Cobalt and zinc columbite compounds as new anode materials for Na-ion batteries

Niobium-based oxide anode materials are promising anode materials for sodium ion batteries because of their high structural stability, safety, fast energy storage kinetics, and promising capacity. In this work, niobate compounds of the columbite type, ANb₂O₆ (A = Co²⁺ and Zn²⁺), prepared by a facile sol-gel method, are investigated as new anode materials for sodium ion batteries. The poor intrinsic electronic conductivity of the niobate anode materials is successfully overcome by coating the active materials with carbon in order to enhance Na-ion transport and storage. High-resolution transmission electron microscopy images reveal uniform coating of the carbon layer around the active particles to prevent agglomeration in both materials. While the carbon-coated material CoNb₂O₆ (CNO) at a current density of 25 mA g^-1 delivered a high reversible capacity of 295 mA h g^-1, the carbon-coated ZnNb₂O₆ (ZNO) material yielded 262 mA h g^-1. However, the ZNO material delivered a significant cycling stability at a current density of 200 mA g^-1, which corresponds to a capacity retention of 72% after 50 cycles. The difference in their electrochemical performances is related to the structural defects, specific surface areas, charge transfer resistances during charge-discharge cycles, Na⁺-ion diffusion coefficients, and the contribution of capacitive and diffusive behaviours to the total capacity. The present work not only introduces columbite compounds as anode materials with promising electrochemical properties but also provides insights into the development of potential anode materials for sodium ion battery technology and offers the foundation for designing new electrode materials with enhanced ion-diffusion pathways.