Mitigating Cation Diffusion Limitations and Intercalation-Induced Framework Transitions in a 1D Tunnel-Structured Polymorph of V2O5
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2017 American Chemical Society. The design of cathodes for intercalation batteries requires consideration of both atomistic and electronic structure to facilitate redox at specific transition metal sites along with the concomitant diffusion of cations and electrons. Cation intercalation often brings about energy dissipative phase transformations that give rise to substantial intercalation gradients as well as multiscale phase and strain inhomogeneities. The layered -V2O5 phase is considered to be a classical intercalation host but is plagued by sluggish diffusion kinetics and a series of intercalation-induced phase transitions that require considerable lattice distortion. Here, we demonstrate that a 1D tunnel-structured -phase polymorph of V2O5 provides a stark study in contrast and can reversibly accommodate Li-ions without a large distortion of the structural framework and with substantial mitigation of polaronic confinement. Entirely homogeneous lithiation is evidenced across multiple cathode particles (in contrast to -V2O5 particles wherein lithiation-induced phase transformations induce phase segregation). Barriers to Li-ion as well as polaron diffusion are substantially diminished for metastable -V2O5 in comparison to the thermodynamically stable -V2O5 phase. The rigid tunnel framework, relatively small changes in coordination environment of intercalated Li-ions across the diffusion pathways defined by the 1D tunnels, and degeneracy of V 3d states at the bottom of the conduction band reduce electron localization that is a major impediment to charge transport in -V2O5. The 1D -phase thus facilitates a continuous lithiation pathway that is markedly different from the successive intercalation-induced phase transitions observed in -V2O5. The results here illustrate the importance of electronic structure in mediating charge transport in oxide cathode materials and demonstrates that a metastable polymorph with higher energy bonding motifs that define frustrated coordination environments can serve as an attractive intercalation host.
