Chiolite Na₅Ti₃F₁₄: A novel sodium titanium fluoride anode for low-cost and high-performance Na-ion batteries

Fluoride-based conversion reaction electrode materials offer exceptional theoretical capacity merit for Na-ion batteries. Nevertheless, it has rarely been considered as potential anode material candidate due to (i) excessive redox potential (> 3 V) and (ii) intrinsically low reaction kinetics related to sluggish structural reorganization process. In this work, we demonstrate that chiolite Na₅Ti₃F₁₄/carbon nanocomposite can deliver the outstanding electrochemical performances as the promising anode for Na-ion batteries, such as a large specific capacity of ∼425 mAh g⁻¹ at 10 mA g⁻¹ with a low average operating voltage, the capacity retention of ∼78 % compared to the initial capacity after 300 cycle with a high Coulombic efficiency of above 99 %, etc. We demonstrate that the chiolite Na₅Ti₃F₁₄ phase can store the ∼8.33 mol Na ions through the following conversion reaction; Na₅Ti₃F₁₄ + 9Na + 9e⁻ ⇔ 3Ti + 14NaF, which is clearly confirmed by various ex-situ analyses using X-ray diffraction, synchrotron-based X-ray adsorption spectroscopy, etc. We expect that this research can provide guidance toward the development of a new class of low-cost and high-performance anode materials, not only for Na-ion batteries but also for other rechargeable batteries.