Rechargeable aqueous Na-air batteries: Highly improved voltage efficiency by use of catalysts

Sahgong, S.H.,Senthilkumar, S.T.,Kim, K.,Hwang, S.M.,Kim, Y. Elsevier Science 2015 Electrochemistry communications Vol.61 No.-

The first rechargeable aqueous Na-air battery has been fabricated, and its electrochemical properties and reversibility are reported herein. The charge-discharge properties of the battery were tested using both Vulcan XC72R- and Pt/C-coated carbon paper as the air electrode. Pt/C-coated carbon paper exhibited a voltage efficiency of 84.3%, whereas, for Vulcan XC72R-coated carbon paper and uncoated carbon paper, the observed efficiencies were 78.0% and 72.4%, respectively. Use of Pt/C-coated carbon paper led to a high and stable discharging voltage of 2.85V. The reported rechargeable aqueous Na-air battery is a potential candidate for high energy density batteries in the future.

Upcycling of nonporous coordination polymers: controllable-conversion toward porosity-tuned N-doped carbons and their electrocatalytic activity in seawater batteries

Jeoung, S.,Sahgong, S.,Kim, J.,Hwang, S.,Kim, Y.,Moon, H. Royal Society of Chemistry 2016 Journal of Materials Chemistry A Vol.4 No.35

<P>Herein, we report the preparation of highly porous N-doped carbons (PNCs) via thermolysis of nonporous Zn-based coordination polymers (CPs) constructed with nitrogen-containing ligands. So far, the thermal conversion of Zn-CPs, including metal-organic frameworks (MOFs), has mainly yielded microporous carbon materials, and to change the textural properties of end carbons, new CPs/MOFs with different properties were introduced. However, present studies show that just varying the conversion conditions of a parent CP results in porosity-tuned PNCs, in which especially mesoporosity is developed, and this is applicable for even nonporous CPs. This is conducted based on the understanding of conversion phenomena which is that during thermal conversion of Zn-based CPs, the in situ generated Zn metal species act as porogens and their agglomeration can be controlled by the reaction conditions. Different reaction temperatures, ramping rates and retention times allow control over the ratio of micro-to meso-pore volume, while a slower ramping rate and longer retention time at lower heating temperature induced the agglomeration of the porogens, yielding greater mesoporosity, and holding the Zn-CPs at high temperature for a short period afforded the micropore-dominant PNCs due to rapid porogen elimination. The superiority of the mesopore-developed PNCs as electrocatalysts, attributed to greater mass-transport-accessible surfaces, was examined for the electrodes in a rechargeable seawater battery system as an example of a practical application. Therefore, our synthetic approach provides a facile method for the preparation of PNCs with suitable hierarchical pore distributions for use as energy-related materials without exerting significant effort in the design of coordination compounds.</P>

Comparative electrochemical sodium insertion/extraction behavior in layered Na_xVS₂ and Na_xTiS₂

Lee, E.,Sahgong, S.,Johnson, C.S.,Kim, Y. Pergamon Press 2014 ELECTROCHIMICA ACTA Vol.143 No.-

This study investigates the electrochemical sodium insertion/extraction of Na<SUB>x</SUB>VS<SUB>2</SUB>, and Na<SUB>x</SUB>TiS<SUB>2</SUB> in the voltage range where either intercalation (0.2@?x@?1) or displacement-conversion reaction (x>1) occurs. Both Na<SUB>x</SUB>VS<SUB>2</SUB> and Na<SUB>x</SUB>TiS<SUB>2</SUB> showed good reversible capacities, as high as ~160mAh/g at an average voltage of ~1.9V vs. Na in the region for the intercalation reaction (0.2@?x@?1). When sodium (Na) insertion was forced further to the x>1 composition, Na<SUB>x</SUB>VS<SUB>2</SUB> exhibited the direct displacement-conversion reaction at 0.3V vs. Na without further Na intercalation, which contrasted with the wider lithium intercalation range of 0<x@?2 for Li<SUB>x</SUB>VS<SUB>2</SUB>. The displacement-conversion reaction for Na<SUB>x</SUB>VS<SUB>2</SUB> (x>1) was reversible with a specific capacity of above 200mAh/g up to 15 cycles, but the displacement reaction for Na<SUB>x</SUB>TiS<SUB>2</SUB> (x>1) was not observed.

Ammonium Fluoride Mediated Synthesis of Anhydrous Metal Fluoride–Mesoporous Carbon Nanocomposites for High-Performance Lithium Ion Battery Cathodes

Chun, Jinyoung,Jo, Changshin,Sahgong, Sunhye,Kim, Min Gyu,Lim, Eunho,Kim, Dong Hyeon,Hwang, Jongkook,Kang, Eunae,Ryu, Keun Ah,Jung, Yoon Seok,Kim, Youngsik,Lee, Jinwoo American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.51

<P>Metal fluorides (MFx) are one of the most attractive cathode candidates for Li ion batteries (LIBs) due to their high conversion potentials with large capacities. However, only a limited number of synthetic methods, generally involving highly toxic or inaccessible reagents, currently exist, which has made it difficult to produce well-designed nanostructures suitable for cathodes; consequently, harnessing their potential cathodic properties has been a challenge. Herein, we report a new bottom-up synthetic method utilizing ammonium fluoride (NH4F) for the preparation of anhydrous MFx (CuF2, FeF3, and CoF2)/mesoporous carbon (MSU-F-C) nanocomposites, whereby a series of metal precursor nanoparticles preconfined in mesoporous carbon were readily converted to anhydrous MFx through simple heat treatment with NH4F under solventless conditions. We demonstrate the versatility, lower toxicity, and efficiency of this synthetic method and, using XRD analysis, propose a mechanism for the reaction. All MFx/MSU-F-C prepared in this study exhibited superior electrochemical performances, through conversion reactions, as the cathode for LIBs. In particular, FeF3/MSU-F-C maintained a capacity of 650 mAh g(FeF3)(-1) across 50 cycles, which is similar to 90% of its initial capacity. We expect that this facile synthesis method will trigger further research into the development of various nanostructured MFx for use in energy storage and other applications.</P>

Metal-free hybrid seawater fuel cell with an ether-based electrolyte

Kim, H.,Park, J. S.,Sahgong, S.,Park, S.,Kim, J. K.,Kim, Y. Royal Society of Chemistry 2014 Journal of Materials Chemistry A Vol.2 No.46

New Chemical Route for the Synthesis of β-Na_0.33V₂O₅ and Its Fully Reversible Li Intercalation

Kim, Jae-Kwang,Senthilkumar, B.,Sahgong, Sun Hye,Kim, Jung-Hyun,Chi, Miaofang,Kim, Youngsik American Chemical Society 2015 ACS APPLIED MATERIALS & INTERFACES Vol.7 No.12

<P>To obtain good electrochemical performance and thermal stability of rechargeable batteries, various cathode materials have been explored including NaVS<SUB>2</SUB>, β-Na<SUB>0.33</SUB>V<SUB>2</SUB>O<SUB>5</SUB>, and Li<SUB><I>x</I></SUB>V<SUB>2</SUB>O<SUB>5</SUB>. In particular, Li<SUB><I>x</I></SUB>V<SUB>2</SUB>O<SUB>5</SUB> has attracted attention as a cathode material in Li-ion batteries owing to its large theoretical capacity, but its stable electrochemical cycling (i.e., reversibility) still remains as a challenge and strongly depends on its synthesis methods. In this study, we prepared the Li<SUB><I>x</I></SUB>V<SUB>2</SUB>O<SUB>5</SUB> from electrochemical ion exchange of β-Na<SUB>0.33</SUB>V<SUB>2</SUB>O<SUB>5</SUB>, which is obtained by chemical conversion of NaVS<SUB>2</SUB> in air at high temperatures. Crystal structure and particle morphology of β-Na<SUB>0.33</SUB>V<SUB>2</SUB>O<SUB>5</SUB> are characterized by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, in combination with electrochemical data, suggest that Na ions are extracted from β-Na<SUB>0.33</SUB>V<SUB>2</SUB>O<SUB>5</SUB> without irreversible structural collapse and replaced with Li ions during the following intercalation (i.e., charging) process. The thus obtained Li<SUB><I>x</I></SUB>V<SUB>2</SUB>O<SUB>5</SUB> delivers a high discharge capacity of 295 mAh g<SUP>–1</SUP>, which corresponds to <I>x</I> = 2, with crystal structural stability in the voltage range of 1.5–4.0 V versus<SUB>.</SUB> Li, as evidenced by its good cycling performance and high Coulombic efficiency (under 0.1 mA cm<SUP>–2</SUP>) at room temperature. Furthermore, the ion-exchanged Li<SUB><I>x</I></SUB>V<SUB>2</SUB>O<SUB>5</SUB> from β-Na<SUB>0.33</SUB>V<SUB>2</SUB>O<SUB>5</SUB> shows stable electrochemical behavior without structural collapse, even at a case of deep discharge to 1.5 V versus Li.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2015/aamick.2015.7.issue-12/acsami.5b01260/production/images/medium/am-2015-01260h_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am5b01260'>ACS Electronic Supporting Info</A></P>