DIRECT Sampling for Robust MLPs

Ji’s work on “Robust training of machine learning interatomic potentials with dimensionality reduction and stratified sampling” is now out in npj Computational Materials! Machine learning interatomic potentials (MLIPs) enable accurate simulations of materials at scales beyond that accessible by ab initio methods. In this work, we present DImensionality-Reduced Encoded Clusters with sTratified (DIRECT) sampling as an approach to select a robust training set of structures from a large and complex configuration space. By applying DIRECT sampling on the Materials Project relaxation trajectories dataset with over one million structures and 89 elements, we develop an improved materials 3-body graph network (M3GNet) universal potential that extrapolates more reliably to unseen structures. We further show that molecular dynamics (MD) simulations with the M3GNet universal potential can be used instead of expensive ab initio MD to rapidly create a large configuration space for target systems. We combined this scheme with DIRECT sampling to develop a reliable moment tensor potential for titanium hydrides without the need for iterative augmentation of training structures. Check out this work here. If you want to use DIRECT sampling for your work, please check out our implementation available on our MAML repository on Github.

Li diffusivity at the grain boundaries

Randy’s work on “Lithium dynamics at grain boundaries of β-Li3PS4 solid electrolyte” has just been published in Energy Advances! Randy was a visiting scientist in the Materials Virtual Lab from NIMS Japan in 2021-2023. Lithium diffusivity at the grain boundaries of solid electrolytes (SEs) can strongly impact the final performance of all-solid-state Li ion batteries (SSLBs). In this study, we systematically investigate the Li ion transport in tilt and twist GBs as well as amorphous/crystal interfaces of β-Li3PS4 by performing large-scale molecular dynamics (MD) simulations with a highly accurate moment tensor interatomic potential (MTP). We find that the Li ion conductivities at the GBs and amorphous/crystal interfaces are 1–2 orders of magnitude higher than that in the bulk crystal. The Li pathway network in twist GBs and amorphous/crystal interfaces comprises persisting large Li ring sub-networks that closely resemble those found in the bulk amorphous structure, whereas more smaller and short-lived Li ring sub-networks are detected in tilt GBs and the bulk crystal. The concentration of persisting large Li ring sub-networks in the GB and amorphous/crystal interfaces is directly proportional to the degree of Li site disordering which in turn correlates with GB conductivity. Our findings provide useful insights that can […]

Compositionally complex perovskite solid electrolytes

CCPO

The Materials Virtual Lab is proud to be part of a collaborative work titled “Compositionally complex perovskite oxides: Discovering a new class of solid electrolytes with interface-enabled conductivity improvements” recently published in Cell Press Matter! This work was performed under the Materials Research Science and Engineering Center (MRSEC) at UC Irvine together with the groups of Prof Xiaoqing Pan @ UCI and Prof Jian Luo @ UCSD. Compositionally complex ceramics (CCCs), including high-entropy ceramics, offer a vast, unexplored compositional space for materials discovery. Here, we propose non-equimolar compositionally complex perovskite oxide (CCPO) solid electrolytes with improved lithium ionic conductivities beyond the limit of conventional doping. For example, we demonstrate that the ionic conductivity can be improved by >60% in (Li0.375Sr0.4375)(Ta0.375Nb0.375Zr0.125Hf0.125)O3-d. MAVRL group member, Ji Qi, developed a machine learning interatomic potential (MLIP) for this 7-component CCPO using an active learning protocol, and demonstrated this enhanced ionic conductivity can be attributed to enhanced GB diffusitivity that is related to the absence of Li depletion at GB regions, which is observed in the resistive GB of the LLTO. This work suggests new routes for discovering and tailoring CCCs for energy storage and many other applications. Check out this work here.

243rd ECS Meeting Presentation on Machine Learning for Solid State Batteries: Progress vs Hype

Prof Shyue Ping Ong gave a talk at the 243rd Electrochemical Society (ECS) Meeting in Boston on May 30 2003 on “Machine Learning for Solid State Batteries: Progress vs Hype:. Machine learning in materials science and solid-state batteries are two topics that have captured the imagination of researchers in recent years. Therefore, it is unsurprising that many researchers have attempted to apply the advances in ML to the discovery and study of materials for solid-state batteries. In this talk, Prof Ong discusses the importance of going back to fundamentals in the application of ML to materials for solid-state batteries. I will highlight areas where ML has had a transformative impact on our understanding and discovery of solid electrolytes, and what are some of the remaining challenges that remain to be surmounted.

Multi-scale investigation of MoNbTi and TaNbTi MPEAs

Hui’s swansong collaborative work on “Multi-scale investigation of short-range order and dislocation glide in MoNbTi and TaNbTi multi-principal element alloys” is now out in npj Computational Materials! This is a really exciting work that showcases an electron (DFT) to atom (machine learning interatomic potentials) to continuum (phase field dislocation dynamics) approach to the study of materials. It is a collaborative work between Hui and Lauren Fey of the group of Irene Beyerlein at UC Santa Barbara. Refractory multi-principal element alloys (RMPEAs) are promising materials for high-temperature structural applications. In this work, we performed an electron-to-atom-to-continuum study of the role of short-range ordering (SRO) on dislocation glide in the MoNbTi and TaNbTi RMPEAs. Monte carlo/molecular dynamics simulations with a moment tensor potential show that MoNbTi exhibits a much greater degree of SRO than TaNbTi and the local composition has a direct effect on the unstable stacking fault energies (USFEs). From mesoscale phase-field dislocation dynamics simulations, we find that increasing SRO leads to higher mean USFEs and stress required for dislocation glide. The gliding dislocations experience significant hardening due to pinning and depinning caused by random compositional fluctuations, with higher SRO decreasing the degree of USFE dispersion and hence, amount of hardening. […]

GB resistance in oxide solid electrolytes

Ji Qi’s co-first author paper on “Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolyte Li0.375Sr0.4375Ta0.75Zr0.25O3 (LSTZ0.75)” has been published in Nature Communications! This is a highly-collaborative work under the Center for Complex and Active Materials (CCAM), an NSF MRSEC. Perovskite solid electrolytes for all-solid-state lithium-ion batteries are often plagued by grain boundary (GB) resistance. In this work, the CCAM team use aberration-corrected scanning transmission electron microscopy and spectroscopy, along with an active learning moment tensor potential, to reveal the atomic scale structure and composition of LSTZ0.75 grain boundaries. A key finding is that Li depletion in the GB is mitigated in LSTZ0.75 compared to the typical LLTO peroskite SE. Instead, a nanoscale defective cubic perovskite interfacial structure that contained abundant vacancies is formed. Ji’s contribution is the development of an accurate machine learning interatomic potential to study the highly complex LLTO and LSTZ perovskites, including the GB structures. Using MC and MC simulations, we demonstrate that Li enrichment and Sr vacancies in the GBs of LSTZ play a key role in fast diffusion in LSTZ. Check out this work here.

Polarons in Vanadium Oxides

Jasleen’s first-author paper on “Polaron-induced metal-to-insulator transition in vanadium oxides from density functional theory calculations” is out in Physical Review B! Vanadium oxides are promising phase-change memory units for neuromorphic computing due to their metal-insulator transitions (MIT) at or near room temperature. In this work, we show that V3O5 exhibits very low hole and electron polaron migration barriers (< 100 meV) compared to V2O3 and VO2, leading to much higher estimated polaronic conductivity. The relative migration barriers are found to be related to the amount of distortion that has to travel when the polaron migrate from one site to another. Polarons in V3O5 also have smaller binding energies to vanadium and oxygen vacancy defects. These results explain recent experiment studies showing the injection of charge carriers into vanadium oxides as an alternative switching mechanism and also potentially as a means to tune the MIT temperature. Check out this work here.

The intercalation chemistry of DRX-Li3V2O5 anode

Xingyu’s swansong work in the Materials Virtual Lab, “Intercalation Chemistry of the Disordered Rocksalt Li3V2O5 Anode from Cluster Expansions and Machine Learning Interatomic Potentials” has been published in Chemistry of Materials! We revisited the intercalation chemistry of the highly promising DRX-Li3V2O5 using machine learning-based computational techniques that enable much larger scale simulations. DRX Li3V2O5 is a promising anode candidate for rechargeable lithium-ion batteries because of its low voltage, high rate capability, and good cycling stability. In contrast to previous DFT studies, we show that insertion of Li primarily occurs in the tetrahedral sites and that the voltage profile depends critically on the initial Li/V disorder. MD simulations also show that DRX-Li3V2O5 has a fast Li diffusivity, which depends on the concentration of Li. We propose tuning the Li:V ratio as a means of trading off increased lithiation capacity and decreased anode voltage in this system. This work provides in-depth insights into the high-performance DRX-Li3V2O5 anode and paves the way for the discovery of other disordered anode materials. Check out the work here.

Matterverse.ai and M3GNet Universal IAP

Dr Chi Chen’s swansong work in our group, “A Universal Graph Deep Learning Potential for the Periodic Table” is now published in Nature Computational Science! Interatomic potentials (IAPs), which describe the potential energy surface of atoms, are a fundamental input for atomistic simulations. However, existing IAPs are either fitted to narrow chemistries or too inaccurate for general applications. In this work, we combine graph neural networks with traditional 3-body interactions to develop a flexible, yet accurate architecture for machine learning of materials properties. Using the massive database of structural relaxations performed by the Materials Project over the past ten years, we train a universal IAP for 89 elements of the periodic table with broad applications in structural relaxation, dynamic simulations and property prediction of materials across diverse chemical spaces. Using the new capabilities of the M3GNet universal IAP, we are proud to launch matterverse.ai, a ML database of yet-to-be-synthesized materials. Matterverse.ai currently contains about 31 million hypothetical crystal structures, of which about 1.8 million materials were identified to be potentially stable. The database also provides ML properties using state-of-the-art multi-fidelity MEGNet models, such as experimental, HSE and PBE band gaps, bulk and shear moduli, etc. Check out the article here. […]

Interfacial Stability of NMC cathodes

Congrats to Hideyuki Komatsu, our visiting scientist from Nissan, on his first author work “Interfacial Stability of Layered LiNixMnyCo1−x−yO2 Cathodes with Sulfide Solid Electrolytes in All-Solid-State Rechargeable Lithium-Ion Batteries from First-Principles Calculations” published in the Journal of Physical Chemistry C! In this work, we explore the relationship between the composition of layered LiNixMnyCo1−x−yO2 (NMC) cathodes and interfacial stability in all-solid-state lithium-ion batteries. A key insight is that the broader commercial trend towards high Ni content to reduce cost leads to significantly more reactive interfaces with the Li6PS5Cl argyrodite solid electrolyte. This suggests that current efforts to reduce the Co content in cathodes may compromise potential applications in all-solid-state architectures. Nevertheless, we find that common SEI phases such as Li2CO3, surface phases such as NiO, and oxide buffer layers such as LiNbO3 can provide effective protection between NMC and LPSCl. Check out the work here.