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 a low-temperature calcination at 800 ºC. Further, 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 the carbon-coated Nb18W16O93 (NWO) delivered a higher reversible capacity of 163.95 mA.h.g-1 compared to that of the carbon-coated Nb7W10O47 (WNO) (127.55 mA.h.g-1). Interestingly, the WNO material showed a 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.