Limitations in electrochemical performance as well as supply chain challenges have rendered positive electrode materials a critical bottleneck for Li-ion batteries. State-of-the-art Li-ion batteries fall short of accessing theoretical capacities. As such, there is intense interest in the design of strategies that enable the more effective utilization of active intercalation materials. Pre-intercalation with alkali-metal ions has attracted interest as a means of accessing higher reversible capacity and improved rate performance. However, the structural basis for improvements in electrochemical performance remains mostly unexplored. Here we use topochemical single-crystal-to-single-crystal transformations in a tunnel-structured ζ-V2O5 positive electrode to illustrate the effect of pre-intercalation in modifying the host lattice and altering diffusion pathways. Furthermore, operando synchrotron X-ray diffraction is used to map Li-ion site preferences and occupancies as a function of the depth of discharge in pre-intercalated materials. Na- and K-ion intercalation 'props open' the one-dimensional tunnel, reduces electrostatic repulsions between inserted Li ions and entirely modifies diffusion pathways, enabling orders of magnitude higher Li-ion diffusivities and accessing higher capacities. Deciphering the atomistic origins of improved performance in pre-intercalated materials on the basis of single-crystal-to-single-crystal topochemical transformation and operando diffraction studies paves the way to site-selective modification approaches for positive electrode design.
Stereoactive electron lone pairs derived from filled 5/6s2 states of p-block cations are an intriguing electronic and geometric structure motif that have been exploited for diverse applications such as thermoelectrics, thermochromics, photocatalysis, and nonlinear optics. Layered trivanadates are dynamic intercalation hosts, where the insertion of cations can be used to tune electron correlation, charge localization, and magnetic ordering. However, the interaction of 5/6s2 stereoactive electron lone pairs with layered trivanadates remains unexplored. In this study, we contrast s- and p-block trivanadates and map off-centering in the coordination environment and reduction in symmetry arising from the stereochemical activity of lone pair cations to the emergence of filled antibonding lone-pair 6s2-O 2p hybridized states. The former is studied by high-resolution single-crystal X-ray diffraction studies of TlV3O8 and isostructural RbV3O8 to probe distinct differences in Tl and Rb coordination environments and the resulting modulation of V-V interactions in V3O8 slabs. The latter has been probed by variable-energy hard X-ray photoelectron spectroscopy (HAXPES) measurements, which manifest orbital-specific contributions from bonding and antibonding interactions of stereoactive Tl 6s2 electron lone pairs in TlV3O8. The spectroscopic assignment of valence band states to stereoactive lone pairs is further corroborated by first-principles electronic structure calculations, crystal orbital Hamilton population analyses, and electron localization function maps. The presence of the Tl 6s2 electron lone pair in TlV3O8 brings about the off-centering of Tl+ cations, which leads to anisotropy in Tl-O bonds. The off-centering of Tl ions weakens V-O bonds in one direction, which subsequently strengthens directional V-V coupling. Magnetic measurements reveal ferromagnetic signatures for both RbV3O8 and TlV3O8. However, the differences in V···V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 140 K for TlV3O8 as compared to 125 K for RbV3O8. The results demonstrate the distinctive effects of stereochemically active lone pairs in modifying electronic structure near the Fermi level and for mediating superexchange interactions.
