Ziheng Lu

Structural origin of the superionic Na conduction in Na2B10H10closo-borates and enhanced conductivity by Na deficiency for high performance solid electrolytes

Recently reported superionic Na closo-borates have drawn considerable attention due to their potential as solid-state electrolytes for Na ion batteries. However, a fundamental understanding of the ion transport mechanism in these materials is still missing. We studied Na conduction in Na2B10H10, a model material, using first-principles calculations. We found that the superior Na diffusivity is closely linked to the behavior of the large B10H102− anionic groups. The reorientations and disorder of these groups facilitate the Na+ hopping to the octahedral (Oh) sites, which link the tetrahedral (Td) sites to form a connected diffusion network. We also found that, in spite of the frequent Na hopping events, the diffusional paths are often blocked, thereby hindering Na transport. Such a mechanism suggests that the Na conductivity can be improved by introducing extrinsic vacancies. By simulating Na-deficient systems, we found that at high temperatures, even a minimal amount of additional Na vacancies significantly increases the ionic diffusivity.

Data Mining of Molecular Dynamics Data Reveals Li Diffusion Characteristics in Garnet Li 7 La 3 Zr 2 O 12

Understanding Li diffusion in solid conductors is essential for the next generation Li batteries. Here we show that density-based clustering of the trajectories computed using molecular dynamics simulations helps elucidate the Li diffusion mechanism within the Li7La3Zr2O12 (LLZO) crystal lattice. This unsupervised learning method recognizes lattice sites, is able to give the site type, and can identify Li hopping events. Results show that, while the cubic LLZO has a much higher hopping rate compared to its tetragonal counterpart, most of the Li hops in the cubic LLZO do not contribute to the diffusivity due to the dominance of back-and-forth type jumps. The hopping analysis and local Li configuration statistics give evidence that Li diffusivity in cubic LLZO is limited by the low vacancy concentration. The hopping statistics also shows uncorrelated Poisson-like diffusion for Li in the cubic LLZO, and correlated diffusion for Li in the tetragonal LLZO in the temporal scale. Further analysis of the spatio-temporal correlation using site-to-site mutual information confirms the weak site dependence of Li diffusion in the cubic LLZO as the origin for the uncorrelated diffusion. This work puts forward a perspective on combining machine learning and information theory to interpret results of molecular dynamics simulations.

Formation and evolution of a hairpin vortex induced by subharmonic sinuous low-speed streaks

In this paper, a process of the formation and evolution of hairpin vortices, which originated from the interaction between the spanwise-aligned low-speed streaks with a subharmonic sinuous (SS) oscillation mode, is studied using a direct numerical simulation method in a small periodic local region of an incompressible plane channel flow. The initial artificial perturbations are used to excite the SS-mode oscillation of two spanwise-aligned low-speed streaks in such a flow. A new mechanism of formation and decay of the hairpin vortices is proposed in which the shear layer induced by the spanwise collision and merging between the low-speed streaks is emphasized. Our results show that the streamwise vortices can be induced by the SS-mode streaks and then developed into an X-like pattern at the initial stage due to the mutual induction effect. The X-like vortices further enhance the spanwise oscillation and lift-up of the two streaks that thus lead to the spanwise collision and merging of the low-speed streaks and produce a low-speed region in high-speed fluid. The strong shear layer between the high- and low-speed fluids gives rise to the spanwise vorticity that connects the X-like streamwise vortices and forms the Λ-like vortex. Once the low-speed region entirely enters the high-speed fluid, the shear layer shows the ring shape and results in the transition from a Λ-like vortex to Ω-like one. After that, the viscous diffusion of the low-speed region in the high-speed fluid leads to the decay of the Ω-like vortex; the collision and merging of the low-speed streaks simultaneously reoccur upstream and give birth to a secondary Λ-like vortex, which exhibits behavior that is nearly similar with that of the primary one. Although the hairpin vortex packet is not observed in the present plane channel flow, the regeneration of the hairpin vortex suggests that this type of vortical structure plays an important role in the wall-bounded flow.

Anti-perovskite cathodes for lithium batteries

It was recently discovered that Li2FeChO (Ch = S, Se, Te) anti-perovskites exhibit an outstanding rate capability and a good discharge capacity as Li-ion battery cathodes. In this work, we use density functional theory calculations to study the origin of the electrochemical characteristics of anti-perovskite cathodes using Li2FeSO as a model material. We evaluate the phase stability, ion-transport, and structural stability of this material and, interestingly, found that Li2FeSO is meta-stable at 0 K. The experimentally observed anti-perovskite phase likely originates from either entropy stabilization or sluggish decomposition kinetics. When delithiated, the stability of the material worsens. Additionally, over-delithiation may lead to irreversible structural change. The good rate capability of Li2FeSO is attributed to the high Li conductivity, where the low Li ion diffusion barrier (∼0.32 eV) rivals those of state-of-art superionic Li conductors. In the search for materials with a voltage higher than that of Li2FeSO, we screen Co, Cr, Cu, Mn, Mo, Ni, and V substitutions into the Fe site of Li2FeSO. While Cu and Ni substitutions may offer higher average voltages, the corresponding materials are less stable than Li2FeSO.

Energetics of Nanoparticle Exsolution from Perovskite Oxides

The presence of active metal nanoparticles on the surface significantly increases the electrochemical performance of ABO3 perovskite oxide materials. While conventional deposition methods can improve the activity, in situ exsolution produces nanoparticles with far greater stability. The migration of transition metal atoms toward the surface is expected to affect the exsolution process. To study the energetics, we use ab initio computations combined with experiments in a SrTiO3-based model system. Our calculations show that Ni preferentially segregates toward the (100)-oriented and SrTiO-terminated surfaces, note that this orientation is identical to one reported by the Irvine and Gorte groups. Vacancies in the Sr-site and O-site promote the segregation of Ni, while placing La on the Sr-site has an opposite effect. The corresponding experiments are in agreement with the computational predictions. Fast nanoparticle growth and activity enhancement are found in STO system with Sr vacancies and without La. The approach developed in this Letter could be used to study the mechanism of exsolution in other material systems, and possibly lead to the development of new compositions capable of nanoparticle exsolution with higher activity and stability.