Hexagonal Tungsten Bronze H_0.25Cs_0.25Nb_2.5W_2.5O₁₄ as a Negative Electrode Material for Li-Ion Batteries

Oxides derived from the ReO3 structure, such as bronzes or Wadsley–Roth phases, have re-emerged from the past as they offer very attractive properties as negative electrodes for high-power Li-ion batteries. Here, we revisit the hexagonal bronze Cs0.5Nb2.5W2.5O14 and its protonated H0.25Cs0.25Nb2.5W2.5O14 derivative, which was newly obtained after ion exchange. A panel of characterization techniques (HAADF-STEM imaging, EDX mapping, and XRD) revealed a specific cation ordering in the material and a preferential ion exchange in the heptagonal tunnels of this oxide. When used as Li-ion battery electrodes, H0.25Cs0.25Nb2.5W2.5O14 exhibits higher specific capacities as well as better capacity retention at high currents than Cs0.5Nb2.5W2.5O14. The protonated phase provides specific capacities of 132 and 111 mAh·g–1 at, respectively, 0.02 and 0.2 A·g–1. Kinetic analysis reveals that the good capacity at a high rate is due to favorable diffusion of lithium ions into the tunnels of H0.25Cs0.25Nb2.5W2.5O14. In addition, using in situ XRD, a solid solution mechanism occurring during lithium insertion was proposed, as well as the different lithiation sites of the material.