Accelerating the Design of Multishell Upconverting Nanoparticles through Bayesian Optimization.

The photon upconverting properties of lanthanide-doped nanoparticles drive their applications in imaging, optoelectronics, and additive manufacturing. To maximize their brightness, these upconverting nanoparticles (UCNPs) are often synthesized as core/shell heterostructures. However, the large numbers of compositional and structural parameters in multishell heterostructures make optimizing optical properties challenging. Here, we demonstrate the use of Bayesian optimization (BO) to learn the structure and design rules for multishell UCNPs with bright ultraviolet and violet emission. We leverage an automated workflow that iteratively recommends candidate UCNP structures and then simulates their emission spectra using kinetic Monte Carlo. Yb3+/Er3+- and Yb3+/Er3+/Tm3+-codoped UCNP nanostructures optimized with this BO workflow achieve 10- and 110-fold brighter emission within 22 and 40 iterations, respectively. This workflow can be expanded to structures with higher compositional and structural complexity, accelerating the discovery of novel UCNPs while domain-specific knowledge is being developed.

Evaluation of Amorphous Oxide Coatings for High-Voltage Li-Ion Battery Applications Using a First-Principles Framework.

Cathode surface coatings are widely used industrially as a means to suppress degradation and improve electrochemical performance of lithium-ion batteries. However, developing an optimal coating is challenging, as different coating materials may enhance one aspect of performance while hindering another. To elucidate the fundamental thermodynamic and transport properties of amorphous cathode coating materials, here, we present a framework for calculating and analyzing the Li+ and O2- transport and the stability against delithiation in such materials. Our framework includes systematic workflows of ab-initio molecular dynamics calculations to obtain amorphous structures and diffusion trajectories coupled with an analysis of critical changes of the active-ion local environment during diffusion. Based on these data, we provide an estimate of room-temperature diffusivities, including statistical error bars, and the evaluation of the coating suitability in terms of its ability to facilitate Li+ transport while blocking O2- transport. Finally, we add the thermodynamic stability analysis of the coating chemistry within the operating voltage of common Li-ion cathodes. We apply this framework to two commonly used amorphous coating materials, Al2O3 and ZnO. We find that (1) in general, a higher Li+ content increases both Li+ and O2- diffusivities in both Al2O3 and ZnO. Also, Li+ and O2- diffuse much faster in ZnO than in Al2O3. (2) However, neither Al2O3 nor ZnO is expected to retain a significant concentration of Li+ at high charge. (3) ZnO performs much more poorly in terms of O2- blocking, and hence, Al2O3 is preferred for high-voltage cathode applications. These results will help to quantitatively evaluate amorphous materials, such as metal oxides and fluorides, for different performance metrics and facilitate the development of optimal cathode coatings.

Mechanical Properties and Chemical Reactivity of Li xSiO y Thin Films.

Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO2 on Si is often inevitable. However, it is not clear if this layer has a positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties of the SiO2 lithiation products and by the convolution of chemical and electrochemical effects during the anode lithiation process. In this study, Li xSiO y thin films as model materials for lithiated SiO2 were deposited by magnetron sputtering at ambient temperature, with the goal of (1) decoupling chemical reactivity from electrochemical reactivity and (2) evaluating the physical and electrochemical properties of Li xSiO y. X-ray photoemission spectroscopy analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO2 can be achieved through sputtering. Our density functional theory calculations also confirm that the possible phases formed by lithiating SiO2 are very close to the measured film compositions. Scanning probe microscopy measurements show that the mechanical properties of the film are strongly dependent on lithium concentration, with a ductile behavior at a higher Li content and a brittle behavior at a lower Li content. The chemical reactivity of the thin films was investigated by measuring the AC impedance evolution, suggesting that Li xSiO y continuously reacts with the electrolyte, in part because of the high electronic conductivity of the film determined from solid-state impedance measurements. The electrochemical cycling data of the sputter-deposited Li xSiO y/Si films also suggest that Li xSiO y is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties.

Mechanical Properties and Chemical Reactivity of LixSiOy Thin Films.

Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO2 on Si is often inevitable. However, it is not clear if this layer has positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties, and by convolution of chemical and electrochemical effects during the anode lithiation process. In this study, LixSiOy thin films as model materials for lithiated SiO2 were deposited by magnetron sputtering at ambient temperature, with the goal of 1) decoupling chemical reactivity from electrochemical reactivity, and 2) evaluating the physical and electrochemical properties of LixSiOy. XPS analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO2, can be achieved through sputtering. Our density functional theory calculations also confirm that possible phases formed by lithiating SiO2 are very close to the measured film compositions. Scanning probe microscopy measurements show the mechanical properties of the film are strongly dependent on lithium concentration, with ductile behavior and higher Li content and brittle behavior at lower Li content. Chemical reactivity of the thin films was investigated by measuring AC impedance evolution, suggesting that LixSiOy continuously reacts with electrolyte, in part due to high electronic conductivity of the film determined from solid state impedance measurements. Electrochemical cycling data of sputter deposited LixSiOy/Si films also suggest that LixSiOy is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties.