Atomistic Insights into Lithium Storage Mechanisms in Anatase, Rutile, and Amorphous TiO2 Electrodes

Density functional theory calculations were used to investigate the phase transformations of Li_xTiO₂ (at 0 ≤ x ≤ 1), solid-state Li⁺ diffusion, and interfacial charge-transfer reactions in both crystalline and amorphous forms of TiO₂. It is shown that in contrast to crystalline TiO₂ polymorphs, the energy barrier to Li⁺ diffusion in amorphous TiO₂ decreases with increasing mole fraction of Li⁺ due to the changes of chemical species pair interactions following the progressive filling of low-energy Li⁺ trapping sites. Sites with longer Li-Ti and Li-O interactions exhibit lower Li⁺ insertion energies and higher migration energy barriers. Due to its disordered atomic arrangement and increasing Li⁺ diffusivity at higher mole fractions, amorphous TiO₂ exhibits both surface and bulk storage mechanisms. The results suggest that nanostructuring of crystalline TiO₂ can increase both the rate and capacity because the capacity dependence on the bulk storage mechanism is minimized and replaced with the surface storage mechanism. These insights into Li⁺ storage mechanisms in different forms of TiO₂ can guide the fabrication of TiO₂ electrodes to maximize the capacity and rate performance in the future.

Bi-Arrhenius Diffusion and Surface Trapping of ^{8}Li^{+} in Rutile TiO_{2}

We report measurements of the diffusion rate of isolated ion-implanted ^{8}Li^{+} within ∼120  nm of the surface of oriented single-crystal rutile TiO_{2} using a radiotracer technique. The α particles from the ^{8}Li decay provide a sensitive monitor of the distance from the surface and how the depth profile of ^{8}Li evolves with time. The main findings are that the implanted Li^{+} diffuses and traps at the (001) surface. The T dependence of the diffusivity is described by a bi-Arrhenius expression with activation energies of 0.3341(21) eV above 200 K, whereas at lower temperatures it has a much smaller barrier of 0.0313(15) eV. We consider possible origins for the surface trapping, as well the nature of the low-T barrier.

Design and synthesis of interconnected hierarchically porous anatase titanium dioxide nanofibers as high-rate and long-cycle-life anodes for lithium-ion batteries

We suggest an efficient and simple synthetic strategy to prepare interconnected hierarchically porous anatase TiO2 (IHP-A-TiO2) nanofibers by two synergetic effects: phase separation between polymers and relative humidity control during electrospinning. The macro channels formed by polystyrene decomposition were interconnected by numerous mesopores that were formed by evaporation of infiltrated water vapor in the structure. The resulting IHP-A-TiO2 nanofibers showed better Li+ ion storage performances than the TiO2 materials reported in the literature. The discharge capacity of IHP-A-TiO2 nanofibers for the 3000th cycle at 1.0 A g-1 and corresponding coulombic efficiency from the 20th cycle onward were 142 mA h g-1 and >99.0%, respectively. Well-interconnected, ultrafine TiO2 nanocrystals within the nanofiber showed structural stability during cycling and facilitated facile charge transfer at the electrode-electrolyte interface.

The structural and electronic properties of reduced amorphous titania

Reduced amorphous titania has been modeled by removing oxygen atoms to clarify the properties of these materials.

Insights into the Li Diffusion Dynamics and Nanostructuring of H2Ti12O25 To Enhance Its Li Storage Performance

Dodecatitanate H2Ti12O25 crystal has a condensed layered structure and exhibits noteworthy Li storage performance that makes it an anode material with great potential for use in Li-ion batteries. However, an unknown Li diffusion mechanism and a sluggish level of Li dynamics through elongated diffusion paths inside this crystal has impeded any forward development in resolving its limited rate capability and cyclic stability. In this study, we investigated the Li diffusion dynamics inside the H2Ti12O25 crystal that play an essential role in Li storage performance. A study of density functional theory combined with experimental evaluation confirmed a strong dependence of Li storage performance on its diffusion. In addition, a nanostructured H2Ti12O25 containing a bundle of nanorods is developed via the introduction of a kinetic gap during the structural transformation, which conferred a significantly shortened diffusion time/length for Li in H2Ti12O25. The nanostructured H2Ti12O25 has high specific capacity (∼230 mAh g(-1)) and exhibits enhanced cyclic stability and rate capability compared with conventional bulky H2Ti12O25. The H2Ti12O25 proposed in this study has high potential for use as an anode material with excellent safety and stability.

Radial alignment of c-channel nanorods in 3D porous TiO2 for eliciting enhanced Li storage performance

Motivated by anisotropic Li mobility inside a rutile crystal, the c-channel specialized nanorods are radially assembled to form a 3D dendritic TiO₂ sphere, which facilitate Li movement during the charge/discharge process.

localized order-disorder transitions induced by Li segregation in amorphous TiO2 nanoparticles

Li segregation and transport characteristics in amorphous TiO2 nanoparticles (NPs) are studied using molecular dynamics (MD) simulations. A strong intraparticle segregation of Li is observed, and the degree of segregation is found to correlate with Li concentration. With increasing Li concentration, Li diffusivity and segregation are enhanced, and this behavior is tied to the structural response of the NPs with increasing lithiation. The atoms in the amorphous NPs undergo rearrangement in the regions of high Li concentration, introducing new pathways for Li transport and segregation. These localized atomic rearrangements, in turn, induce preferential crystallization near the surfaces of the NPs. Such rich, dynamical responses are not expected for crystalline NPs, where the presence of well-defined lattice sites leads to limited segregation and transport at high Li concentrations. The preferential crystallization in the near-surface region in amorphous NPs may offer enhanced stability and fast Li transport for Li-ion battery applications, in addition to having potentially useful properties for other materials science applications.