Synthesis and Characterization of W-Nb Oxides (WO3-Nb2O5) for Large-Volumetric-Capacity Negative Electrodes

To develop lithium insertion materials used in lithium-ion batteries in terms of reversible capacity, it is desirable for crystal structures that can reversibly accommodate more Li ions in the lattice. Conventionally, lithium insertion materials with the densest close-packed structures should have the highest volumetric capacity. However, in recent years, materials with non-closest-packed structures, such as TiNb2O7, have been paid much attention as high-capacity materials because they can accommodate an extremely large number of Li ions. In this study, we synthesized and characterized a series of binary W-Nb oxides for the high-volumetric-capacity negative electrode because it has many binary compounds having non-close-packed structures. All of the W-Nb oxides reversibly accommodate Li ions at the ratio of Li/M > 1, although the crystal structures are different between the W-rich phase with flower-type structures and the Nb-rich phase with block-type structures. The Nb-rich phases with block-type structures exhibit larger reversible capacity than the W-rich phases. The reaction mechanism of W3Nb14O44, which has the highest capacity among the binary oxides, was investigated by ex situ X-ray diffraction (XRD), and it was found that its volume change is small owing to a large free space in the crystal structure with non-close packing. Finally, to examine the feasibility of W3Nb14O44 as the high-capacity negative electrode, we fabricated a lithium-ion cell of W3Nb14O44 combined with a 5 V positive electrode material of Li[Ni1/2Mn3/2]O4 and performed a cycle test.