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 Ax(PO2)2(NbO3)m (where A = Na/Li and m = 4), specifically Na2Nb4P2O16 and Li2Nb4P2O16, are investigated. The crystal structure of Na2Nb4P2O16 is revisited using a combination of laboratory and synchrotron X-ray powder diffraction. It is found to crystallize in the P21/a space group (different from the previously reported P21 space group), with lattice parameters a = 13.2503(6) Å, b = 5.3498(2) Å, c = 19.0807(7) Å, β = 109.9574o (3) and V/Z = 317.833(9) Å3. Additionally, we synthesized and solved the crystal structure of Li2Nb4P2O16 for the first time, introducing it as a lithium-based phosphate niobium bronzoid. It crystallizes in the Pc21n orthorhombic cell with lattice constants a = 6.7031(4) Å, b = 5.1936(2)  Å , c = 17.4260(8) Å and V/Z = 303.324(5) Å3. As negative electrodes in Li batteries, Li2Nb4P2O16 and Na2Nb4P2O16 exhibited average discharge capacities of 386 mAh/g and 277 mAh/g, respectively, at C/15 in a voltage window of 3.0 V to 0.1 V.