High-Rate Electrochemistry in Bulk (um) Particles: NMR Insights into Nb2O5
TT (pseudohexagonal), T (orthorhombic), B (monoclinic), and H (monoclinic) polymorphs of Nb2O5 were synthesised by calcining NbO2 for 24 hours over a range of temperatures. In addition to low cost and ease of synthesis, solid state preparative methods were selected in an effort to characterise inherent properties of the bulk crystal structure of the various polymorphs, as it is well known that nanostructuring can fundamentally change the kinetics and thermodynamics of intercalation reactions. Samples were prepared at every 50 °C interval from 200 °C to 1100 °C and analysed by x-ray diffraction (XRD). Electrochemically, the Wadsley–Roth type shear structure of H-Nb2O5 exhibited the highest capacity, reversibly intercalating more than one electron per transition metal ion. Akin to the nanostructured mesoporous oxides, TT-Nb2O5 and T-Nb2O5 from NbO2displayed impressive intercalation kinetics and no phase change upon discharge-charge cycling; this result is somewhat unexpected considering the large (μm) scale of the particles in this investigation.
Solid state NMR was employed to examine lithiation mechanisms and energy barriers within the high-rate (T) and high-capacity (H) phases. Results will be presented from ex situ magic angle spinning (MAS) 93Nb, 17O, and 7Li solid state NMR experiments, which were performed on pristine niobia samples and cycled coin cell cathodes to examine the different local atomic environments within each structural moiety and to correlate local and long range structural changes and lithium transport with rate performance for the different polymorphs. Changes in NMR chemical shift, peak shape, intensity, and relaxation parameters were observed as a function of degree of lithiation and in variable temperature experiments.
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