Research
Research Statement
The Li Group's long term goal is to understand and develop new (electro)chemical systems that will play a major role in renewable energy, sustainability, and global climate change, all of which represent grand challenges for the 21st century. To accomplish this, our research program pursues two main thrusts: (1) to develop state-of-the-art experimental tools to uncover new insights and understandings for how electrochemical systems operate and fail, and (2) to demonstrate novel materials chemistry and engineering methodologies to impart enhanced performance for electrochemical systems based on the new understandings gained previously. These research thrusts are intimately linked and provide natural synergy with each other.
1. Cryo-EM for materials and energy research
Batteries
Cryogenic-electron microscopy (cryo-EM) received the 2017 Nobel Prize in Chemistry for its profound impact on the field of structural biology. By rapidly freezing fragile biomolecules at cryogenic temperatures (77 K) and imaging with low dose rates, 3-dimensional structures with atomic resolution can be obtained. Indeed, the structure of the COVID-19 virus was quickly determined by cryo-EM. Here, we pioneer this powerful technique to uncover key discoveries for next-generation batteries (Y. Li, Y. Cui, et al. Science 2017) and show that cryo-EM can have a significant impact in materials research beyond biology.
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Electrocatalysts
Since early efforts by our group to adapt cryo-electron microscopy (cryo-EM) towards battery research, significant breakthroughs have been made in our fundamental understanding of how batteries operate and fail. Now, our present work further advances cryo-EM towards the field of electrocatalysis for the first time, providing new findings that have eluded past studies. Combined with a multiscale approach, our cryo-EM experiments reveal the precise role of metallic lithium and its solid electrolyte interphase (SEI) in the electrosynthesis of ammonia. Contrary to conventional wisdom, we discover that the SEI layer prevents nitrogen fixation and must be disrupted for catalytic activity. This surprising finding completely revises our previous understanding of lithium-mediated ammonia synthesis and provides new insights for designing electrochemical systems to decarbonize future fertilizer production.
Perovskite Solar Cells
Despite rapid progress of hybrid organic-inorganic halide perovskite solar cells, using transmission electron microscopy to study their atomic structures has not been possible because of their extreme sensitivity to electron beam irradiation and environmental exposure. Here, we develop cryoelectron microscopy (cryo-EM) protocols to preserve an extremely sensitive perovskite, methylammonium lead iodide (MAPbI3) under various operating conditions for atomic-resolution imaging. We discover the precipitation of lead iodide nanoparticles on MAPbI3 nanowire’s surface after short UV illumination and surface roughening after only 10 s exposure to air, while these effects remain undetected in conventional X-ray diffraction. Our results highlight the importance of cryo-EM since traditional techniques cannot capture important nanoscale changes in morphology and structure that have important implications for perovskite solar cell stability and performance.
Nanomaterials for carbon capture
For the first time, we have used cryo-EM to obtain images of individual carbon dioxide molecules trapped in a series of molecular “cages." Metal-organic frameworks (MOFs) are exceptionally porous polymers designed to capture particular gas molecules, letting scientists separate or purify various vapors. Even small amounts can slurp up a lot of gas: a single gram can have a gas-grabbing surface area nearly the size of two football fields. MOFs have been proposed for holding hydrogen in automobile tanks or fuel cells (without the need for extra cooling) and for grabbing and storing planet-warming carbon dioxide emissions, among many other uses.
2. Materials innovations and data-driven diagnostics
Ultrafast electrodeposition for fast-charging
Dendritic deposition of metallic lithium during battery fast charging has long been understood as a critical failure mode limiting charging speeds. When mass-transport limitations are avoided at ultrafast current densities, we discover the intrinsic deposition morphology of metallic lithium to be a non-dendritic rhombic dodecahedron (e.g., a 12-sided polygon). This morphology persists independent of electrolyte chemistry and current collector substrate, revising our previous understanding of Li metal electrodeposition. New insights can potentially guide improved battery charging protocols for safety and stability.
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X. Yuan, Y. Li^, et al. Nature, 620, (2023): 86–91 (link)
Next-generation Li-ion batteries
High-energy lithium battery chemistries (e.g. silicon, lithium metal, sulfur) have the potential to facilitate our transition away from fossil fuels and towards renewable energy resources (solar, wind). In particular, silicon has more than ten times the capacity of conventional battery materials. Unfortunately, batteries using silicon cannot be recharged because the material fractures and loses electrical contact during charging and discharging, rendering the broken particles inactive. For the first time, we’ve demonstrated that by encapsulating each silicon particle within a graphene cage, the ruptured fragments continue to be electrochemically active (Y. Li, Y. Cui, et al. Nature Energy 2016). This graphene cage encapsulation strategy (patent) allows us to achieve specific capacities more than four times that of conventional materials and enable the stabilized silicon to be recharged over 300 times.
Aqueous battery chemistries
We have developed a disruptive battery innovation based on inexpensive Zn chemistry that can simultaneously achieve extreme fast charging (80% SOC in 5-15 min), long cycle life (90% capacity after 3000 cycles), low temperature performance, and enhanced safety (aqueous chemistry). This breakthrough technology will eliminate major barriers for widespread electric vehicle adoption. While previous approaches were able to demonstrate promising advances in a subset of the above requirements, no existing technology can achieve them all simultaneously. We collaborate with prof. Ric Kaner, who's commercially proven materials synthesis (Science 335, 1326, 2012) can produce novel 3D battery architectures at scale for high performance. Specifically, our engineered electrode contains a vast network of interpenetrating pores that facilitate ultrafast ion and electron transport and is agnostic to battery chemistry. Our materials approach will produce a transformative battery technology with the potential to challenge existing Li-ion technologies.
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coming soon...
Real-time battery diagnostics for dynamic control
Fast-charging capabilities are critical to accelerate the mainstream adoption of electric vehicles. However, current battery charging protocols mainly consist of conservative rate steps to avoid hazardous lithium plating and its associated parasitic reactions. A highly sensitive onboard detection method could enable battery fast-charging without reaching the lithium plating regime. Here, we demonstrate a novel differential pressure sensing method to precisely detect the lithium plating event. By measuring the real-time change of cell pressure per unit of charge (dP/dQ) and comparing it with the threshold defined by the maximum of dP/dQ during lithium-ion intercalation into the negative electrode, the onset of lithium plating before its extensive growth can be detected with high precision. Our differential pressure sensing approach can serve as an early nondestructive diagnostic tool to enable dynamic control of current densities during extreme fast charging. This will allow batteries to reach their fundamental materials limit without compromising safety.