Abstract
The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.
Highlights
The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery
V2O5 is intercalated with Li ions, it undergoes a series of phase transformations, to a puckered e-phase (a); on further lithiation, the e-phase transforms with an in-plane shift to a d-phase (b). (c) Scanning electron microscopy images depict V2O5 nanowires with lengths spanning hundreds of micrometres. (d) High-resolution transmission electron microscopy (TEM) image of an individual V2O5 wire, indicating the separation between the (711) lattice planes of orthorhombic V2O5
The experimental results in concert with the calculations indicate that local structural distortions and the stabilization of small polarons impede electron diffusion within V2O5 and give rise to distinctive lithiation gradients[42,59]
Summary
The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. V2O5 was first proposed as a Li-ion intercalation host by Whittingham[18], owing to the following: the abundance of interlayer sites that can accommodate Li ions; the readily accessible V5 þ /V4 þ and V4 þ /V3 þ redox couples; and the strong enthalpic driving forces for Li-ion insertion within this structure[12,17,18,19] Despite these promising attributes, the poor high-rate performance of these materials and issues with retention of capacity over prolonged cycling have limited the widespread commercial development of this material. The top inset shows a low-magnification TEM image of several nanowires (scale bar, 0.2 mm), whereas the bottom inset indicates an indexed selected-area electron diffraction pattern (scale bar, 5 nm À 1). (e) XANES measurements of stoichiometrically lithiated V2O5 depict a reduction of the 3dxy resonance at the V L-edge and a diminution of the t2g to eg* ratio at the O K-edge with increasing lithiation
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