High capacity spinel@layered Li_1.5MnTiO_4+<i>δ</i> as thermally stable core-shell-driven cathode materials for lithium-ion batteries

Vu, Ngoc Hung,Arunkumar, Paulraj,Im, Jong Chan,Im, Won Bin Elsevier 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.704 No.-

<P><B>Abstract</B></P> <P>The recently developed composite cathode can solve the disadvantages of single-component Lithium-ion batteries such as low capacity, low rate performance, and poor cycling stability. Spinel-layered Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> composites synthesized by a solid-state reaction showed high capacity and excellent cycling stability at room temperature. However, this material showed very poor cycling stability at 50 °C. A novel approach was used to synthesize spinel@layered composites, with the thermally stable layered component located in the outer part, and the high-capacity spinel located in the inner part of the composite particles. The effects of annealing temperature on electrochemical performance of cathodes were studied at both room temperature and 50 °C. The optimized sample, which was annealed at 700 °C, showed excellent thermal stability at 50 °C with 92% capacity retention after 100 cycles at 1C, compared to the value of 87% shown by the solid-state sample. At room temperature, the optimized cathode exhibited enhanced capacities of 209 and 157 mAh g<SUP>−1</SUP> at C/5 and 1C, respectively. Moreover, the optimized sample showed improved performance at different C-rates compare to the solid-state sample.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A promising cathode Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB>: excellent cycling stability at room temperature. </LI> <LI> A novel approach to synthesize thermal stability spinel@layered Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB>. </LI> <LI> Spinel@layered Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> possess high capacity and excellent thermal stability. </LI> </UL> </P>

Effect of synthesis temperature on the structural defects of integrated spinel-layered Li_1.2Mn_0.75Ni_0.25O_2+δ: a strategy to develop high-capacity cathode materials for Li-ion batteries

Vu, Ngoc Hung,Arunkumar, Paulraj,Im, Jong Chan,Ngo, Duc Tung,Le, Hang T. T.,Park, Chan-Jin,Im, Won Bin Royal Society of Chemistry 2017 Journal of Materials Chemistry A Vol.5 No.30

<▼1><P>An integrated layered-spinel of (1 − <I>x</I>)Li1.2Mn0.6Ni0.2O2·<I>x</I>LiMn1.5Ni0.5O4 (0.15 < <I>x</I> < 0.3) was synthesized by a hydrothermal reaction followed by firing at different temperatures.</P></▼1><▼2><P>An integrated layered-spinel with a nominal composition of (1 − <I>x</I>)Li1.2Mn0.6Ni0.2O2·<I>x</I>LiMn1.5Ni0.5O4 (0.15 < <I>x</I> < 0.3) was synthesized by a hydrothermal reaction followed by firing at different temperatures. The effects of firing temperature on the phase components, cation disorder, and crystal defects, and their relationship with the electrochemical performance of the cathode material were studied. The sample fired at 650 °C showed the highest capacity of up to 320 mA h g<SUP>−1</SUP> and highest initial coulombic efficiency (98%, 2–4.9 V), but the capacity decreased dramatically to only 55% after 50 cycles. The sample fired at 850 °C showed the slowest activation of the layered phase, requiring up to several dozen cycles. The intermediate firing temperature of 750 °C showed a balance between the activation rate, capacity, initial coulombic efficiency, and cycling stability, with 270 mA h g<SUP>−1</SUP> after 10 cycles and a 99% capacity retention after 50 cycles. All samples showed different rates of the layered-to-spinel phase transformation, which depends on the activation rate. This study reports the relationships between synthesis conditions, structure, and electrochemical performance, providing a strategy to develop high-capacity cathode materials based on the (1 − <I>x</I>)Li1.2Mn0.6Ni0.2O2·<I>x</I>LiMn1.5Ni0.5O4 system.</P></▼2>

Template-engaged synthesis of spinel-layered Li_1.5MnTiO_4+<i>δ</i> nanorods as a cathode material for Li-ion batteries

Vu, Ngoc Hung,Unithrattil, Sanjith,Hoang, Van Hien,Chun, Sangeun,Im, Won Bin Elsevier Sequoia 2017 Journal of Power Sources Vol. No.

<P><B>Abstract</B></P> <P>Spinel-layered composites of Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> were studied for their use as high-energy, low-cost, and environmentally benign cathode materials. The bulk particles showed an attractive specific capacity of up to 250 mAh g<SUP>−1</SUP> at C/10. To improve the performance of this cathode at a high C-rate, a spinel-layered Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> nanorod was successfully synthesized using a β-MnO<SUB>2</SUB> nanorod template. The nanorod, which had an average diameter of 200 nm and a length of 1 μm, showed specific capacity as high as the bulk particle at C/10. However, owing to a one-dimensional nanostructure with a large effective contact area for Li<SUP>+</SUP> diffusion, the nanorod sample exhibited enhanced capacities 11% (170 mAh g<SUP>−1</SUP>) and 167% higher (80 mAh g<SUP>−1</SUP>) at 1C and 10C rates, respectively, compared to the bulk particles. Moreover, both samples showed good cycle stability and capacity retention of over 85% after 100 cycles at 1C.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A promising Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> cathode: spinel-layered structure with high capacity. </LI> <LI> A Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> nanorod was synthesized by using a β-MnO<SUB>2</SUB> nanorod template. </LI> <LI> The Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> nanorods exhibit improved rate performance at high C-rate. </LI> <LI> The structure and electrochemical performances of Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> were studied. </LI> </UL> </P>

Effects of excess Li on the structure and electrochemical performance of Li_1+<i>z</i>MnTiO₄₊ <i> _δ </i> cathode for Li-ion batteries

Vu, Ngoc Hung,Arunkumar, Paulraj,Won, Seob,Kim, Ha Jun,Unithrattil, Sanjith,Oh, Yoong,Lee, Jong-Won,Im, Won Bin Elsevier 2017 ELECTROCHIMICA ACTA Vol.225 No.-

<P><B>Abstract</B></P> <P>Spinel-based LiMn<SUB>2</SUB>O<SUB>4</SUB> is the most attractive cathode for Li-ion battery due to high voltage, low cost, and non-toxicity. The cycle life of the spinel cathodes could be improved by replacing Mn<SUP>4+</SUP> with Ti<SUP>4+</SUP> leading to the formation of new spinel cathode, LiMnTiO<SUB>4</SUB>. However, its application is restricted due to the associated loss in the specific capacity. In this work, spinel-layered Li<SUB>1+<I>z</I> </SUB>MnTiO<SUB>4+<I>δ</I> </SUB> (<I>z</I> =0, 0.5, and 1.0; <I>δ</I> is the value to reflect the composite character of the material) cathodes were fabricated to achieve long cycle life, without compromising on the specific capacity. Cathodes with excess Li (<I>z</I> =0.5 and 1.0) formed a spinel-layered composite structure with notation (1-<I>a</I>)LiMn<SUB>2-<I>x</I> </SUB>Ti<I> <SUB>x</SUB> </I>O<SUB>4</SUB>.<I>a</I>Li<SUB>2</SUB>Mn<I> <SUB>y</SUB> </I>Ti<SUB>1-<I>y</I> </SUB>O<SUB>3</SUB> [<I>y</I> =0.5–((1/<I>a </I>−1)(1− <I>x</I>))]. These cathodes exhibited an enhanced specific capacity of ∼218mAhg<SUP>−1</SUP> (20% higher), with a capacity retention of 94% after 60 cycles. The structural and electrochemical properties of these cathodes were investigated using X-ray diffraction, galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique to understand the mechanisms underlying the enhanced capacity and cycle stability. The effect of the Li-rich layered phase on the electrochemical performance of the Li<SUB>1+<I>z</I> </SUB>MnTiO<SUB>4+<I>δ</I> </SUB> cathodes was also investigated.</P>

Synergic coating and doping effects of Ti-modified integrated layered-spinel Li_1.2Mn_0.75Ni_0.25O_2+δ as a high capacity and long lifetime cathode material for Li-ion batteries

Vu, Ngoc Hung,Im, Jong Chan,Unithrattil, Sanjith,Im, Won Bin The Royal Society of Chemistry 2018 Journal of materials chemistry. A, Materials for e Vol.6 No.5

<P>An integrated layered-spinel material with a nominal composition of (1 − <I>x</I>)Li1.2Mn0.6Ni0.2O2·<I>x</I>LiMn1.5Ni0.5O4 (0.15 < <I>x</I> < 0.3) and crystal defects has been found to be a promising cathode material with a high capacity of 280 mA h g<SUP>−1</SUP>. However, capacity fading arising from Mn<SUP>2+</SUP> dissolution occurred at low voltages and long cycling times. To improve the cycling stability while preserving the advantages of this cathode material, a synergic coating and doping approach was studied. This method yields a coating with a similar, but more stable, structure to that of the pristine sample. This coating is achieved by the bulk doping of the surface while maintaining the ratio of layered to spinel phases. The coating layer had a thickness of 12 to 18 nm, which increased with increasing Ti doping, and protected the sample at low voltages while maintaining the ion and charge transport channels on the surface. The Ti-doped sample enhanced the capacity retention by up to 97% after 100 cycles at C/10 and 89% after 200 cycles at 1C compared to 75% and 74% of the pristine sample, respectively. The optimized sample delivered a stable capacity of 270, 250, and 145 mA h g<SUP>−1</SUP> at C/20, C/10, and 1C respectively. This study provides an effective approach to improve the cycling performance of integrated spinel-layered cathode materials.</P>

Logistics Service Quality, Customer Satisfaction, and Loyalty During the COVID-19 Pandemic in Vietnam

Xuan Hung NGUYEN(Xuan Hung NGUYEN ),Thuy Dương TRAN(Thuy Dương TRAN ),Phan Hoai Diem VU(Phan Hoai Diem VU ),Yen Chi DINH(Yen Chi DINH ),Thi Mai Huong DUONG(Thi Mai Huong DUONG ),Thi Ngoc Hien THAN(Thi 한국유통과학회 2022 The Journal of Asian Finance, Economics and Busine Vol.9 No.9

The goal of the study is to examine and assess how the factors of logistics service quality provided by logistics service providers influence customer loyalty and satisfaction in Vietnam. This study employs samples of the Kansei technique. The article’s data was gathered through an online survey of 388 distinct Vietnamese customers who used logistics services before and during the Covid-19 outbreak. After the data was obtained, it was evaluated using SmartPLS 3.0’s Cronbach’s Alpha test and structural equation model (SEM). Staff service quality, operational service quality, and technical service quality are the three factors that determine the quality of logistics services in the study. Out of the 10 proposed hypotheses, research findings show that five are acceptable. The Covid-19 Pandemic in Vietnam had a significant impact on all dimensions, but it had no effect on how satisfied customers were with the logistics service providers. Nevertheless, loyalty is considerably influenced by only one factor, which is customer satisfaction, and customer trust moderates the relationship between these two factors. As a consequence of that, several managerial implications have been proposed to help logistics service providers improve their services to attract and keep Vietnamese customers using their service in the long term.

Fiscal Decentralization, Corruption, and Income Inequality: Evidence from Vietnam

Hung Thanh NGUYEN,Thuy Hoang Ngoc VO,Duc Doan Minh LE,Vu Thanh NGUYEN 한국유통과학회 2020 The Journal of Asian Finance, Economics and Busine Vol.7 No.11

The objective of this research paper is to study the simultaneous relationship between fiscal decentralization, corruption, and income inequality among Vietnamese provinces. We use a balanced panel data set of 63 provinces/cities in Vietnam in the period from 2011 to 2018. The study used 3SLS-GMM (Three Stage Least Squares - Generalized Method of Moments estimator) and GMM-HAC (Generalized Method of Moments - Heteroskedastic and Autocorrelation Consistent estimator). Empirical evidence shows a strong simultaneous relationship: increased corruption will increase regional income disparities, income inequality, and increase fiscal decentralization. In addition, the results also suggest that an increase in per-capita income will reduce the level of corruption, or better control corruption of each province. The degree of increase in income inequality, which reduces fiscal decentralization, is the same for trade liberalization. All demonstrate that there is a simultaneous relationship between fiscal decentralization, corruption, and income inequality. In a region of high public governance quality, fiscal decentralization positively effects its economic growth. This issue will indirectly increase income inequality between provinces within a country. Our findings imply that a country’s fiscal decentralization strategy should be linked to improving corruption control and local governance effectiveness, indirectly improving income inequality between localities or regions.

The Growing Problem of Radiologist Shortage: Vietnam’s Perspectives

Vu Luu Dang,Nguyen Hang Thi Thu,Nguyen Trang Ngoc,Pham Thong Minh 대한영상의학회 2023 Korean Journal of Radiology Vol.24 No.11

Impacts of Ownership Structure on Systemic Risk of Listed Companies in Vietnam

VU, Van Thi Thuy,PHAN, Nghia Trong,DANG, Hung Ngoc Korea Distribution Science Association 2020 The Journal of Asian Finance, Economics and Busine Vol.7 No.2

The research objective of the paper is to clarify the factors influencing system risks of listed companies in Vietnam, with a focus on clarifying the relationship and quantifying the impacts of ownership structure on systemic risk of listed companies. The data used in this study included financial statements and stock price data of listed companies on the Ho Chi Minh City Stock Exchange and Hanoi Stock Exchange of Vietnam stock market in the period from 2010 to 2017. The paper used the method of estimation in establising the regression models to choose among three models: Random Effect Model, Fixed Effect Model or Pooled OLS for regression using Stata statistical software. The research results showed that state ownership and ownership by foreign investors were positively related to systemic risk, while ownership by domestic investors had a reverse relationship with systemic risk of listed companies in Vietnam. In addition, as a control variable, both company size and profitability had an effect on the systemic risk of listed companies in the research sample. Based on the research results, the authors interpreted some of the implications in order to minimize systemic risks in the operation of listed companies in Vietnam.

Fabrication of An Antibacterial Non-Woven Mat of a Poly(lactic acid)/Chitosan Blend by Electrospinning

Hang Thi Au,박준서,Lan Ngoc Pham,Thu Ha Thi Vu 한국고분자학회 2012 Macromolecular Research Vol.20 No.1

Nonwoven mats made of a poly(lactic acid)/chitosan (PLA/CS) blend and a PLA/CS blend containing silver (Ag) nanoparticles (Ag/PLA/CS) were prepared using an electrospinning technique. The morphology of electrospun fibers was observed by field emission scanning electron microscopy. The addition of AgNO3 to the PLA/CS blend solution improved the electrospinning ability of the PLA/CS blend. The average diameters of the electrospun PLA/CS and Ag/PLA/CS blend fibers decreased as CS content increased. The Ag particles were evenly distributed in PLA/CS ultrafine fibers observed under transmission electron microscopy. Ag nanoparticles were spontaneously generated during the electrospinning process. When the CS content in the blend increased, the size of the Ag nanoparticles on the surface of the electrospun fibers increased as well. The thermal and mechanical properties of the nonwoven mats were examined by differential scanning calorimetry and a tensile tester. Fourier transform infrared spectroscopy was used to characterize the molecular interactions among PLA, Ag, and CS in the blends. The antibacterial activity of the nonwoven mats against Escherichia coli and Staphylococcus aureus was studied using an optical density method.