Exploration of the Lithium Storage Mechanism in Monoclinic Nb₂O₅ as a Function of the Degree of Lithiation

Insertion-type Nb2O5 is a promising candidate for high-power lithium-ion anodes. Among the various polymorphs, monoclinic Nb2O5 (H-Nb2O5) is considered as one of the most promising materials. Herein, the impact of decreasing the lower cutoff potential, i.e., increasing the amount of lithium that is inserted into the crystal structure, from the commonly used 1.0 V versus Li+/Li to 0.8 V and even 0.01 V is explored, yielding reversible specific capacities of 260, 280, and 400 mAh g(-1), respectively, at a specific current of 0.05 A g(-1). Remarkably, such increase in capacity does not come along with a deterioration of the cycling stability-at least initially. In fact, the comprehensive investigation of the reaction mechanism via operando/ex situ X-ray diffraction, operando/ex situ X-ray absorption spectroscopy, ex situ high-resolution transmission electron microscopy, and operando isothermal microcalorimetry reveals that the extension of the voltage range does not affect the crystal structure during the first couple of cycles, but there is a continuous evolution upon long-term cycling.