We study the physics and chemistry of materials using atomistic computational methods and high-performance computing technology.
The Materials Project is a multi-institution, multi-national effort to compute the properties of all inorganic materials and provide the data and associated analysis algorithms for every material researcher free of charge. The ultimate goal of the initiative is to drastically reduce the time needed to invent new materials by focusing costly and time-consuming experiments on compounds that show the most promise computationally.
The Materials Project has already computed over 30,000 distinct inorganic compounds (requiring over 10 million CPU hours), and has registered over 4500 users from around the world. Leveraging the Materials Project data and capabilities, users have published papers on Li battery anodes, rare earth materials, and magnetic materials, amongst other applications. Several Python libraries developed by Materials Project are also released open source and are used by researchers around the world. The current focus of the project is to expand the range of predicted properties to mechanical, thermal, and surface behavior, as well as to predict never-before-seen materials in the area of clean energy production, harvesting and storage.
The Electrolyte Genome & Multivalent Cathode Screening projects aim to accelerate the next generation of energy storage systems, through discovery of novel solid and liquid materials.
The Electrolyte Genome uses both density functional theory (DFT) and molecular dynamics (MD) simulations to uncover the properties of potential electrolyte molecules. DFT computations are performed on a large scale to reveal the electrochemical windows of tens of thousands of molecules. MD calculations are being semi-automated to be run over dozens of the most promising candidates to determine kinetic properties such as diffusion coefficient.
The Multivalent Cathode Screening project uses high-throughput DFT to search for new chemical compounds that can serve as electrodes by exchanging multivalent cations such as Mg2+, Ca2+, and Y3+. Such formulations can potentially offer large improvements in energy density over today’s Li+ based batteries, but suitable electrode compounds must be identified amongst tens of thousands of potential candidates.
In both applications, the role of computations is to guide JCESR experimental collaborators towards next-generation, blockbuster battery materials. Both projects are therefore in close contact with experimental teams across the country to feed back information between computational and experimental channels.
Interfaces and surfaces add complexity and engineering avenues to the properties of materials, and can dominate the performance of nanostructured materials, as for example in photovoltaic (PV) nanowires, photocatalysts, or energy storage electrodes. Furthermore, surface energies and related properties are often used in higher length scale theories (e.g. nucleation and growth theories, coarsening, stability, device performance). The objective of our research is to develop a grand canonical approach whereby surfaces are equilibrated under the chemical potentials of select species in the environment and make the data and algorithms available through the Materials Project.
We focus on materials with energy applications, especially the electrode of Li-ion batteries, which is supported by the Batteries for Advanced Transportation Technologies (BATT) program.
Since its discovery, graphene, a single layer of honeycomb structured carbon atoms composing the bulk graphite, has attracted attention because of its ballistic electronic transport, optoelectronic properties as well as its unique two-dimensional geometry. In particular, some experimental and computational studies on the Li capacity of graphene are notable as they predict that graphene may absorb Li ions through a specialized Li ordering on both sides of the graphene, resulting in a higher theoretical capacity than graphite. However, there is conflicting experimental evidence that the Li capacity of graphene is significantly less than that of bulk graphite. This controversy and the interest in graphene systems for electrode materials motivate a systematic study of Li absorption on single layer graphene as well as Li absorption and intercalation in few layer graphene.
We perform a exhaustive investigation of the stability of Li-absorbed single layer graphene against the two-phase separation into a single layer graphene and metallic Li, using the cluster expansion method and density functional theory calculations. We also examine Li interactions, both absorption and intercalation, with few layer graphene to study how the Li-graphene system evolves from Li-graphene to Li-graphite. A semi-empirical potential is calibrated to account for van der Waals interactions, which has a significant effect on the interactions between carbon atoms in neighboring graphene layers.
The lithium manganese spinel, LiMn2O4, has received attention as a cathode material for the Li-ion batteries due to its cheap price, non-toxicity, and good rate capability. However, commercialization of LiMn2O4 has been delayed due to capacity fade upon cycling, which has been attributed to the dissolution of Mn3+ produced by the redox process during lithiation/delithiation cycling. Furthermore, it has been speculated that the strong Jahn–Teller distortion of the Mn3+ ion degrades the structural stability of the material and inhibits Li migration. Substituting some Mn with Ni to form LiNi0.5Mn1.5O4 suggests an attractive material. In LiNi0.5Mn1.5O4, the redox process occurs on the Ni site only, which prevents the creation of the Mn3+ cation and its related problems. Furthermore, the reduction of Ni4+to Ni2+ occurs at 4.7 V, which increases the total energy density of LixNi0.5Mn1.5O4 as compared to LiMn2O4 from 440 Wh/kg to 686 Wh/kg.
Despite the promising qualities of LiNi0.5Mn1.5O4, some of its fundamental properties have remained unexplained. For example, the origin of the voltage step at Li0.5Ni0.5Mn1.5O4 and its pronounced dependence on the cation ordering, the difference in rate capability, and the occurrence of reversible phase transitions between ordered and disordered Ni/Mn arrangement during charge/discharge cycling are debated. We present first-principles energy calculations and a coupled cluster expansion model of the ionic ordering and its effect on the electrochemical behavior and phase diagram of LiNi0.5Mn1.5O4.
Li-rich composite materials, such as layered-layered and layered-spinel, is attracting attentions in terms of their promising electrochemical performance as the cathode materials for the Li-ion batteries far beyond the current cathode materials. However, the Li-rich composite materials also have several issues: significant capacity loss after the first charge, structural instability, oxygen gas evolution/leakage, etc, which delay the commercialization of the composite materials.,We aims to understand the electrochemical behaviors in the Li-rich composite materials and identify the origins of the issues and predict the way to improve the properties for better cathode materials.
Currently, we are focusing on the Li2MnO3, which is commonly included in the Li-rich composite materials and believed as a key material to explain the high Li ion capacity in the layer-layer or layer-spinel composite materials. The material is intriguing as it shows electrochemical activity even though the oxidation state of Mn is already Mn4+. Different studies point to oxygen release, conversion reactions and peroxide formation as possible oxidation mechanisms.
We develop an ab-initio cluster expansion model of the ionic interactions in LixMnO3 (0≤x≤2) and predict the ground state as a function of the lithiation/delithiation and the corresponding voltage profiles. Furthermore, the oxygen stability during the lithiation/delithiation will be examined and discussed. By comparing our computational predictions with others’ experimental observations, we derive the mechanisms of the lithiation and delithiation in the Li2MnO3 and provide the evidence of the phase transformation to the spinel structure after the 1st charge.