New and higher-performing materials play a key role in enabling new technology. Central to achieving this goal is a detailed understanding of the impact that structure and composition have on physical properties. To that end, the research conducted in our group broadly focuses on understanding the interplay between spin, charge, and lattice degrees of freedom in the solid state so as to better design new and improve existing functional materials. A few examples of which are outlined below.
We use a large variety of synthetic methods to prepare new materials; including high-temperature ceramic as well as hydrothermal routes. We place equal emphasis on the preparation of materials and their complete physical characterization which includes an accurate determination of: the underlying crystal structure, magnetism, electrical transport, dielectric properties, and electrochemical performance. We also make extensive use of a variety of density functional theory (DFT) computational tools in order to understand the nature of chemical bonding in these materials.
New Materials for Li-ion Batteries
Facing dwindling supplies of fossil fuels and skyrocketing prices associated with such shortages, the most important challenge our society must overcome is how we will establish a sustainable and environmentally benign energy infrastructure. The challenge as it stands today is not to understand how to convert and store the energy we will need, but rather what materials we will use to produce and manage the energy required while making world-wide implementation a reality. Despite fervent research efforts, lithium batteries, which have been heavily investigated for nearly forty years, have been primarily restricted to small consumer devices such as laptops and cellphones. However, provided that the cost, safety, and energy density can be further enhanced, these batteries have great potential in the fields of automotive transportation and electrical grid storage. In order to circumvent intrinsic materials limitations, it becomes increasingly clear that we must continue the search for new compounds if this technology is to approach feasibility in large scale applications. To help meet this demand, our group works on the preparation and elaboration of new compounds for use Li-ion batteries. In particular, we focus on modifying the electronegativity of constituent anions as a way to increase the open circuit voltage of individual cells and thereby increase their power density.
Structure-Property Relationships in Perovskite Light Absorbers
As early as 1980, halide perovskites, with generic composition AMX3, had been under investigation because their optical properties were recognized to match closely with the solar spectrum. It was not until 2009, however, that the first solar cell based on these materials was constructed by Miyasaka et al. While these first devices only exhibited efficiencies around 3-4%, it was not long until the groups of Graetzel and Kanatzidis reported devices with efficiencies of 9% and 10%. The subsequent three years has seen hundreds of publications and recently culminated in the report of cells with efficiencies near 18% - a mere stone's throw from the 25% of single crystal silicon. Despite these rapid gains in performance, the best perovskite cells currently rely on compositions such as CH3NH3PbI3. Setting aside the challenges associated with manufacturing Pb-based devices in a safe and environmentally benign fashion, these halide salts are highly soluble in water and therefore present significant danger with respect to environmental and public health if panels containing these materials were to crack and leak their contents in to the surrounding area. My group currently focuses on developing new compositions for the light-absorbing and charge-transporting layers in perovskite solar cells that can be made inexpensive out of sustainable compositions.
Enhancing Ionic Conductivity in Solid Electrolytes
The ultimate Li-ion conductor would allow Li-ions to move as quickly as they can in the non-aqueous liquids currently used but with enhanced stability under large applied voltages. Fast Li-ion conductors, like the stuffed garnet Li7La3Zr2O12, have attracted considerable interest because of their exceptional ionic conductivity at room temperature (10-4 S cm-1). Our group recently demonstrated that solid solutions of Li6MLa2Ta2O12, with compositions where M = Ba, Ba0.5Sr0.5, Sr, Sr0.5Ca0.5 and Ca, exhibited a continuous increase of Li mobility due to a lowering of the activation energy for Li-ions migration as the average ionic radius of the M site increased. This was first demonstration that increasing the unit cell volume of these materials facilitates faster ion transport.
Frustrated Magnetism and Magnetodielectricity
Magnetodielectrics are materials in which the dielectric properties couple to changes in the magnetic order. The ability to find and design new single-phase materials which exhibit this kind of coupling has significant technological implications in the development of magnetic sensors and field-tunable dielectrics. One route which has shown great promise for the identification of such materials is investigate materials which exhibit magnetic frustration. Frustrated systems are those in which the spin, charge, or orbital degrees of freedom are unable to attain a unique low-energy ground state due to either competing interactions or the geometry of the lattice. Our group focuses on developing ways to understand and thereby gain some control over the degree of frustration in these systems with the hope of identifying new magnetodielectric materials.