Journal features Wenhao Sun in "Emerging Investigator" series

Materials Horizons highlighted Sun for being an "exceptional early-career materials researcher" for his work on developing new quantitative and predictive theories of inorganic materials synthesis.
Journal features Wenhao Sun in "Emerging Investigator" series

Assistant Professor Wenhao Sun was featured in the journal Materials Horizons

Assistant Professor Wenhao Sun was recently featured as an Emerging Investigator by the journal Materials Horizons. The publication's Emerging Investigator Series features exceptional early-career researchers working in the field of materials science.

The interview that Materials Horizons conducted with Wenhao is reprinted below, followed by the abstract for his article, "Selective metathesis synthesis of MgCr2S4 by control of thermodynamic driving forces."

MH: Your recent Materials Horizons Communication focuses on metathesis reactions as an alternative synthesis route to inorganic materials for applications in batteries and photocatalysis. How has your research evolved from your first article to this most recent article and where do you see your research going in future?

WS: When I began my PhD in 2010, the U.S. Materials Genome Initiative was just being announced, and there was a great deal of hype and excitement to use high-throughput ab initio (HT-DFT) methods to discover novel functional materials. Over the last ten years, many HT-DFT papers have been written—often predicting dozens of new materials at a time, for applications spanning batteries, solar cells, thermoelectrics, and more.

Unfortunately, making even just one of these predicted materials in the lab can take a year or more, and even after all that effort, synthesis is sometimes still unsuccessful! From my perspective, the most urgent question in the computational materials design pipeline is shifting from “Which materials should I make?” to “How do I make these predicted compounds?”

For this reason, I have focused my research on developing new quantitative and predictive theories of inorganic materials synthesis. When I started my PhD, I primarily aimed to understand metastable phases—which appear ubiquitously during synthesis, but are difficult to anticipate from traditional phase diagrams. Over time, my interests have grown into more generally trying to understand the interplay between thermodynamics and kinetics during materials nucleation and growth.

Our Materials Horizons paper on MgCr2S4 excites me because it helps to rationalize a powerful but underutilized strategy for materials synthesis. Like many other computationally-designed materials, MgCr2S4 was predicted to have great performance (for Mg-battery applications) but numerous research groups have struggled to synthesize MgCr2S4 efficiently. I met my co-author, Professor Akira Miura, at a Gordon Research Conference, and he designed this brilliant metathesis reaction to drive the formation of MgCr2S4 using NaCl as a highly-stable and easily-removable byproduct. As the paper shows, our metathesis reaction produces MgCr2S4 in 30 minutes at 500 °C, as opposed to the conventional route (MgS + Cr2S3), which takes 2 weeks at 800 °C.

I love this metathesis work for two reasons: (1) the strategy is conceptually simple and satisfying, and it is also readily computable; (2) metathesis reactions offer a broad reaction design space, where volatile or removable ‘byproduct’ phases can be cleverly chosen to drive synthesis reactions to compounds that are otherwise difficult to make. I am inspired by elegant and powerful synthesis reactions like these, and hope to design more such reactions in my future work.

MH: What aspect of your work are you most excited about at the moment?

WS: As I work to develop new theoretical frameworks for predictive synthesis, I foresee many opportunities to derive new ‘textbook theory’. Thermodynamics textbooks today emphasize classical thermodynamics developed in the early 1900s, and kinetics textbooks focus on metallurgy kinetics formulated in the mid 20th-century. However, materials science has developed so much over the last twenty years, with incredible experimental observations enabled by advanced characterization techniques, performed over broad classes of materials. There is therefore this wealth of empirical data just waiting to be interpreted and rationalized—not from computation—but from humble pencil-and-paper theoretical work. One of my ambitions is to develop new fundamental conceptual frameworks that can appear as core chapters in future thermodynamics and kinetics textbooks.

MH: In your opinion, what are the most important questions to be asked/answered in this field of research?

WS: Now that it is easy to create fantastical new materials in the computer, I think the most urgent questions have become: (1) which computationally-designed materials are actually synthesizable? (2) Which synthesis method—solid-state, solvothermal, vapour deposition, etc.—is best to realize a predicted compound? (3) Within the parameter space of that synthesis method, what is the synthesis recipe needed to realize the target material in a phase-pure form?

From a broader computational materials design perspective, I often ask myself: “Why is it that, despite tremendous high-throughput computational efforts, there have been so few best-in-class materials designed purely in silico?” I hope our community can take a moment to reflect and deconstruct the computational materials design process—figuring out where are the leaks in this materials discovery pipeline, and innovating on ways to fix those leaks.

MH: What do you find most challenging about your research?

WS: To validate my theories of predictive synthesis, I often collaborate closely with experimentalists. Together, we spend a lot of effort ‘translating’ computable thermodynamic quantities into experimental ‘knobs’, and vice versa. This task requires us to all be intimately familiar with each other's techniques, and their limitations. The end result is very rewarding, as it results in a cohesive team fluent in both experiment and computation, allowing us to focus on the underlying theories of materials synthesis.

MH: In which upcoming conferences or events may our readers meet you?

WS: I try to attend the Materials Research Society (MRS) and the American Physical Society (APS) meetings every year. I’m also a big fan of Gordon Research Conferences and Faraday Discussions. Given the current COVID situation, though, anyone interested in my work can always email me directly (at whsun@umich.edu), I’m always excited to discuss new ideas and potential collaborations.

MH: How do you spend your spare time?

WS: My wife and I have a seven-month-old baby boy, so I’m actually quite grateful that I get to spend a lot of time with my family during this self-quarantine COVID period. It is exciting to watch him grow and develop new skills every day.

MH: Can you share one piece of career-related advice or wisdom with other early career scientists?

WS: Early career scientists need to quickly establish a strong and distinct research identity. We must therefore be very strategic in deciding which research projects we invest our time into. It is easy to think, “Ah, this project will be a quick and easy paper!” But even the simplest papers can take a year to go from conception to publication. Instead of chasing ‘low-hanging fruit,’ I think it is better to pursue more ambitious research projects that establish your unique expertise in the scientific ecosystem. This helps you to distinguish yourself from all the other excellent scientists out there in the world.

Abstract

MgCr2S4 thiospinel is predicted to be a compelling Mg-cathode material, but its preparation via traditional solid-state synthesis methodshas proven challenging. Wustrow et al. [Inorg. Chem., 2018, 57, 14] found that the formation of MgCr2S4 from MgS + Cr2S3 binaries requires weeks of annealing at 800 °C with numerous intermediate regrinds. The slow reaction kinetics of MgS + Cr2S3 → MgCr2S4 can be attributed to a miniscule thermodynamic driving force of ΔH = −2 kJ mol−1. Here, we demonstrate that the double ion-exchange metathesis reaction, MgCl2 + 2NaCrS2 → MgCr2S4 + 2NaCl, has a reaction enthalpy of ΔH = −47 kJ mol−1, which is thermodynamically driven by the large exothermicity of NaCl formation. Using this metathesis reaction, we successfully synthesized MgCr2S4 nanoparticles (<200 nm) from MgCl2 and NaCrS2 precursors in a KCl flux at 500 °C in only 30 minutes. NaCl and other metathesis byproducts are then easily washed away by water. We rationalize the selectivity of MgCr2S4 in the metathesis reaction from the topology of the DFT-calculated pseudo-ternary MgCl2–CrCl3–Na2S phase diagram. Our work helps to establish metathesis reactions as a powerful alternative synthesis route to inorganic materials that have otherwise small reaction energies from conventional precursors.