Influence of carbon and fluorine on potassium niobate anode material for sodium-ion battery applications

Potassium niobate, KNb3O8 (KNO) material is investigated as a potential anode material for sodium ion batteries owing to its layered structure and excellent electrochemical stability. However, the poor electrical conductivity of the material is addressed by surface modification with fluorine-doped carbon utilizing polyvinylidene fluoride as both a carbon and fluorine source. High resolution transmission electron microscopy results reveal that the active material is successfully embedded in the carbon matrix and X-ray photoelectron spectroscopy analysis confirms the tight attachment of carbon and fluorine bonding with the bulk material. As a result, the KNO@F-C material delivers a high reversible capacity of 173 mA h g(-1) at a current density of 10 mA g(-1), a superior rate performance of 137 mA h g(-1) at 200 mA g(-1) and a remarkable capacitance recovery rate (>100%). In addition, the coated material exhibits 90% capacity retention, demonstrating its long term cycling stability even after 200 cycles. The enhanced electrochemical performance of the coated sample over the pristine material is attributed to its large specific surface area, and a high Na+-ion diffusion coefficient, which facilitates a rapid transfer of electrons and improves grain-to-grain conductivity.