>V</i>/<i>Z</i> = 317.833 (9) ų. Additionally, we synthesized and determined the crystal structure of Li₂Nb₄P₂O₁₆ for the first time, introducing it as a lithium-based phosphate niobium bronzoid. It crystallizes in the <i>Pc</i>2₁<i>n</i> orthorhombic cell with lattice constants <i>a</i> = 6.7031 (4) Å, <i>b</i> = 5.1936 (2) Å, <i>c</i> = 17.4260 (8) Å, and <i>V</i>/<i>Z</i> = 303.324 (5) ų. As negative electrodes in Li batteries, Li₂Nb₄P₂O₁₆ and Na₂Nb₄P₂O₁₆ exhibited average discharge capacities of 386 and 277 mAh/g, respectively, at C/15 in a voltage window of 3.0-0.1 V.