Chemomechanics in Li-ion batteries
Li-ion batteries play a starring role in the era of portable information. Of equal importance is the implementation of Li-ion batteries in the population of electric vehicles. Mechanics and chemistry are intimitely coupled at the heart of high-energy-density Li-ion batteries. Our goal is to create a foundation for how mechanical stress regulates chemical reactions, ion diffusion, and phase transitions, and how the latter modulate stress generation and mechanical failure in battery electrodes.
The first-principles modeling relies on the Density Functional Theory and provides fundamental understanding at the levels of the electronic structure, crystal lattice, energy landscape for diffusion and interfacial reactions, as well as chemical and mechanical stabilities of competing phases. The following figures show the bonding and charge transfer during Li reaction with Si anode (left) and density of states of sulfur cathode (right).
We program finite element models to simulate the chemomechanical behaviors of three-dimensional composite electrodes and assist design of resilient batteries. The following figures show the multiple particles (left) and nanowires (right) under the coupled process of diffusion and large elasto-plastic deformation.
We integrate a nanoindenter (Agilent G200) into an argon-filled glovebox. The system is dedicated to measure the mechanical properties of thin-film and composite electrodes under the in-situ conditions of lithiation cycles. The platform provides the input parameters for the FEM program, and validates the continuum theories on coupled mechanical and electrochemical behaviors of electrodes.
We set up in-situ testing of small-scale batteries inside TEM using a single nanowire/nanoparticle as the working electrode. The in-situ TEM diagnosis provides direct observables on the real-time electrochemical process, phase transition, defect growth, and chemomechanical instability of nanostructured electrodes.
First-principles modeling on transition metal diborides
Transition metal diborides attract intense interest as structural materials for hypersonic flight, super-hard and incompressible materials, and as materials for hydrogen storage or low-temperature fuel cells. However, their high melting points, chemical, and mechanical stability require extreme processing conditions that hinder their widespread use. We use first-principles theoretical approaches to determine the energetics, structural evolution, and phase transitions during solid-state alloying of B with transition metals, as well as the electronic, ionic, and mechanical properties of diborides. We compare the atomistic modeling with nanocalorimetry experiments.
Micromechanics of nanotwinned metals
Nanotwinned metals exhibit unique properties of ultrahigh mechanical strength, enhanced ductility, superior thermal stability, and excellent electrical conductivity. Prior studies are focused on materials of low stacking-fault energy. We synthesize nanotwinned metals of medium to high stacking fault energy (Ni) with controlled twin patterns – parallel versus parallelogram nanotwins under different electrodeposition conditions. The patterned nanotwins provide a new strategy of designing high-strength and high-ductility metals.