Honors & Awards
NSF Graduate Research Fellowship, National Science Foundation (May 2017)
Ford Foundation Fellowship, National Academy of Sciences, Ford Foundation (May 2017)
EDGE Fellowship, Stanford University (May 2017)
Education & Certifications
B.S., Pennsylvania State University, Materials Science and Engineering (2016)
A.S., Cochise College, Engineering (2013)
Rafael A. Vilá. "United States Patent 62/398,751 Vertical 2D Structures for Advanced Electronic and Optoelectronic Systems", Pennsylvania State University, Oct 14, 2015
Incorporating the nanoscale encapsulation concept from liquid electrolytes into solid-state lithium-sulfur batteries.
Lithium-sulfur (Li-S) batteries are attractive due to their high specific energy and low-cost prospect. Most studies in the past decade are based on these batteries with liquid electrolytes, where many exciting material/structural designs are realized at the nanoscale to address problems of Li-S chemistry. Recently, there is a new promising direction to develop Li-S batteries with solid polymer electrolytes, although it is unclear whether the concepts from liquid electrolytes are applicable in the solid state to improve battery performance. Here we demonstrate that the nanoscale encapsulation concept based on Li2S-TiS2 core-shell particles, originally developed in liquid electrolytes, is very effective in solid polymer electrolytes. Using in situ optical cell measurement and sulfur K-edge X-ray absorption near edge spectroscopy, we find that polysulfides form and are well trapped inside individual particles by the nanoscale TiS2 encapsulation. This TiS2 encapsulation layer also functions to catalyze the oxidation reaction of Li2S to sulfur, even in solid-state electrolytes, proved by both experiments and density functional theory calculations. A high cell-level specific energy of 427 W∙h∙kg-1 at 60 °C (including the mass of the anode, cathode, and solid-state electrolyte, but excluding the current collector and packaging) is achieved by integrating TiS2 encapsulated Li2S cathode with ultrathin polyethylene oxide-based solid polymer electrolyte (10~20 m) and lithium metal anode. The solid-state cells show excellent stability over 150 charge/discharge cycles at 0.8 C at 80 °C. This study points to the fruitful direction of borrowing concepts from liquid electrolytes into solid-state Li-S batteries.
View details for DOI 10.1021/acs.nanolett.0c02033
View details for PubMedID 32515973
- Tortuosity Effects in Lithium-Metal Host Anodes JOULE 2020; 4 (4): 938–52
Electrochemical generation of liquid and solid sulfur on two-dimensional layered materials with distinct areal capacities.
It has recently been shown that sulfur, a solid material in its elementary form S8, can stay in a supercooled state as liquid sulfur in an electrochemical cell. We establish that this newly discovered state could have implications for lithium-sulfur batteries. Here, through in situ studies of electrochemical sulfur generation, we show that liquid (supercooled) and solid elementary sulfur possess very different areal capacities over the same charging period. To control the physical state of sulfur, we studied its growth on two-dimensional layered materials. We found that on the basal plane, only liquid sulfur accumulates; by contrast, at the edge sites, liquid sulfur accumulates if the thickness of the two-dimensional material is small, whereas solid sulfur nucleates if the thickness is large (tens of nanometres). Correlating the sulfur states with their respective areal capacities, as well as controlling the growth of sulfur on two-dimensional materials, could provide insights for the design of future lithium-sulfur batteries.
View details for DOI 10.1038/s41565-019-0624-6
View details for PubMedID 31988508
Electrochemical generation of liquid and solid sulfur on two-dimensional layered materials with distinct areal capacities
View details for DOI 10.1038/s41565-019-0624-6
- Unravelling Degradation Mechanisms and Atomic Structure of Organic-Inorganic Halide Perovskites by Cryo-EM JOULE 2019; 3 (11): 2854–66
- Cryo-EM Structures of Atomic Surfaces and Host-Guest Chemistry in Metal-Organic Frameworks MATTER 2019; 1 (2): 428–38
Wrinkled Graphene Cages as Hosts for High-Capacity Li Metal Anodes Shown by Cryogenic Electron Microscopy.
Lithium (Li) metal has long been considered the "holy grail" of battery anode chemistry but is plagued by low efficiency and poor safety due to its high chemical reactivity and large volume fluctuation, respectively. Here we introduce a new host of wrinkled graphene cage (WGC) for Li metal. Different from recently reported amorphous carbon spheres, WGC show highly improved mechanical stability, better Li ion conductivity, and excellent solid electrolyte interphase (SEI) for continuous robust Li metal protection. At low areal capacities, Li metal is preferentially deposited inside the graphene cage. Cryogenic electron microscopy characterization shows that a uniform and stable SEI forms on the WGC surface that can shield the Li metal from direct exposure to electrolyte. With increased areal capacities, Li metal is plated densely and homogeneously into the outer pore spaces between graphene cages with no dendrite growth or volume change. As a result, a high Coulombic efficiency (CE) of 98.0% was achieved under 0.5 mA/cm2 and 1-10 mAh/cm2 in commercial carbonate electrolytes, and a CE of 99.1% was realized with high-concentration electrolytes under 0.5 mA/cm2 and 3 mAh/cm2. Full cells using WGC electrodes with prestored Li paired with Li iron phosphate showed greatly improved cycle lifetime. With 10 mAh/cm2 Li metal deposition, the WGC/Li compositeanodewas able to provide a high specific capacity of 2785 mAh/g. With its roll-to-roll compatible fabrication procedure, WGC serves as a highly promising material for the practical realization of Li metal anodes in next-generation high energy density secondary batteries.
View details for PubMedID 30676759
In situ crystallization kinetics of two-dimensional MoS2
2017; 5 (1)
View details for DOI 10.1088/2053-1583/aa9674
- Bottom-up synthesis of vertically oriented two-dimensional materials 2D MATERIALS 2016; 3 (4)
Two-dimensional gallium nitride realized via graphene encapsulation
2016; 15 (11): 1166-?
The spectrum of two-dimensional (2D) and layered materials 'beyond graphene' offers a remarkable platform to study new phenomena in condensed matter physics. Among these materials, layered hexagonal boron nitride (hBN), with its wide bandgap energy (∼5.0-6.0 eV), has clearly established that 2D nitrides are key to advancing 2D devices. A gap, however, remains between the theoretical prediction of 2D nitrides 'beyond hBN' and experimental realization of such structures. Here we demonstrate the synthesis of 2D gallium nitride (GaN) via a migration-enhanced encapsulated growth (MEEG) technique utilizing epitaxial graphene. We theoretically predict and experimentally validate that the atomic structure of 2D GaN grown via MEEG is notably different from reported theory. Moreover, we establish that graphene plays a critical role in stabilizing the direct-bandgap (nearly 5.0 eV), 2D buckled structure. Our results provide a foundation for discovery and stabilization of 2D nitrides that are difficult to prepare via traditional synthesis.
View details for DOI 10.1038/NMAT4742
View details for Web of Science ID 000386377000011
View details for PubMedID 27571451
Growth and Tunable Surface Wettability of Vertical MoS2 Layers for Improved Hydrogen Evolution Reactions
ACS APPLIED MATERIALS & INTERFACES
2016; 8 (34): 22190-22195
Layered materials, especially the transition metal dichalcogenides (TMDs), are of interest for a broad range of applications. Among the class of TMDs, molybdenum disulfide (MoS2) is perhaps the most studied because of its natural abundance and use in optoelectronics, energy storage and energy conversion applications. Understanding the fundamental structure-property relations is key for tailoring the enhancement in the above-mentioned applications. Here, we report a controlled powder vaporization synthesis of MoS2 flower-like structures consisting of vertically grown layers of MoS2 exhibiting exposed edges. This growth is readily achievable on multiple substrates, such as graphite, silicon, and silicon dioxide. The resulting MoS2 flowers are highly crystalline and stoichiometric. Further observations using contact angle indicate that MoS2 flowers exhibit the highest reported contact angle of ∼160 ± 10°, making the material super hydrophobic. This surface wettability was further tuned by changing the edge chemistry of the MoS2 flowers using an ozone etching treatment. Hydrogen evolution reaction (HER) measurements indicate that the surface treated with UV-ozone showed a reduction in the Tafel slope from 185 to 54 mV/dec, suggesting an increase in the amount of reactive surface to generate hydrogen.
View details for DOI 10.1021/acsami.6b05848
View details for Web of Science ID 000382514100041
View details for PubMedID 27500662