Carbon-coated niobium tungsten oxides as new and efficient anode materials for Na-ion batteries

Abstract

Tetragonal tungsten bronze (TTB)-type niobium–tungsten oxides are very interesting anode candidates for Na-ion battery applications due to their compositional flexibility, structural stability and large open-tunnel networks, which enable fast ion transport and pseudo capacitive behaviour. The present work introduces two TTB-type niobium tungstate anode materials, namely Nb₁₈W₁₆O₉₃ (Nb dominant) and Nb₇W₁₀O₄₇ (W dominant), prepared by a one-step hydrothermal method followed by low-temperature calcination at 800 °C. Furthermore, the low inherent electronic conductivity of the anode materials is effectively enhanced by incorporating the active materials with a carbon matrix to improve Na⁺-ion transport and storage. The presence of uniform carbon-coating surrounding the active particles in both materials is confirmed by high-resolution transmission electron microscopy images. At a current density of 10 mA g⁻¹, carbon-coated Nb₁₈W₁₆O₉₃ (NWO) delivered a higher reversible capacity of 163.95 mA h g⁻¹ compared to that of carbon-coated Nb₇W₁₀O₄₇ (WNO) (127.55 mA h g⁻¹). Interestingly, the WNO material showed remarkable cycling stability over 420 cycles, at a current density of 100 mA g⁻¹, recording a higher capacity retention of 81%, compared to NWO (69%). The promising electrochemical performance of the materials is related to structural defects, specific surface areas, charge transfer resistances during charge/discharge cycles and Na⁺-ion diffusion coefficients. Thus, the current work introduces niobium tungsten oxides as new and efficient anode candidates for sodium ion battery technology, and also enables safe, economical, and long-cycling Na-ion batteries, ultimately supporting the transition toward more sustainable energy technologies.