Comparative Insight into Crystal Evolutions of Monodisperse MNb2O6 (M = Ni, Cu, Zn) Spheres During Electrochemical Cycling in Li/Al-ion Batteries

Niobates have emerged as advanced anode materials for lithium-ion batteries, owing to their inherently higher voltage and the occurrence of multiple one-electron redox reactions. However, there is a lack of comprehensive comparative analysis regarding their performance in lithium-ion and aluminum-ion battery applications. In this study, a facile solvothermal process is developed to synthesize uniform submicrospheres of three niobates: NiNb2O6, CuNb2O6, and ZnNb2O6 with large specific surface area and narrow size distribution. Their superior textural properties consisting of interconnected nanograins offer numerous active sites, delivering enhanced capacity and rate performance for the constructed half and full cells. Our in-situ XRD analysis reveals that all three niobates exhibit ultralow crystal volume expansion (< 6 %). But they exhibit distinct ion storage mechanisms: CuNb2O6 undergoes a phase transition to LiNb3O8 and NbO2 upon cyclic insertion of Li+ ions; NiNb2O6 mainly involves a classic hybrid-controlled process, facilitating easy ion insertion/extraction into/from the crystal lattices, especially at low sweep rates; while ZnNb2O6 stores charges via a pseudocapacitive-controlled process. Density functional theory (DFT) calculation and ex-situ XRD analysis confirm that CuNb2O6 outperforms the others, displaying the best electrochemical performance and holding promise as a high-energy electrode material thanks to favorable phase conversion, enhanced transport channels, porous structure and spherical morphology.