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

Insertion-type Nb2O5is a promising candidate for high-power lithium-ion ano-des. Among the various polymorphs, monoclinic Nb2O5(H-Nb2O5) is consideredas one of the most promising materials. Herein, the impact of decreasing thelower cutoff potential, i.e., increasing the amount of lithium that is inserted intothe crystal structure, from the commonly used 1.0 V versus Liþ/Li to 0.8 V andeven 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, suchincrease in capacity does not come along with a deterioration of the cycling stability—at least initially. In fact, the comprehensive investigation of the reactionmechanism viaoperando/ex situX-ray diffraction,operando/ex situX-rayabsorption spectroscopy,ex situhigh-resolution transmission electron micros-copy, andoperandoisothermal microcalorimetry reveals that the extension of thevoltage range does not affect the crystal structure during thefirst couple of cycles,but there is a continuous evolution upon long-term cycling.