Peter Khalifah

MATERIALS CHEMISTRY, SOLID STATE CHEMISTRY

Periodic solids are at the center of high-tech industry due to the strong hybridization between the atomic constituents of their crystalline lattices which enable cooperative properties that are generally absent from molecular solids. The Khalifah group focuses on designing functionality into crystalline solids using elemental substitution and structural control to fine-tune the energy states and functionality of bulk materials. Our group has wide expertise in materials synthesis, structural characterization, and physical properties measurements. As a result, we can comprehensively attack the challenges of the materials design and discovery process, and can take a lead in identifying the next generation of functional materials. While these fundamental skills can be applied to many problems, our recent focus has been on materials for energy applications.

Batteries:

Li-ion battery systems remain unsurpassed for high-power rechargeable battery applications, such as those central to the mobile electronics industry. Present challenges that are being addressed include increasing the power density (voltage, specific capacity), safety, and lifetime of Li-ion batteries through the design and discovery of next-generation battery materials, as well as achieving an improved understanding of the energy storage mechanisms and limitations of current industrially relevant battery systems. In many emerging applications for energy storage (grid-scale storage, HEV and PHEV automobiles), cost and safety are critically important performance metrics.  It is therefore desirable to investigate alternatives to conventional Li-ion systems which may have improved performance in these areas. We have therefore been investigating potential cathodes for both Na-ion and Mg-ion battery systems.

Solar Fuels:

Semiconductors of certain metal oxides and oxynitrides have been demonstrated to be capable of harnessing solar energy to split water, producing hydrogen gas. This process has the potential to alleviate two looming energy crises (limited supply of hydrocarbon fuels and global warming from steeply rising CO₂ levels), provided that the overall efficiency of this process can be raised. We are pursuing strategies to drastically raise the efficiency of photoelectrolysis by tuning the energy levels of oxide semiconductors to optimal positions, studying methods for effectively measuring and controlling the carrier concentration in these semiconductors, and evaluating the absolute optical constants of important semiconductors systems as well as the chemical origins of their absorption.

Electrocatalysis:

Fuel cells represent one of the most efficient technologies for obtaining work from chemical fuels, with efficiencies nearly double that of typical internal combustion engines. The widespread deployment of fuel cells will require an improvement in electrocatalysts for the oxygen reduction reaction since the best current electrocatalysts face significant challenges associated with their performance and their cost/abundance. The Khalifah group has been working to design and evaluate new non-noble metal electrocatalysts for the oxygen reduction reaction, and for other important catalytic reactions.

Direct metal-metal bonding oxides:

While the functionality of most metal transition metal oxides is best understood by thinking metal-oxygen interactions, there is a small subset of oxides in which direct metal-metal bonding is also present. These extra bonding interactions cause radical changes in the electronic structure of these materials relative to conventional oxides, and represent an intriguing opportunity for designing functionality for a variety of technological applications.

Crystal growth and crystallography:

The Khalifah group has expertise in preparing not just powder samples, but also thin film and bulk single crystal samples (up to ~1 cm in size) that allow the fundamental properties of solids to be most effectively measured. The structure of crystalline solids are routinely evaluated using a wide variety of structural probes (X-ray and neutron powder diffraction, X-ray and neutron single crystal diffraction, pair distribution function analysis, transmission electron microscopy, density functional theory), allowing us to obtain exceptionally detailed direct insights into the crystal structure, purity, and defects (down to the 0.1% level) of these materials. We actively collaborate with a variety of groups to perform structural characterization, and have recently launched a national school to teach best practices for structural analysis using cutting-edge national facilities and software tools.

S.-H. Bo, C. P. Grey, and  P. Khalifah, “Defect-Tolerant Diffusion Channels for Mg²⁺ Ions in Ribbon-Type Borates: Structural Insights into Potential Battery Cathodes MgVBO₄ and Mg_xFe_2–xB₂O₅”, Chem. Mater., 27, 4630–4639 (2015).

J. Ma, S.-H. Bo, L. Wu, Y. Zhu, C. P. Grey, and  P. Khalifah, “Ordered and Disordered Polymorphs of Na(Ni_2/3Sb_1/3)O₂: Honeycomb-Ordered Cathodes for Na-Ion Batteries”, Chem. Mater., 27, 2387-2399 (2015).

D. Colabello, F. Camino, A. Huq, M. Hybertsen, and P. Khalifah, “Charge disproportionation in tetragonal La₂MoO₅, a small band gap semiconductor influenced by direct Mo–Mo bonding”, JACS, 137, 1245-1257 (2015).

S.-H. Bo, K.-W. Nam, O. Borkiewicz, Y.-Y. Hu, X.-Q. Yang, P. Chupas, K. Chapman, L. Wu, L. Zhang, F. Wang, C. Grey, and P. Khalifah, “Structures of delithiated and degraded LiFeBO₃, and their distinct changes upon electrochemical cycling”, Inorg. Chem., 53, 6585-6595 (2014).

J. Liu, D. Chang, P. Whitfield, Y. Janssen, X. Yu, Y. Zhou, J. Bai, J. Ko, K.-W. Nam, L. Wu, Y. Zhu, M. Feygenson, G. Amatucci, A. Van der Ven, X.-Q. Yang, and P. Khalifah,  “Ionic conduction in cubic Na₃TiP₃O₉N, a secondary Na-ion battery cathode with extremely low volume change”, Chem. Mater., 26, 3295-3305 (2014).

B. Cao, G. Veith, J. Neuefeind , R. Adzic, and P. Khalifah,  “Mixed close packed cobalt molybdenum nitrides as non-noble metal electrocatalysts for the hydrogen evolution reaction”, JACS, 135, 19186-192 (2013).

Y. Janssen, D. Santhanagopalan, D. Qian, M. Chi, X. Wang, C. Hoffmann, Y. S. Meng, and P. Khalifah, “Reciprocal Salt Flux Growth of LiFePO₄ single crystals with controlled defect concentrations”, Chem. Mater., 25, 4574-84 (2013).

J. Liu, X. Yu, E. Hu, K.-W. Nam, X.-Q. Yang, and P. Khalifah, “Divalent iron nitridophosphates: a new class of cathode materials for Li-ion batteries”, Chem. Mater., 25, 3929-31 (2013).  (Communication)

B. Cao, G. M. Veith, R. E. Diaz, J. Liu, E. A. Stach, R. R. Adzic, and P. Khalifah, “Cobalt Molybdenum Oxynitrides: Synthesis, Structural, Characterization, and Catalytic Activity for the Oxygen Reduction Reaction”, Angew. Chem. IE., 52, 10753-57 (2013).

L. Wang, B. Cao, W. Kang, M. Hybertsen, K. Maeda, K. Domen, and P. Khalifah, “Design of medium band gap Ag-Bi-Nb-O and Ag-Bi-Ta-O semiconductors for driving direct water splitting with visible light”, Inorg. Chem., 52, 9192-9205 (2013).

A. A. Reinert, C. Payne, L. Wang, J. Ciston, Y. Zhu, and P. Khalifah, “Synthesis and Characterization of Visible Light Absorbing (GaN)_1-x(ZnO)_x semiconductors nanorods”, Inorg. Chem., 52, 8389-8398 (2013).

Y. Janssen, D. Middlemiss , S.-H. Bo, C. Grey, and P. Khalifah, “Structural Modulation in the High Capacity Battery Cathode Material LiFeBO₃”, JACS, 134, 12516-27 (2012).

FULL PUBLICATION LIST