Research

Here, I list a few areas that I'm currently working on. You can refer to my publication list for more complete picture of what I've done in the past and where I may be headed in near future. 

Operando and in-depth characterization in energy materials 

© Hyun-Wook Lee. All rights reserved. This image is protected by copyright and may not be reproduced, distributed, or used without explicit permission.

1. In-situ transmission electron microscopy on battery materials

- Related papers: *Chemical Communications 59 11963 (2023), *Energy & Environmental Science 16 2003 (2023), *Nano Letters 22 7423 (2022), *ACS Materials Letters 4 831 (2022), *Nano Letters 21 1530 (2021), *Nano Letters 19 8793 (2019), *Advanced Energy Materials 9 1803121 (2019), *Journal of the American Chemical Society 139 10133 (2017), etc. 

2. Side-view operando optical microscopy on battery materials

- Related papers: *In preparation, *ACS Energy Letters 10 3112 (2025), *Energy Storage Materials 64 103074 (2024), *ACS Energy Letters 8 3962 (2023), *Energy Storage Materials 57 269 (2023), *Korean Journal of Chemical Engineering 40 488 (2023), Nature Communications, 11 829 (2020), *ChemSusChem 13 1480 (2020), *Advanced Materials 30 1801745 (2018), *ACS Applied Materials & Interfaces 10 15270 (2018), etc.   

3. Cryogenic transmission electron microscopy on battery materials

- Related papers: Advanced Energy Materials 13 2301600 (2023), Nature Communications 14 2459 (2023), *ACS Energy Letters 8 2193 (2023), *Nano Letters 23 3582 (2023), Advanced Energy Materials 13 2203292 (2023), *Advanced Materials 34 2200083 (2022), *Nano Letters 20 4337 (2020), etc.  

4. Operando pressiometry on battery materials

- Related papers: *ACS Energy Letters 10 3112 (2025), *Energy Storage Materials 66 103196 (2024), ACS Applied Materials & Interfaces 14 4051 (2022), *ACS Energy Letters, 6 3261 (2021), Joule 5 2450 (2021), Nature Materials 20 503 (2021), etc.

Materials for rechargeable energy storage 

1. Metallic lithium-based anodes for high-energy batteries

 Dendritic lithium (Li), characterized by vertically grown metallic structures, poses a serious challenge to the development of anode-free Li batteries. These needle-like formations not only create voids on current collectors but also raise significant safety concerns. Extensive dendritic growth increases the interfacial area exposed to electrolytes, accelerating side reactions and resulting in low Coulombic efficiency (CE) due to the accumulation of inactive Li. Traditionally, such growth has been attributed to sluggish Li-ion (Li⁺) transport in the electrolyte. When Li⁺ electro-adsorption at the electrode surface outpaces its movement through the bulk electrolyte, steep concentration gradients emerge, driving uneven Li⁺ flux and uncontrolled deposition. While many studies have attempted to alleviate this issue by introducing lithiophilic agents or optimizing electrolyte formulations, the definition of "lithiophilicity" has often been misinterpreted—its original meaning referred to the wettability of molten Li on host materials, rather than its influence on deposition behavior.

As an example, we take a fundamentally different approach by focusing on the surface migration of Li adatoms—a previously overlooked factor in lithium growth dynamics. Using single-crystal Cu(111) foil as a model current collector, we demonstrate that Li adatoms experience minimal migration barriers, allowing them to move laterally and merge into uniform layers. This lateral growth, rather than vertical dendritic protrusion, significantly suppresses dendrite formation. We directly visualized this behavior as the formation and migration of rhombic dodecahedral Li clusters across the copper surface. Our findings shift the paradigm of Li metal growth from vertical dendrite formation to controlled horizontal deposition via surface migration, paving the way for safer and more efficient anode-free lithium metal batteries.

- Related papers: *In preparation, *Energy & Environmental Science 17 6521 (2024), *ACS Energy Letters 8 2193 (2023), *Nano Letters 23 3582 (2023), *Nano Letters 19 1504 (2019), *Advanced Materials 30 1801745 (2018), etc.

© Hyun-Wook Lee. All rights reserved. This image is protected by copyright and may not be reproduced, distributed, or used without explicit permission.

2. New strategies for high-energy and stable cathodes

The capacity limitations of conventional layered oxide cathodes are primarily dictated by the oxidation states accessible to transition metals (TMs). To overcome this boundary, researchers have actively explored incorporating anionic redox—particularly oxygen redox—into the charge compensation mechanism. The combination of cationic and anionic redox processes can deliver discharge capacities exceeding 250 mAh g⁻¹ at high cutoff voltages above 4.6 V. However, high-voltage oxygen redox introduces several critical challenges, including first-cycle voltage hysteresis, voltage fading, cationic disorder, and irreversible phase transitions. A major contributor to irreversible capacity loss is oxygen gas evolution from lattice oxygen (O²⁻ → O₂ + 2e⁻), which limits practical implementation despite numerous mitigation strategies such as superstructure tuning, compositional engineering, phase control, and morphology optimization. These approaches have provided only partial solutions, and a comprehensive understanding of the underlying redox chemistry remains incomplete.

As an example, our study demonstrates a strategy to suppress first-cycle voltage hysteresis and irreversible O₂ evolution in Li-rich layered oxide cathodes by leveraging covalency competition, induced through the substitution of electropositive groups. We show that in an asymmetric MA–O–MB configuration, nonequivalent electron distribution facilitates charge transfer between metal sites via oxygen ligands. This redistribution enhances the electron density on electronegative TMs, preventing them from reaching unstable oxidation states within the operating voltage range. This phenomenon is broadly observed across diverse TM combinations and offers new insight into controlling undesirable oxygen redox activity. Our findings provide a pathway toward rational design of high-energy, stable Li-rich oxide cathodes through intrinsic redox control.

- Related papers: *In preparation, *In preparation, *Submitted, *Advanced Materials 37 e08602 (2025), *Science Advances 11 eadt0232 (2025), *Angewandte Chemie International Edition 62 e202312928 (2023), *Advanced Materials 35 2208423 (2023), *Small 17 2005605 (2021), etc.

Prussian Blue analogues for post–Li-ion battery technologies

1. Electrode materials based on Prussian Blue open-framework structures

- Related papers: *ChemSusChem 18 e202500564 (2025), *Nano Letters 24 7783 (2024), *Journal of Materials Chemistry A 11 13535 (2023), *Advanced Functional Materials 32 2111901 (2022), etc.

2. New redox flow battery system

- Related papers: *In preparation, *Angewandte Chemie International Edition 64 e202507119 (2025), *EES Catalysis 2 522 (2024), *ACS Energy Letters 8 3702 (2023), *Advanced Energy Materials 13 2300707 (2023), etc.

3. New solid electrolytes or membranes

- Related papers: *EES Batteries online published, *Angewandte Chemie International Edition 62 e202309852 (2023), *Nano Letters 22 1804 (2022), etc.

4. Energy harvesting applications

- Related papers: *Advanced Materials 35 2303199 (2023), Advanced Materials 33 2004717 (2021), Nano Letters 20 1800 (2020), Advanced Functional Materials 28 1803129 (2018), etc.