Exfoliated graphene nanosheets as high‑power anodes for lithium‑ion batteries

Duc‑Luong Vu,Yeon Ju Kwon,Soon Chang Lee,Jea Uk Lee,Jae‑Won Lee 한국탄소학회 2019 Carbon Letters Vol.29 No.1

High-quality and solution processable graphene sheets are produced by a simple electrochemical exfoliation method and employed as a high-power anode for lithium-ion batteries (LIBs). The electrochemically exfoliated graphene (EEG) composed of a few layers of graphene sheets, have low oxygen content and high C/O ratio (~ 14.9). The LIBs with EEG anode exhibit ultrafast lithium storage and excellent cycling stability, but low initial efficiency. The excellent rate capability and cycling stability are attributed to the favorable structural and chemical properties of the EEG, but the large irreversibility needs to be overcome for practical applications.

Layered LiNi_0.8Co_0.1Mn_0.1O₂ Prepared through Calcination in Air with Preoxidized Precursor

Vu, Duc-Luong,Choi, Jae-Young,Kim, Woo-Byoung,Lee, Jung Ju,Lee, Jae-Won The Electrochemical Society 2017 Journal of the Electrochemical Society Vol.164 No.12

<P>The layered Ni-rich oxide LiNi0.8Co0.1Mn0.1O2 is very attractive as a cathode material owing to its large reversible capacity. However, the inherent poor cycling stability of Ni-rich cathode materials has always been a serious issue limiting their commercialization. Moreover, LiNi0.8Co0.1Mn0.1O2 should be calcined under oxygen flow to promote the oxidation of Ni2+ to Ni3+, which makes it costly. In this study, we introduce a technique for preoxidation of the LiNi0.8Co0.1Mn0.1O2 precursor by KMnO4 to partially oxidize Ni2+ in the precursor (KMnO4-treatment) and eliminate the need for calcination in pure oxygen atmosphere. X-ray photoelectron spectroscopy analysis of the KMnO4-treated precursor and LiNi0.8Co0.1Mn0.1O2 obtained from the KMnO4-treated precursor in air (KMnO4-LiNi0.8Co0.1Mn0.1O2) indicates that the valence states of nickel ions are higher than those in the untreated (pristine) precursor and LiNi0.8Co0.1Mn0.1O2 obtained from the pristine precursor in air (Air-LiNi0.8Co0.1Mn0.1O2). The structure of the KMnO4-LiNi0.8Co0.1Mn0.1O2 shows no distinct differences except for a slight expansion along the c-axis and lower cation disorder. Electrochemical measurements demonstrate that KMnO4-LiNi0.8Co0.1Mn0.1O2 has better rate capability and cycling stability at cutoff voltages of 4.3 and 4.5 V than Air-LiNi0.8Co0.1Mn0.1O2. The improved cycling performance of KMnO4-LiNi0.8Co0.1Mn0.1O2 can be attributed to the favorable structural change due to the more oxidized trivalent nickel ion. (C) 2017 The Electrochemical Society. All rights reserved.</P>

A Separator with Activated Carbon Powder Layer to Enhance the Performance of Lithium-Sulfur Batteries

Vu, Duc-Luong,Lee, Jae-Won The Korean Powder MetallurgyMaterials Institute 2018 한국분말재료학회지 (KPMI) Vol.25 No.6

The high theoretical energy density ($2600Wh\;kg^{-1}$) of Lithium-sulfur batteries and the high theoretical capacity of elemental sulfur ($1672mAh\;g^{-1}$) attract significant research attention. However, the poor electrical conductivity of sulfur and the polysulfide shuttle effect are chronic problems resulting in low sulfur utilization and poor cycling stability. In this study, we address these problems by coating a polyethylene separator with a layer of activated carbon powder. A lithium-sulfur cell containing the activated carbon powder-coated separator exhibits an initial specific discharge capacity of $1400mAh\;g^{-1}$ at 0.1 C, and retains 63% of the initial capacity after 100 cycles at 0.2 C, whereas the equivalent cell with a bare separator exhibits a $1200mAh\;g^{-1}$ initial specific discharge capacity, and 50% capacity retention under the same conditions. The activated carbon powder-coated separator also enhances the rate capability. These results indicate that the microstructure of the activated carbon powder layer provides space for the sulfur redox reaction and facilitates fast electron transport. Concurrently, the activated carbon powder layer traps and reutilizes any polysulfides dissolved in the electrolyte. The approach presented here provides insights for overcoming the problems associated with lithium-sulfur batteries and promoting their practical use.

Effect of temperature on the electrochemical oxidation of ash free coal and carbon in a direct carbon fuel cell

Duc-Luong Vu,이충곤 한국화학공학회 2016 Korean Journal of Chemical Engineering Vol.33 No.5

The present study proposes the application of ash-free coal (AFC) as a primary fuel in a direct carbon fuel cell (DCFC) based on a molten carbonate fuel cell (MCFC). AFC was produced by solvent extraction using microwave irradiation. The influence of AFC-to-carbonate ratio (3 : 3, 3 : 1, 3 : 0 and 1 : 3 g/g) on the DCFC performance at different temperatures (650, 750 and 850 oC) was systematically investigated with a coin-type cell. The performance of AFC was also compared with carbon and conventional hydrogen fuels. AFC without carbonate (AFC-to-carbonate ratio=3 : 0 g/g) gave a comparable performance to other compositions, indicating that the gasification of AFC readily occurred without a carbonate catalyst at 850 oC. The ease of gasification of AFC led to a much higher performance than for carbon fuel, even at 650 oC, where carbon cannot be gasified with a carbonate catalyst.

Properties of LiNi0.8Co0.1Mn0.1O2 as a high energy cathode material for lithium-ion batteries

Duc-Luong Vu,Jae-won Lee 한국화학공학회 2016 Korean Journal of Chemical Engineering Vol.33 No.2

Nickel-rich layered materials are prospective cathode materials for use in lithium-ion batteries due to their higher capacity and lower cost relative to LiCoO2. In this work, spherical Ni0.8Co0.1Mn0.1(OH)2 precursors are successfully synthesized through a co-precipitation method. The synthetic conditions of the precursors - including the pH, stirring speed, molar ratio of NH4OH to transition metals and reaction temperature - are investigated in detail, and their variations have significant effects on the morphology, microstructure and tap-density of the prepared Ni0.8Co0.1Mn0.1 (OH)2 precursors. LiNi0.8Co0.1Mn0.1O2 is then prepared from these precursors through a reaction with 5% excess LiOH· H2O at various temperatures. The crystal structure, morphology and electrochemical properties of the Ni0.8Co0.1Mn0.1 (OH)2 precursors and LiNi0.8Co0.1Mn0.1O2 were investigated. In the voltage range from 3.0 to 4.3 V, LiNi0.8Co0.1Mn0.1O2 exhibits an initial discharge capacity of 193.0mAh g−1 at a 0.1 C-rate. The cathode delivers an initial capacity of 170.4 mAh g−1 at a 1 C-rate, and it retains 90.4% of its capacity after 100 cycles.

Gasification of ash-free coal prepared with microwave method

이충곤,Won-Ki Kim,Duc-Luong Vu 한국화학공학회 2015 Korean Journal of Chemical Engineering Vol.32 No.9

The production of ash-free coal as a clean fuel for high temperature fuel cell was investigated. The ash-free coal was made from a bituminous coal. It was prepared by solvent extraction using a microwave. The solvent-to-coal ratio and microwave irradiation time of raw coal were parameters for the extraction yield. The microwave method showed merits in terms of faster extraction and higher extraction yield than conventional heating methods. In addition, gasification behaviors of the ash-free coal were investigated by gas chromatography. Contrary to carbon, ash-free coal showed hydrogen as a dominant gas species in its gasification and did not require carbonate catalyst. The electrical conductivity of ash-free coal was found very low close to zero.

Oxidation of ash-free coal from sub-bituminous and bituminous coals in a direct carbon fuel cell

Choong-Gon Lee,Duc-Luong Vu 한국화학공학회 2016 Korean Journal of Chemical Engineering Vol.33 No.2

The present study proposes the production of ash-free coal (AFC) and its oxidation as a primary fuel in direct carbon fuel cells (DCFCs). The AFC was produced by the extraction of Arutmin sub-bituminous coal (AFC1) and Berau bituminous coal (AFC2) using polar solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). It was carried out at a temperature of around 202 oC under atmospheric conditions and using a microwave irradiation method. Using NMP as the solvent showed the highest extraction yield, and the values of 23.53% for Arutmin coal and 33.80% for Berau coal were obtained. When NMP was added to DMSO, DMA and DMF, the extraction yield in the solvents was greatly increased. The yield of AFC from a sub-bituminous coal was slightly lower than that from a bituminous coal. The AFC was evaluated in a coin-type DCFC with a mixture of AFC and carbonate electrolyte (3 g/3 g) at 850 oC. The AFC and gaseous H2 fuels were compared using the electrochemical methods of steady-state polarisation and step chronopotentiometry. The DCFC ran successfully with the AFCs at 850 oC. The open-circuit voltages were about 1.35 V (AFC1) and 1.27 V (AFC2), and the voltages at 150 mA cm−2 were 0.61 V (AFC1) and 0.74 V (AFC2).