Li⁺ Diffusion in Li_nCoNb₂O₆ (0 < n ≤ 6) Anode with High Capacity Density: Fast Kinetics and Mechanistic Insights

The potential of high power/capacity density and Li⁺ solid diffusion mechanisms of niobium-based binary metal oxide (CoNb₂O₆) anode material are investigated by combining high-rate Nb₂O₅ with the redox-active 3d transition metal Co. CoNb₂O₆ exhibited exceptional rate capability and cycling stability, which is attributed to anisotropic expansion during cycling and dual diffusion mechanisms at high and low lithium concentrations. The anisotropic expansion of crystals ensures structural stability, whereas the organic combination of a direct-hopping diffusion mechanism in Li_nCoNb₂O₆ (0 ≤ n ≤ 3) and a knock-off diffusion mechanism in Li_nCoNb₂O₆ (3 < n ≤ 6) based on the nudged elastic band (NEB) calculations reveals rapid Li⁺ solid diffusion and excellent rate performance during lithiation/delithiation. The electrochemical performance of CoNb₂O₆ also depends on its morphology, where different structures modulate synergistic Nb and Co interactions, influencing Li⁺ diffusion in the Nb layers. Specifically, the micron-scale structure formed by secondary particle attachment (CoNb₂O₆-MP) provides space for anisotropic expansion, fully utilizing the dual ion diffusion mechanism, enhancing diffusion efficiency, and delivering both high-capacity density and excellent rate performance. This work not only introduces CoNb₂O₆ with superior electrochemical properties but also provides insights into the solid diffusion mechanisms under various lithium concentrations, offering a foundation for designing electrode materials with enhanced ion diffusion pathways.