The Influence of Size on Phase Morphology and Li‐Ion Mobility in Nanosized Lithiated Anatase TiO₂

Abstract

Sustainable energy storage in the form of Li-ion batteries requires new and advanced materials in particular with a higher power density. Nanostructuring appears to be a promising strategy, in which the higher power density in nanosized materials is related to the dramatically shortened Li-ion diffusion paths. However, nanosizing materials also changes intrinsic material properties, which influence both ionic and electronic conductivity. In this work neutron diffraction is used to show that in addition to these two aspects, nanostructuring changes the phase behavior and morphology. Lithiated 40-nm TiO(2) anatase crystallites become single phase, either having the Li-poor original anatase phase, or the Li-rich Li-titanate phase, in contrast to microsized crystallites where these two phases coexist in equilibrium within one crystal particle. In addition, Li(x)TiO(2) compositions occur with stoichiometries that are not stable in micron-sized crystallites, indicating enhanced solid solution behavior. Reduced conduction electron densities at the sites of the Li ions are observed by NMR spectroscopy. This is accompanied by reduced spontaneous Li-ion mobility, suggesting a correlation between the electron density at the Li-ion site and the Li-ion mobility. The present results show that in the case of lithiated anatase TiO(2), significant effects on phase composition, morphology, and electronic configurations are induced, as well as slower intracrystallite Li diffusion.

GGA+Udescription of lithium intercalation into anataseTiO2

“…10,11 In nanoscale samples it has been reported that this phase separation is suppressed with the two phases coexisting within a single domain. Stoichiometries that are normally unstable in the bulk become accessible, suggesting a synthetic route to achieving increased charge densities, 12,13 and the electrochemical behavior of lithium insertion into nanotubular anatase has been studied by a number of groups. [14][15][16][17] The historical interest in anatase TiO 2 means that lithium intercalation is relatively well characterized experimentally for the bulk system, providing useful reference data for theoretical studies, and making it a prototypical system when considering the properties of more exotic polymorphs and morphologies.…”

Equilibrium Lithium‐Ion Transport Between Nanocrystalline Lithium‐Inserted Anatase TiO2 and the Electrolyte

“…The mobile lithium-ion species in the electrolyte result in a relatively narrow central resonance with a linewidth of approximately 500 Hz, whereas lithium in the electrode material gives rise to a broad signal with a much lower peak height and a linewidth of approximately 5 kHz consistent with earlier findings. [22] It should be realized that at this composition, Li 0.5 TiO 2 is fully converted into the lithium titanate structure and hence has a single crystalline phase. The T 1 relaxation time for lithium in the solid Li 0.5 TiO 2 was determined to be 3-5 s and for lithium in the electrolyte to range from 0.3-1 s depending on the composition of the electrolyte and temperature.…”

Phase separation in lithium intercalated anatase: A theory

“…Recent NMR spectroscopy study [35] of nanosized lithiated anatase revealed further important details of the phase behavior and morphology. The coexistence of the Li-poor and the Li-rich phases is possible only in the particles of the size exceeding 120 nm due to the surface strain (occurring between the phases) which becomes energetically unfavourable in small particles.…”