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2. Utilizing Oxygen Redox in Layered Cathode Materials from Multiscale Perspective

Author

Lee, Gi‐Hyeok, Lau, Vincent Wing‐hei, Yang, Wanli, and Kang, Yong‐Mook

Subjects
Affordable and Clean Energy, dopant, layered cathodes, microstructure, oxygen redox, surface modifications, Macromolecular and Materials Chemistry, Materials Engineering, Interdisciplinary Engineering
Abstract

In high-capacity layered oxide cathode materials, utilization of lattice oxygen as a redox center is considered to be one of the most promising approaches to overcome the capacity limitation set by conventional transition metal redox centers. However, rapid material degradation is often associated with oxygen oxidation, leading to formidable challenges in utilizing oxygen redox. Further mechanistic understanding of the oxygen activities thus becomes critical to better control oxygen redox reactions. This review summarizes recent advances for investigating oxygen redox reactions in cathode materials from a multiscale perspective, i.e., from the atomistic level to the microstructure regime. First the mechanistic aspects of oxygen redox and the consequences of this reaction on various electrode degradation pathways during battery operation (e.g., oxygen loss, transition metal migration, irreversible phase transition), relating structural changes at the crystallographic scale to those at the macro scale, are discussed. Then recent developments based on atomic and microstructure modifications that are promising for improving the reversibility of oxygen redox reaction or mitigating the harmful processes arising from oxidation of the oxygen centers under high operating voltage are recounted. The analysis is concluded with a commentary on further research directions toward optimizing the oxygen activity for high-capacity charge storage.

Published
2021

4. Towards rational design of carbon nitride photocatalysts: Identification of cyanamide 'defects' as catalytically relevant sites

Author

Lau, Vincent Wing-hei, Moudrakovski, Igor, Botari, Tiago, Weinburger, Simon, Mesch, Maria B., Duppel, Viola, Senker, Jürgen, Blum, Volker, and Lotsch, Bettina V.

Subjects
Condensed Matter - Materials Science
Abstract

The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C3N4), is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches due to a lack of fundamental understanding of its mechanistic operation. Here, we employ molecular heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant "defect". We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12- and 16-times the hydrogen evolution rate and apparent quantum efficiency (400 nm), respectively, compared to the benchmark melon. Computational modelling and material characterization suggest this moiety improves co-ordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the separation of the photo-generated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high performance heptazine-based photocatalysts., Comment: Main article: 15 pages, 5 figures

Published
2016
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7. Reversible Li Intercalation in Layered Cathodes Enabled by Dopant-Induced Medium-Range Orders [Dataset]

Author

Ou, Xun, Luo, Junfeng, Zhang, Jiliang, Chen, Chi Liang, Yu, Jing, Hu, Jiawei, Chang, Chung Kai, Moudrakovski, Igor, Lau, Vincent Wing-hei, Zhao, Ruirui, Ou, Xun, Luo, Junfeng, Zhang, Jiliang, Chen, Chi Liang, Yu, Jing, Hu, Jiawei, Chang, Chung Kai, Moudrakovski, Igor, Lau, Vincent Wing-hei, and Zhao, Ruirui

Abstract

Doping could effectively tune the electrochemical performance of layered oxide cathodes in Li-ion batteries, whereas the working mechanism is usually oversimplified (i.e., a “pillar” effect). Although the Jahn–Teller effect is generally regarded as the fundamental origin of structural instability in some oxides, more polyhedral distortions are associated with pseudo-JTE (PJTE), which involves vibronic couplings. In this work, the atomic structures of doped LiCoO2 by Mg cations, F anions, and both were investigated thoroughly to reveal the atomic environments of these dopants and their influence on electrochemical performance. The function of these dopants as pillars is well discussed from the view of PJTE manipulation. Briefly, the MgO4 tetrahedra in Mg-doped LiCoO2 could suppress the charge transfer from the ligand to Co in neighboring octahedra, thus depressing PJTE. Although F doping does increase the ligand-field strength, the induced octahedral distortion reduces the structural stability dramatically. Comparatively, Mg/F co-doping generates the CoO5F–MgO4F2–CoO5F medium-range orders (MROs), which could depress both structural distortion and charge transfer in Co-centered octahedra for reduced PJTE. The reduced PJTE accounts for the improved electrochemical performance, making the co-doped LiCoO2 offer the best performance: a 70% capacity retention after 200 cycles within the potential range of 2.8–4.6 V, followed by Mg-doped LiCoO2. In contrast, although F-doping could induce an extra rock salt-like surface layer for higher capacity, its cycling improvement is rather limited. These results highlight the importance of structural modulation in enhancing the material performance and propose that the manipulation of PJTE would be an effective strategy in developing novel high-performance oxide cathodes.

Published
2023

8. Reversible Li Intercalation in Layered Cathodes Enabled by Dopant-Induced Medium-Range Orders

Author

National Natural Science Foundation of China, South China Normal University, TINCI, Chen, Chi Liang [0000-0001-8509-0655], Moudrakovski, Igor [0000-0002-8919-4766], Zhao, Ruirui [0000-0002-6016-5275], Ou, Xun, Luo, Junfeng, Zhang, Jiliang, Chen, Chi Liang, Yu, Jing, Hu, Jiawei, Chang, Chung Kai, Moudrakovski, Igor, Lau, Vincent Wing-hei, Zhao, Ruirui, National Natural Science Foundation of China, South China Normal University, TINCI, Chen, Chi Liang [0000-0001-8509-0655], Moudrakovski, Igor [0000-0002-8919-4766], Zhao, Ruirui [0000-0002-6016-5275], Ou, Xun, Luo, Junfeng, Zhang, Jiliang, Chen, Chi Liang, Yu, Jing, Hu, Jiawei, Chang, Chung Kai, Moudrakovski, Igor, Lau, Vincent Wing-hei, and Zhao, Ruirui

Abstract

Doping could effectively tune the electrochemical performance of layered oxide cathodes in Li-ion batteries, whereas the working mechanism is usually oversimplified (i.e., a “pillar” effect). Although the Jahn-Teller effect is generally regarded as the fundamental origin of structural instability in some oxides, more polyhedral distortions are associated with pseudo-JTE (PJTE), which involves vibronic couplings. In this work, the atomic structures of doped LiCoO2 by Mg cations, F anions, and both were investigated thoroughly to reveal the atomic environments of these dopants and their influence on electrochemical performance. The function of these dopants as pillars is well discussed from the view of PJTE manipulation. Briefly, the MgO4 tetrahedra in Mg-doped LiCoO2 could suppress the charge transfer from the ligand to Co in neighboring octahedra, thus depressing PJTE. Although F doping does increase the ligand-field strength, the induced octahedral distortion reduces the structural stability dramatically. Comparatively, Mg/F co-doping generates the CoO5F-MgO4F2-CoO5F medium-range orders (MROs), which could depress both structural distortion and charge transfer in Co-centered octahedra for reduced PJTE. The reduced PJTE accounts for the improved electrochemical performance, making the co-doped LiCoO2 offer the best performance: a 70% capacity retention after 200 cycles within the potential range of 2.8-4.6 V, followed by Mg-doped LiCoO2. In contrast, although F-doping could induce an extra rock salt-like surface layer for higher capacity, its cycling improvement is rather limited. These results highlight the importance of structural modulation in enhancing the material performance and propose that the manipulation of PJTE would be an effective strategy in developing novel high-performance oxide cathodes.

Published
2023

11. Addition to “Regulating Pseudo-Jahn–Teller Effect and Superstructure in Layered Cathode Materials for Reversible Alkali-Ion Intercalation”

Author

Zhang, Jiliang, primary, Kim, Jae-Bum, additional, Zhang, Jing, additional, Lee, Gi-Hyeok, additional, Chen, Mingzhe, additional, Lau, Vincent Wing-hei, additional, Zhang, Kai, additional, Lee, Suwon, additional, Chen, Chi-Liang, additional, Jeon, Tae-Yeol, additional, Kwon, Young-Wan, additional, and Kang, Yong-Mook, additional

Published
2022
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12. Regulating Pseudo-Jahn–Teller Effect and Superstructure in Layered Cathode Materials for Reversible Alkali-Ion Intercalation

Author

Zhang, Jiliang, primary, Kim, Jae-Bum, additional, Zhang, Jing, additional, Lee, Gi-Hyeok, additional, Chen, Mingzhe, additional, Lau, Vincent Wing-hei, additional, Zhang, Kai, additional, Lee, Suwon, additional, Chen, Chi-Liang, additional, Jeon, Tae-Yeol, additional, Kwon, Young-Wan, additional, and Kang, Yong-Mook, additional

Published
2022
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13. Activating a Multielectron Reaction of NASICON-Structured Cathodes toward High Energy Density for Sodium-Ion Batteries

Author

Chen, Mingzhe, primary, Hua, Weibo, additional, Xiao, Jin, additional, Zhang, Jiliang, additional, Lau, Vincent Wing-hei, additional, Park, Mihui, additional, Lee, Gi-Hyeok, additional, Lee, Suwon, additional, Wang, Wanlin, additional, Peng, Jian, additional, Fang, Liang, additional, Zhou, Limin, additional, Chang, Chung-Kai, additional, Yamauchi, Yusuke, additional, Chou, Shulei, additional, and Kang, Yong-Mook, additional

Published
2021
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17. Morphology Control in 2D Carbon Nitrides: Impact of Particle Size on Optoelectronic Properties and Photocatalysis

Author

Kröger, Julia, primary, Jiménez-Solano, Alberto, additional, Savasci, Gökcen, additional, Lau, Vincent Wing-hei, additional, Duppel, Viola, additional, Moudrakovski, Igor, additional, Küster, Kathrin, additional, Scholz, Tanja, additional, Gouder, Andreas, additional, Schreiter, Marie Luise, additional, Podjaski, Filip, additional, Ochsenfeld, Christian, additional, and Lotsch, Bettina, additional

Published
2021
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18. Molecular reconfigurations enabling active liquid–solid interfaces for ultrafast Li diffusion kinetics in the 3D framework of a garnet solid-state electrolyte

Author

Wei, Fuxin, primary, Wu, Shufen, additional, Zhang, Jiliang, additional, Fan, Hongyang, additional, Wang, Liuyang, additional, Lau, Vincent Wing-hei, additional, Hou, Sizhou, additional, Zhang, Minmin, additional, Zhang, Jiafeng, additional, Liang, Bo, additional, and Zhao, Ruirui, additional

Published
2021
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22. A Tour‐Guide through Carbon Nitride‐Land: Structure‐ and Dimensionality‐Dependent Properties for Photo(Electro)Chemical Energy Conversion and Storage.

Author

Lau, Vincent Wing‐hei and Lotsch, Bettina V.

Subjects
*CHEMICAL energy conversion, *ENERGY storage, *COMPUTER-assisted molecular design, *MOLECULAR structure, *CONSTRUCTION materials, *HOT carriers
Abstract

Despite the explosion in the number of publications on the graphitic carbon nitride family of materials, much still remains unknown about their structure and the underlying properties responsible for their various applications. This critical review covers the state‐of‐the‐art in the understanding of their structure–property–photocatalysis relationship, from their molecular constituents to stacking as a (quasi) two‐dimensional structure, highlighting the areas in which there is wide agreement and those still unresolved. This review first recounts how the structural understanding of these materials has evolved since the 19th century, followed by a commentary on the best practice for unambiguously characterizing their molecular structure and two‐dimensional stacking arrangements. The recent literature is then examined to elucidate how individual molecular moieties affect their various material properties, particularly their chemical and opto–electronic properties, carrier dynamics, and catalytic reactivity, and how their use for energy applications can be impacted by the structural features across each dimension. Lastly, the translation of the aforementioned fundamental insights to rational molecular design is demonstrated, highlighting the synthesis of heptazine‐based materials for order‐of‐magnitude improvement in photocatalytic reactivity, as well as the unusual phenomenon of stabilization of light‐induced electrons, an effect currently exploited for a new paradigm in solar energy storage. [ABSTRACT FROM AUTHOR]

Published
2022
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27. The origin of heavy element doping to relieve the lattice thermal vibration of layered materials for high energy density Li ion cathodes

Author

Zhao, Ruirui, primary, Zhang, Jiliang, additional, Lee, Gi-Hyeok, additional, Zhang, Kai, additional, Lau, Vincent Wing-hei, additional, Lee, Jey-Jau, additional, Moudrakovski, Igor, additional, Yang, Yue-Lin, additional, Zou, Feng, additional, Park, Mihui, additional, Hung, I.-Ming, additional, and Kang, Yong-Mook, additional

Published
2020
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34. Highly Reversible and Rapid Sodium Storage in GeP3with Synergistic Effect from Outside-In Optimization

Author

Wang, Ting, Zhang, Kai, Park, Mihui, Lau, Vincent Wing-hei, Wang, Haihua, Zhang, Jiliang, Zhang, Jing, Zhao, Ruirui, Yamauchi, Yusuke, and Kang, Yong-Mook

Abstract

The composite GeP3/C@rGO as a sodium ion battery anode material was fabricated by introducing a carbon matrix into GeP3through high-energy ball milling, followed by encapsulating the resultant composite with graphene viaa solution-based ultrasonic method. To delineate the individual role of carbon matrix and graphene, material characterization and electrochemical analyses were performed for GeP3/C@rGO and three other samples: bare GeP3, GeP3with graphene coating (GeP3@rGO), and GeP3with carbon matrix (GeP3/C). GeP3/C@rGO exhibits the highest electric conductivity (5.89 × 10–1S cm–1) and the largest surface area (167.85 m2g–1) among the four samples. The as-prepared GeP3/C@rGO delivered a reversible high capacity of 1084 mA h g–1at 50 mA g–1, excellent rate capacity (435.4 mA h g–1at a high rate of 5 A g–1), and long-term cycling stability (400 cycles with a reversible capacity of 823.3 mA h g–1at 0.2 A g–1), all of which outperform the other three samples. The kinetics investigation reveals a “pseudocapacitive behavior” in GeP3/C and GeP3/C@rGO, where solely faradic reactions took place in bare GeP3and GeP3@rGO with a typical “battery behavior”. Based on ex-situX-ray photoelectron spectroscopy and ex-situelectrochemical impedance spectroscopy, the carbon matrix serves to activate and stabilize the interior of the composite, while the graphene protects and restrains the exterior surface. Benefiting from the synergistic combination of these two components, GeP3/C@rGO achieved extremely stable cycling stability as well as outstanding rate performance.

Published
2020
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35. Lewis acidity/basicity of inorganic interlayers as a descriptor for inhibition of charge shuttling in DMPZ-based organic batteries.

Author

Lau, Vincent Wing-hei, Lee, Chang-Gi, Lee, Hakwoo, and Kang, Yong-Mook

Subjects
*LEWIS acidity, *ELECTRIC batteries, *LITHIUM sulfur batteries, *STATIONARY phase (Chromatography), *ALUMINUM oxide, *LITHIUM cells, *BASICITY
Abstract

A disadvantage of organic battery electrode materials is their solubility in battery electrolytes, leading to the shuttle effect. One solution is to use an additional separator, the interlayer, to inhibit charge shuttling of the solubilized species. The work here aims to identify the interlayer material's properties critical to reducing the shuttle effect so as to provide some guidance on interlayer material selection. Using various alumina as interlayer materials in batteries with N,N′ -dimethylphenazine (DMPZ) as cathode material, surface charge can be identified as the critical descriptor to inhibiting DMPZ shuttling, more so than morphological properties. Specifically, the interlayer material can be selected for shuttling prevention in the same way as how one chooses chromatography stationary phase for analyte retention. Based on further characterizations, short- and long-term shuttling inhibition can be attributed to the interlayer and the solid-electrolyte interface formed in situ, respectively. Inorganic separators are known to alleviate the deleterious shuttle effect in batteries using molecular electrode materials, including the redox-active organic compounds. We identified here the properties of the interlayer material crucial to inhibiting the shuttle effect and elucidated some mechanistic insights, both useful for rational material selection for improving battery performance. [Display omitted] • Al 2 O 3 interlayer significantly inhibited the shuttle effect of organic batteries. • Efficacy of the Al 2 O 3 interlayer was determined by its Lewis acidity/basicity. • Shuttle effect was prevented by Al 2 O 3 interlayer for both short and long time. • The shuttling protection was realized through dynamic protection mechanism. [ABSTRACT FROM AUTHOR]

Published
2024
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37. Urea‐Modified Carbon Nitrides: Enhancing Photocatalytic Hydrogen Evolution by Rational Defect Engineering

Author

Lau, Vincent Wing‐hei, primary, Yu, Victor Wen‐zhe, additional, Ehrat, Florian, additional, Botari, Tiago, additional, Moudrakovski, Igor, additional, Simon, Thomas, additional, Duppel, Viola, additional, Medina, Elise, additional, Stolarczyk, Jacek K., additional, Feldmann, Jochen, additional, Blum, Volker, additional, and Lotsch, Bettina V., additional

Published
2017
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43. Can X-ray Photoelectron Spectroscopy Characterize Photocatalytically Relevant Defects in Graphitic Carbon Nitride? Re-examining Peak Assignment and Quantification Using Model Compounds

Author

Lau, Vincent Wing-hei, Lu, Chi-Feng, Putri Wijaya, Nathania, and Lutan, Michael

Abstract

Heptazine-based compounds, often classified as graphitic carbon nitride or the g-C3N4family of materials, have attracted a significant amount of attention since the seminal report on their photocatalytic reactivity. To achieve further progress, a better understanding of their structure–property–function relationship is necessary, which requires the development of analytical methodologies for characterizing their structure. In this work, we assessed the suitability of X-ray photoelectron spectroscopy (XPS) for the analysis of g-C3N4materials, especially their crystallographic defects that have been implicated in the photocatalytic mechanism. Using molecular model compounds and following the latest recommendation of sample mounting, we first determined the binding energies of key functional groups, which were then used to analyze the XPS spectra of melon, the archetype g-C3N4material. Extensive signal overlaps prevented the resolution of −NH–, −NH2, and “defect” moieties such as guanidine and nitrile/cyanamide, though urea-like defects may be discernible in the O 1s spectrum. Nevertheless, given that the quantitative uncertainty was estimated to range from 14% to 65%, structural information that can be obtained via XPS was therefore limited, necessitating additional analytical techniques for a more comprehensive characterization of g-C3N4materials, especially for identifying the “defect” groups that are crucial to their applications.

Published
2024
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45. Controlling the Valence State of Cu Dopant in α-Fe2O3Anodes: Effects on Crystal Structure and the Conversion Reactions with Alkali Ions

Author

Zhang, Jiliang, Lau, Vincent Wing-hei, Liao, Chang-Zhong, Wong, Kam Wa, Lee, Gi-Hyeok, Zou, Feng, Chang, Chung-Kai, Sheu, Hwo-Shuenn, and Kang, Yong-Mook

Abstract

Doping is one of the most important ways to tailor the performance of energy materials. However, the crystal structure of doped materials is usually oversimplified as a simple substitution of dopants. Here, we characterized the doped α-Fe2O3with different Cu cations using synchrotron X-ray diffraction, X-ray absorption, and X-ray photoelectron spectroscopy, and electrochemically evaluated it as an anode in lithium batteries. The results suggest that doping is not the simple replacement of Fe3+sites by Cu2+or Cu+but induces a complex local structure change, which may be a characteristic of this class of materials. In Cu+-doped samples, Cu+not only replaces the Fe3+site and distorts the FeO6octahedra, but also gives rise to oxygen vacancies in CuO6octahedra in the bulk structure and peroxides at the surface, leading to uniform precipitation of Cu as a conductive and buffering agent. These CuO6octahedra also facilitate homogeneous reactions (electrochemical reduction of Cu+and Fe3+together) and the formation of high quality solid-electrolyte interface (SEI) layers. All these factors account for its improved electrochemical performance (discharge capacity of 841(25) mAh/g against 758(21) mAh/g of undoped one, after 80 cycles at 100mA/g). In Cu2+-doped samples, Cu2+takes both Fe3+and empty octahedral interstitial sites, forming linear clusters of three neighboring CuO6octahedra. Such medium-range phase separation causes electrochemical reduction to metallic Cu before the reduction of Fe3+, leading to inactive surface Cu that contributes to poor SEI layers and deteriorates its electrochemical performances. The present work allows a better understanding of how doping affects the crystallographic structures and offers insights into how this strategy can be employed to improve electrochemical performance, in contrast to the ambiguity over material properties associated with the commonly accepted model of simple atomic replacement.

Published
2019
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47. Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation.

Author

Lau, Vincent Wing ‐ hei, Klose, Daniel, Kasap, Hatice, Podjaski, Filip, Pignié, Marie ‐ Claire, Reisner, Erwin, Jeschke, Gunnar, and Lotsch, Bettina V.

Subjects
*ENERGY storage, *SOLAR energy, *SOLAR radiation, *INTERSTITIAL hydrogen generation, *OXIDATION-reduction reaction
Abstract

While natural photosynthesis serves as the model system for efficient charge separation and decoupling of redox reactions, bio-inspired artificial systems typically lack applicability owing to synthetic challenges and structural complexity. We present herein a simple and inexpensive system that, under solar irradiation, forms highly reductive radicals in the presence of an electron donor, with lifetimes exceeding the diurnal cycle. This radical species is formed within a cyanamide-functionalized polymeric network of heptazine units and can give off its trapped electrons in the dark to yield H2, triggered by a co-catalyst, thus enabling the temporal decoupling of the light and dark reactions of photocatalytic hydrogen production through the radical′s longevity. The system introduced here thus demonstrates a new approach for storing sunlight as long-lived radicals, and provides the structural basis for designing photocatalysts with long-lived photo-induced states. [ABSTRACT FROM AUTHOR]

Published
2017
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50. Cationically Charged MnIIAlIIILDH Nanosheets by ChemicalExfoliation and Their Use As Building Blocks in Graphene Oxide-BasedMaterials.

Author

Werner, Stephan, Lau, Vincent Wing-hei, Hug, Stephan, Duppel, Viola, Clausen-Schaumann, Hauke, and Lotsch, Bettina V.

Subjects
*CHEMICAL peel, *CHARGE exchange reactions, *GRAPHENE oxide, *HYDROXIDES, *MOLECULAR self-assembly, *SUSPENSIONS (Chemistry)
Abstract

We report on the synthesis and exfoliationof MnIIAlIIIsulfonate and sulfate layered doublehydroxides (LDHs)and their combination with graphene oxide by charge-directed self-assembly.The synthesis of the LDH compounds has been accomplished either directlyby coprecipitation of the respective hydroxides with sulfonate anionsor by ion-exchange of the chloride-containing LDH with sodium dodecylsulfate.Exfoliation of the bulk material in formamide yields colloidal suspensionsof positively charged nanosheets with lateral dimensions of tens tohundreds of nanometers and thicknesses down to 1.3 nm, ascertainedby TEM and AFM. Flocculation of the LDH nanosheets with an aqueousgraphene oxide suspension yields a hybrid material that can be convertedto a reduced graphene oxide/LDH composite by hydrazine reduction.The hybrid materials were tested for pseudocapacitive electrochemicalstorage capacity and electrocatalytic oxygen evolution reactions andshowed significant increases compared to the pristine materials. [ABSTRACT FROM AUTHOR]

Published
2013
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