Lithium migration at low concentration in TiO 2 polymorphs

High-Reliability and Self-Rectifying Alkali Ion Memristor through Bottom Electrode Design and Dopant Incorporation

Ionic memristor devices are crucial for efficient artificial neural network computations in neuromorphic hardware. They excel in multi-bit implementation but face challenges like device reliability and sneak currents in crossbar array architecture (CAA). Interface-type ionic memristors offer low variation, self-rectification, and no forming process, making them suitable for CAA. However, they suffer from slow weight updates and poor retention and endurance. To address these issues, the study demonstrated an alkali ion self-rectifying memristor with an alkali metal reservoir formed by a bottom electrode design. By adopting Li metal as the adhesion layer of the bottom electrode, an alkali ion reservoir was formed at the bottom of the memristor layer by diffusion occurring during the atomic layer deposition process for the Na:TiO2 memristor layer. In addition, Al dopant was used to improve the retention characteristics by suppressing the diffusion of alkali cations. In the memristor device with optimized Al doping, retention characteristics of more than 20 h at 125 °C, endurance characteristics of more than 5.5 × 105, and high linearity/symmetry of weight update characteristics were achieved. In reliability tests on 100 randomly selected devices from a 32 × 32 CAA device, device-to-device and cycle-to-cycle variations showed low variation values within 81% and 8%, respectively.

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.