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. 

With cryo-EM, we are able to resolve atomic columns of Li metal for the first time (left). We can also measure the distance between atoms, which matches XRD (right). 
(A) Li metal dendrites are electrochemically deposited directly onto a Cu TEM grid and then plunged into liquid nitrogen after battery disassembly. (B) The specimen is then placed onto the cryo-TEM holder while still immersed in liquid nitrogen and isolated from the environment by a closed shutter. During insertion into the TEM column, temperatures do not increase above -170 °C, and the shutter prevents air exposure to the Li metal. 

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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.

TEM grids are sandwiched between Cu foils at the working electrode to expose the grid to LiMEAS surface conditions. After experiments, TEM grids are sealed under Ar then plunge-frozen in LN2 before transferring to a cryo-transfer holder for imaging.
(a), SEI generated by THF and LiBF4 breakdown in the absence of proton donor inhibits N2 reactivity with lithium. (b), The addition of ethanol leads to organic components of the SEI that are permeable to nitrogen and other electrolyte components, enabling lithium reactivity including nitrogen fixation. (c), Mechanisms by which ethanol-derived SEI materials could result in poor passivation. Top to bottom, poor passivation could result from hydrogen gas generation that induces porosity in the SEI, from a high degree of SEI swelling in electrolyte, or because of partial solubility of SEI components, such as lithium ethoxide (LiOEt).

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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.

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Pristine or UV/moisture exposed perovskite NWs were dropcast onto a quantifoil TEM grid in nitrogen glovebox. Then, the sample was plunge-frozen into liquid nitrogen inside glovebox. EM images of MAPbI3 and MAPbBr3 show smooth surface on NWs. Atomic-resolution TEM images resolving [PbI6]4− octahedral and MA+ molecule of MAPbI3 and MAPbBr3.

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.

Direct cryo-EM images of a metal-organic framework called ZIF-8 (top and middle) and the CO2 molecules trapped within it (bottom)

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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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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. 

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When used in lithium-ion battery anodes, silicon microparticles swell, break apart and react with the battery’s electrolyte to form a thick coating that saps the anode's performance, top. To address these problems, we built a graphene cage around each particle, bottom. The cage gives the particle room to swell during charging, holds its pieces together when it breaks apart, controls the growth of the coating and preserves electrical conductivity and performance. (Y. Li et al., Nature Energy) 

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.

Schematic of laser-scribing process to produce high-rate and high-capacity Zn cathode materials.

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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. 

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(a) The configuration of operando pressure measurement. The stack containing a multilayer pouch cell, a metal force-distribution plate, and a pressure sensor (load cell), is clamped onto a mechanically fixed constraint. (b) Zoomed-in schematic of the graphite anode illustrating that Li-plating induces a higher volume/pressure change than intercalation for the same amount of charge passed. (c) The average interplanar lattice spacing of graphite at different lithiation stages agrees with the pressure profile of a 70 mAh NMC-graphite multilayer pouch cell. The cell was charged and discharged at 0.2 C under which Li-plating is unlikely to happen. (d) The differential pressure (dP/dQ) profile of the pouch cell in a full cycle. The red dashed line is the upper bound of dP/dQ during intercalation defining the Li-plating threshold. When Li-plating happens, the dP/dQ curve will penetrate the upper pink region.

Funding