High-Rate Intercalation without Nanostructuring in Metastable Nb2O5 Bronze Phases

Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb2O5 are inherent properties of the bulk crystal structure. Their unique "room-and-pillar" NbO6/NbO7 framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g–1), while the higher temperature (Wadsley–Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g–1) but only at moderate rates. High-resolution 6/7Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb2O5 revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)–98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb2O5 synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs.