Fluorinated covalent organic polymers for high performance sulfur cathodes in lithium–sulfur batteries

H. Shin§, D. Kim§, H. J. Kim§, J. Kim, K. Char*, C. T. Yavuz*, J. W. Choi*

Chem. Mater., accepted (2019). §: Equal contribution
DOI: 10.1021/acs.chemmater.9b01986

Lithium–sulfur (Li–S) batteries by far offer higher theoretical energy density than that of the commercial lithium-ion battery counterparts, but suffer predominantly from an irreversible shuttling process involving lithium polysulfides. Here, we report a fluorinated covalent organic polymer (F-COP) as a template for high performance sulfur cathodes in Li–S batteries. The fluorination allowed facile covalent attachment of sulfur to a porous polymer framework via nucleophilic aromatic substitution reaction (SNAr), leading to high sulfur content, e.g., over 70 wt %. The F-COP framework was microporous with 72% of pores within three well-defined pore sizes, viz. 0.58, 1.19, and 1.68 nm, which effectively suppressed polysulfide dissolution via steric and electrostatic hindrance. As a result of the structural features of the F-COP, the resulting sulfur electrode exhibited high electrochemical performance of 1287.7 mAh g–1 at 0.05C, 96.4% initial Columbic efficiency, 70.3% capacity retention after 1000 cycles at 0.5C, and robust operation for a sulfur loading of up to 4.1 mgsulfur cm–2. Our findings suggest the F-COP family with the adaptability of SNAr chemistry and well-defined microporous structures as useful frameworks for highly sustainable sulfur electrodes in Li–S batteries.

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EEWS 2016: Progress and Perspectives of Energy Science and Technology

J. Oh, J. W. Choi, C. T. Yavuz, S. Y. Chung, J. Y. Park, Y. Jung*
ACS Energy Lett., 2, 592–594, (2017).
DOI: 10.1021/acsenergylett.6b00640

Established in 2009, the Graduate School of EEWS (Energy, Environment, Water, and Sustainability) at the Korea Advanced Institute of Science and Technology (KAIST) is the first of its kind, an interdisciplinary department at KAIST collectively addressing with interdisciplinary approaches for the emerging and urgent issues in energy, environment, water, and natural resources of the twenty-first century for sustainable society through science, technology, and education (http://eewseng.kaist.ac.kr). Currently housing 12 research groups with diverse backgrounds in chemistry; physics; chemical, electrical, mechanical, and environmental engineering; and materials science, the EEWS is the culmination of unprecedented collaboration under the same roof with close interaction of students and faculty from unlikely backgrounds (Figure). The output in a relatively short period of time is remarkable; the collaborative research combining basic and applied disciplines of seemingly different subjects have produced many novel concepts and approaches in various energy science and technology fields that are otherwise difficult to conceive in a traditional way. In an effort to critically assess the current status of the energy research, identify major challenges, and further stimulate active interactions among the disciplines to solve the challenges, we held the first EEWS forum, “EEWS 2016: Progress and Perspectives of Energy Science and Technology”, in the KI Fusion Hall of KAIST on October 20, 2016. The meeting featured eight internationally recognized energy experts from around the world introducing their cutting-edge research covering a wide range of topics in energy materials, advanced characterization tools, and catalysis, from both experimental and theoretical viewpoints.

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Nanoporous Networks as Caging Supports for Uniform, Surfactant-free Co3O4 Nanocrystals and Their Applications in Energy Storage and Conversion

J. Byun, H. A. Patel, D. J. Kim, C. H. Jung, J. Y. Park*, J. W. Choi*, C. T. Yavuz* 
J. Mater. Chem. A, 3, 15489 - 15497, (2015). DOI: 10.1039/C5TA02825F.
Selected among "Hot Papers of 2015"

We report a new, surfactant-free method to produce Co₃O₄ nanocrystals with controlled sizes and high dispersity by caging templation of nanoporous networks. The morphologies of Co₃O₄ nanoparticles differ from wires to particulates by simply varying solvents. The composites of nanoparticles within network polymers are highly porous and are promising for many applications where accessible surface and aggregation prevention are important. The electrochemical performance of the composites demonstrates superior capacity and cyclic stability at a high current density (∼980 mA h g−1 at the 60th cycle at a current density of 1000 mA g−1). In a catalytic oxidation reaction of carbon monoxide, the composites exhibit a remarkable stability (in excess of 35 hours) and catalytic performance (T100 = 100 °C).

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A Half Millimeter Thick Coplanar Flexible Battery with Wireless Recharging Capability

J. S. Kim, D. Ko, D. J. Yoo, D. S. Jung, C. T. Yavuz, N. I. Kim, I. S. Choi, J. Y. Song*, J. W. Choi*
Nano Lett., 15 (4), 2350–2357, (2015). DOI: 10.1021/nl5045814.
Most read paper in March 2015.

Most of the existing flexible lithium ion batteries (LIBs) adopt the conventional cofacial cell configuration where anode, separator, and cathode are sequentially stacked and so have difficulty in the integration with emerging thin LIB applications, such as smart cards and medical patches. In order to overcome this shortcoming, herein, we report a coplanar cell structure in which anodes and cathodes are interdigitatedly positioned on the same plane. The coplanar electrode design brings advantages of enhanced bending tolerance and capability of increasing the cell voltage by in series-connection of multiple single-cells in addition to its suitability for the thickness reduction. On the basis of these structural benefits, we develop a coplanar flexible LIB that delivers 7.4 V with an entire cell thickness below 0.5 mm while preserving stable electrochemical performance throughout 5000 (un)bending cycles (bending radius = 5 mm). Also, even the pouch case serves as barriers between anodes and cathodes to prevent Li dendrite growth and short-circuit formation while saving the thickness. Furthermore, for convenient practical use wireless charging via inductive electromagnetic energy transfer and solar cell integration is demonstrated.

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