Abstract
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 (6/7)Li 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.
Highlights
There is a growing need for high-power, high-capacity energy storage materials for applications that require faster rate performance than traditional battery materials can offer, along with higher charge storage capability than can be achieved with supercapacitor systems
Thermal gravimetric analysis of NbO2 and Nb (Supporting Information, Figure S2) showed that the onset of oxidation for NbO2 occurs at a temperature significantly below that for oxidation of Nb metal 290 °C versus 420 °C allowing a greater range of metastable phases to be prepared with this starting material
A systematic X-ray diffraction investigation revealed that four from the range of polymorphs with a nominal composition of Nb2O5 were observed upon thermal oxidation of NbO2 (Figure 2)
Summary
There is a growing need for high-power, high-capacity energy storage materials for applications that require faster rate performance than traditional battery materials can offer, along with higher charge storage capability than can be achieved with supercapacitor systems. Electric double-layer capacitors (EDLCs) can deliver high rate performance but are limited to relatively low volumetric and areal energy densities as redox reactions offer the opportunity for 10−100 times greater charge storage than the electrostatic mechanism of EDLCs.[1,2] In lithium-ion batteries, realization of the maximum capacity of an electrode material in a given potential window is inherently dependent upon the ability of lithium to reach the particle interior. This has generally limited high rate performance to materials with short diffusion distances typically achieved via nanoscaling or nanostructuring of the particles. The disadvantages of the synthesis and use of nanoparticles and nanoarchitectures for battery applications are well known: high surface area leading to increased dissolution and increased surface−electrolyte interface (SEI) formation, low packing density, toxicity, high cost, chemical waste generation, scalability issues, and often many-step synthetic complexity.[3−5] Preparation of energy-dense materials with high capacity and good rate performance through a simple and efficient synthetic route is clearly desirable but evidence from, e.g., Li4Ti5O12, LiFePO4, and TiO2 suggests that this is not generally observed
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