Tetragonal Tungsten Bronze Framework as Potential Anode for Na-Ion Batteries

Na-ion batteries (NIBs) are becoming more promising owing to potentially better rate performance than Li-ion batteries apart from resource abundance. However, activation of Na-storage electrochemistry remarkably depends on the modification or expansion of existing structure prototypes by adjusting the substituent and linkage of their ligand moieties or the redox transition metals. Recently the natural existence of mineral phases has inspired us to explore more plentiful artificial analogues, most of which possess open framework properties. Here for the first time we propose a novel bronze phase (tetragonal tungsten bronze, TTB) of oxyfluoride (KNb 2 O 5 F) as a potential NIB anode. This tunnel-type open framework is achieved by equimolar KF doping into T-Nb 2 O 5 with K and F as channel supporter and ligand substituent, respectively. The improvement of intrinsic conductivity and near-zero strain sodiation enable a highly reversible capacity even by employing undecorated samples of less surface defects. Reduction of Nb 5+ may cause a uniform precipitation of fine NbO 2 nanoparticles around larger solidated KNb 2 O 5 F.