Mechanistic Investigation of the Novel Anodic Material of T-Nb2O5 in LIB Technology

November 10, 2021

A lithium-ion battery (LIB) is an indispensable energy storage device that has been extensively studied due to the accelerating evolution of environmentally friendly   electric vehicles and portable electronics. Our recent studies discovered the ultrafast lithium-ion storage system of T phase niobium oxide (T-Nb2O5) and extensively examined the sophisticated mechanism in the charge/discharge processes; the detailed structural changes in delithiated/lithiated states of T-Nb2O5, vibrational characteristics and dynamics of Li ion migration have been systematically resolved by the density functional based calculations. We emphasized the charge storage mechanism and clarified that the loosely packed 4g layers in T-Nb2O5 can minimize the structural changes and are capable of stabilizing lithium ion adoptions, which is responsible for the excellent LIB durability. Also, the ultrafast storage of lithium ions in T-Nb2O5 corresponds to their low diffusion barriers and appropriated adsorption energies in 4g positions on the 2D void formed by the Nb-O bonds. Those findings not only discover the novel materials used for stable and highly efficient LIB, but also provide useful information for the better understanding of intercalation chemistry for those oxide-based materials applicable to the fast-growing market of power storage devices. Mechanistic study of lithium diffusions and related vibrations in T-Nb2O5   The high-power energy storage system of lithium-ion batteries (LIB) has drawn considerable attention recently due to the fast growing adoption of electronic equipment and vehicles. One of the most critical challenges suffered by LIB technology is to find the promising electrodes that can efficiently store and stably release the energy with excellent durability. Numerous materials are promising candidates for LIB anodes, including carbonaceous nanomaterials, semiconductors, and transition metal oxides. Among them, T phase niobium oxide (T-Nb2O5) has been extensively studied because of its large capacity, high working potential, and, most importantly, layered crystal structure. In this selected work, we have utilized the vibrational analysis and dynamic calculations to resolve the detailed mechanism, for the better understanding of chemical insights beneath the ultrafast Li ion storage of T-Nb2O5 and the rational design of niobium oxide-based anodes to promote LIB performance.  Crystal structures of unlithiated and lithiated T-Nb2O5 have been initially optimized to represent the anodic materials in the discharged and charged states, respectively. The 4g layer in unlithiated T-Nb2O5 with loose Nb-O bonds shows a large number of voids, which are preferential sites for Li ion accommodation to ease the stress of structural changes and repulsion from positively charged metallic elements (Figure 1). Accordingly, lithiated T-Nb2O5 has Li ions fully adsorbed on those voids in the 4g layer; the computed adsorption energies are in the range of -3.11 ~ -3.53 eV for single Li ion adsorption, and the averaged adsorption energy is -2.38 eV for the full adsorption. Based on the optimized structures, we further analyzed the vibrations of lithiated and unlithiated T-Nb2O5 to understand the structural evolution in the charge and discharge processes.    The irreducible representation of vibrations for Raman active modes Unlithiated: ΓRaman=28Ag+28B1g+14B2g+14B3g  Lithiated: ΓRaman=32Ag+32B1g+16B2g+16BB3g    Figure 1.Structures and vibrations of unlithiated and lithiated Nb2O5.   Additionally, the continuous distribution of vibrational density of states (VDOS), resulting from phonon dispersion and integrating at the Γ-point, has been examined. Those vibrations can be grouped in the low (100-300 cm-1), mid (300-650 cm-1) and high (>650 cm-1) frequency regions, which correspond to Nb-O stretching modes perpendicular to the 4g layer, parallel to the ab plane and O-Nb-O bending modes (Figure 1), respectively. On lithiated T-Nb2O5 (charging process), high-frequency vibrations diminished obviously, and more vibrations appeared in the mid-frequency region; low-frequency vibrations showed limited changes. The noticeable changes in high and mid-frequency vibrations, confirmed with Raman spectroscopy, are direct evidence of Li ion adsorption in the loosely packed 4g layer during the charging process.  The charge dynamics have been studied by examining the migration of a Li ion from one site to another in T-Nb2O5. According to the vibrational analysis, Li ions preferentially adsorb on the 4g plane; it is thus concluded that the migration pathways of Li ions are sticky on the plane. Additionally, the migration can be categorized into four simple path topologies in terms of the neighboring Nb–O bonding structures; as shown in Figure 2, Li ions migrate to bond with edge-shared O in pathways 1 and 4 and to the corner-shared ones in pathways 2 and 3. Since the void size, that is, the distance between adjacent ab planes, is large enough (approximately 4 Å) to retard the migration, the energetic barriers for Li-ion migration are mainly obstructed by the nonbridged oxygen atoms located on the 4g plane. The computed barriers in those migration pathways are rather small (<0.4 eV), indicating that Li ions can easily diffuse on the 4g plane. Also, the migrations are all on the 4g plane, implying that the migrations of Li ions have directionality. After careful emphasis of the migration topologies and barriers, our result has identified the evident merits responsible for fast Li-ion transport in the novel T-Nb2O5; those unique features distinguish T-Nb2O5 from other classical intercalation-type materials and account for the excellent electrochemical performance. Figure 2. Lithium diffusion pathways and related energetics in T-Nb2O5.   In order gain a comprehensive understanding of the energy storage mechanism of T-Nb2O5 used in LIB technology, our study extensively examined the structural and vibrational characteristics of unlithiated and lithiated models, well verified with the experimental Raman spectroscopy, and analyzed the migrating dynamics of Li ions. Our results identified the preferential adsorption sites and migration pathways located on the loosely packed 4g layer that can stabilize the structure and assist the ultrafast charge/discharge processes, thus resulting in a superior LIB performance. It is believed that our work elucidates the key concepts for the mechanistic understanding and can further be utilized to rationally optimize niobium oxide based anodes in LIB application.   Source: https://pubs.acs.org/doi/abs/10.1021/jacs.7b03141 https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.9b00179 https://doi.org/10.1016/j.nanoen.2021.106398

Research and Development of Highly Color Rendering Fluorescent Powder - Innovative Applications for the LED Industry

November 09, 2021

(This report is provided by Dr. Shu-Fen Hu’s research team in the Department of Physics) Conventional white LED lighting mainly uses blue LED chips with yellow phosphors to produce white light. However, because its spectrum coverage is not complete, the color rendering of white light by this method is not good; color rendering is a measure that describes the ability of a light source to render the color of a real object. The higher the color rendering of a light source, the closer its color performance is to an ideal light source or natural light. Therefore, the research team used different sintering methods to synthesize high-intensity infrared phosphors with wide emission peaks, established a mechanism to regulate wavelengths, mixed different ratios of high-intensity infrared phosphors, and developed high-intensity infrared LED devices suitable for detector measurements. The high color rendering and high stability of the phosphor will be applied in the LED industry. (https://doi.org/10.1021/ja304754b)    Human lighting tools have undergone numerous reforms, from more primitive lamps such as torches, kerosene lamps, and candles to incandescent lamps, fluorescent lamps, and light-emitting diode lamps that require electrical conversion, illustrating the changing needs of humans for lighting. During the second industrial revolution, people were familiar with how to use electricity and how to use it as a source of power. Thomas Alva Edison used it as a power source after many attempts using different heat-resistant materials. In 1880, it was found that carbonized bamboo filaments as filament materials could generate a significant amount of light and heat, and incandescent lamps could illuminate for 1,200 hours, leading mankind to enter the era of electric lighting. However, the incandescent lamp converts too much electrical energy into heat energy and only less than 10% of energy into light energy, so fluorescent lamps with higher efficiency were developed. The glass tube contains inert gas and low-pressure mercury vapor, and by increasing the voltage, the filament releases electrons, and the electron-excited mercury emits 253 nm and 185 nm ultraviolet light. Fluorescent powder coated in the wall of the tube emits visible light by absorbing ultraviolet light, but its disadvantages are mercury pollution, large size, poor color rendering, and flash frequency, which prompted the pursuit of better lighting tools, such as light-emitting diodes (LEDs). At present, the mainstream of commercial white LEDs is to generate white light by mixing phosphor with a single chip, which is called phosphor-converted LED (pc-LED). It can also be divided into UV chips or blue chips. UV chips are often used with blue, green, and red fluorescent powder for better color rendering. However, UV chips are too powerful and can easily damage the epoxy resin of the packaging material, and there are also concerns about the problem of UV light escape. Therefore, after the development of high-efficiency blue wafers, they have become the mainstream of single-crystal type diodes. For example, in 1997, Nichia Chemical applied for a white light patent, US 5,998,925, for the excitation of a yellow fluorescent powder (Y3Al5O12:Ce3+; where YAG is the main lattice and Ce3+ is the luminescence center) by means of an indium gallium nitride (InGaN) diode. Subsequently, various green and red phosphors were developed to meet the needs of various fields to improve color rendering and increase the color gamut.  A phosphor is a substance that produces a cold luminescence phenomenon and is mainly composed of a host lattice and an activator. As shown in Figure 1, the activator is the emitting center and is mixed into the host lattice in small amounts. The emitting characteristics of the activator are determined by the coordination environment of the host lattice, and some of the phosphors are mixed with a trace amount of sensitizer, and the energy of the excitation source is transferred from the sensitizer to the activator by energy transfer to enhance the light emitting efficiency. Figure 1. The main structure of the phosphor and the activation center is illustrated.   Prof. Shu-Fen Hu has established a complete laboratory for the synthesis and characterization of fluorescent powder in the past decade or so. The instruments and analytical methods used in the laboratory are shown in Figure 2. Figure 2. The diagram of the equipment and analytical methods of the phosphor characterization laboratory was established by Prof. Shu-Fen Hu.   Due to the advancement of human-computer programs and technology, artificial intelligence is developing rapidly and is used in many fields such as medicine, mathematics, and neurology. Infrared LEDs are an eye-catching development that has been used in many fields such as health inspection, iris recognition, and food safety, as their accuracy and brightness have increased dramatically. In the early days, if we wanted to analyze basic information about the body such as glucose concentration and blood oxygen level, we had to use large instruments with high precision or even invasive measurements. However, in recent years, we have been able to use infrared irradiation to detect the concentration of substances to be measured by the magnitude of absorption values at different wavelengths, significantly reducing the measurement threshold, measurement time, and cost. However, conventional infrared LEDs are limited by their narrow half-height, and the information provided by a single LED is limited. For measuring multiple messages, multiple infrared LED devices are required. However, this will make the device much more costly and difficult to miniaturize, and the circuit design is more complex, limiting the application. Therefore, the best solution to this problem is to develop a device that uses a single LED chip to excite infrared phosphors. This method of infrared LED can significantly reduce the size of the device, lower the cost and reduce the circuit control design. In recent years, Prof. Shu-Fen Hu has led her team to synthesize high-intensity infrared phosphors with wide emission peaks by using different sintering methods, analyze their structures and emission characteristics, regulate their emission wavelengths, and establish a wavelength control mechanism to mix different ratios of high-intensity infrared phosphors to develop high-intensity infrared LED devices suitable for detector measurements (Figure 3a). The space-confinement method was used to synthesize nanoscale phosphors, which were packaged in mini LEDs to evaluate their practical applications (Fig. 3b). It is expected to introduce a new application directly to the domestic LED industry.   Figure 3. Conceptualization of the infrared phosphor with wide emission peaks from blue light-emitting diode excitation.   Source: Origin of Thermal Degradation of Sr2–xSi5N8:Eux Phosphors in Air for Light-Emitting Diodes, J. Am. Chem. Soc. 2012, 134, 14108−14117. https://doi.org/10.1021/ja304754b

Screening of inorganic gases released from firework-rockets by a milli-whistle

February 05, 2020

The use of an accelerometer for detecting inorganic gases in gas chromatography (GC) is described. A milli-whistle was connected to the outlet of the GC capillary and was used instead of a normal GC detector. When the GC carrier gases and the sample gases pass through the milli-whistle, a sound is produced, leading to vibrational changes, which can be recorded using an accelerometer. Inorganic gases, including SO2, N2 and CO2, which are released from traditional Chinese firework-rockets at relatively high levels as the result of burning the propellant and explosive material inside could be rapidly determined using the GC/whistle-accelerometer system.    When we were young, the happiest thing during Lunar New Year was to set off firecrackers! During Lunar New Year, children bought firecrackers with their New Year's money, adults gathered to play cards, and the streets were very lively! The sound of the whistle soaring through the night sky came to an abrupt end with a "bang," which seemed to be an essential part of the New Year festival. However, in recent years, due to policy factors banning the letting off of firecrackers, the atmosphere of "Nian" has become much less fun. During the "flying" of the sky cannon, and the sound of the final "explosion," the composition of the gunpowder filling the cannon differs. It is generally believed that black gunpowder is made according to the ratio of "Mono Sulphur, Di Nitrate and Three Charcoal." In terms of the ratio, it is 75% Potassium Nitrate, 10% Sulfur powder, and 15% Charcoal powder. As early as the year 1044, the book "Complete Essentials for the Military Classics" had recorded three complex gunpowder formulas, which used gunpowder to make bombs such as thunderbolt fireballs and iron-mouthed/iron-billed fire harriers. It is conceivable that the composition of black gunpowder is complicated, and it may not be easy to analyze clearly even with modern instruments. The general formula for the chemical reaction of black powder can be written as: 2KNO3 + S + 3C → K2S + N2 + 3CO2. In other words, the explosion was caused by a chemical reaction in the enclosed space of the paper tube, causing a large amount of Nitrogen and Carbon Dioxide to ignite instantaneously. As long as you know how much Nitrogen and Carbon Dioxide are produced, you can reverse it to know the proportions of the components in the black powder. Professor Cheng-Huang Lin at our school invented a new gas detection method, which is different from the traditional method. It uses the frequency change of the whistle to detect and quantify the gas. As shown in Figure 2, after the gas sample is injected into the separator tube/column through the injection needle, it is carried by the gas in the gas cylinder and blown to the detection end through the gas separator tube. This method is called gas chromatography and is often used to test gases. Generally, mass spectrometers or other types of detectors are used to detect different types of gases. However, this research uses a mini whistle to detect gas in a physical way. When different gases pass through the whistle, they will emit sounds of different frequencies, which are received by the microphone and sent to the computer for analysis and processing. From the change of frequency, the amount of gas passing through the whistle can be calculated. After taking out the powder from the cupola, we put it in a transparent glass bottle. In order to avoid explosion when the powder burns instantaneously, a pressure relief valve is connected to the top of the glass bottle. The experimental results found that when the laser irradiated a small amount of propellant powder in the glass bottle, the temperature rose instantaneously (1265°C/sec). After the gas was cooled, 10 µL of the burned gas was extracted with an injection needle and injected into the separator column for analysis. Figure 3 shows the appearance, cross-section, and experimental results of the cupola. Inside the cupola, the propellant is about 3/4 of the length and weighs 4.81 grams, while the explosive is about 1/4 of the length and weighs 2.9 grams. After the change in the measurement frequency of the mini whistle and the analysis of the computer program (LabVIEW), it was found that the combustion components of propellant powder were mainly Carbon Dioxide and Nitrogen (lower right of Figure 3). In this measurement, the gas cylinder was filled with Nitrogen, but no spectral peaks were seen for the Nitrogen itself, so the peaks were marked as Nitrogen and the remaining Oxygen was from the combustion. The spectral peaks of Nitrogen and Carbon Dioxide were observed when the gas in the gas cylinder was replaced by Oxygen. According to the result of the calculations, the proportion of charcoal in the propellant powder is 20.0%. The proportion of the ideal black powder as mentioned above is 75% Potassium Nitrate, 10% Sulfur powder, and 15% charcoal powder, so it is inferred that increasing the Carbon content contributes to the propulsive force of the cupola. In contrast, the explosive powder burns components other than Carbon Dioxide and Nitrogen, as well as a large amount of Sulfur Dioxide (lower left of Figure 3). Similarly, when the gas in the gas cylinder was replaced with Oxygen, a spectrum in the Nitrogen, Carbon Dioxide, and Sulfur Dioxide was observed. As a result of the calculations, the proportion of charcoal powder in the explosive powder is only 3.4 %, while the Sulfur powder is as high as 24.8 %. It is therefore inferred that an increase in the content of Sulfur to produce a loud blast is very helpful. In this experiment, because of the rapid burning rate, the presence of Carbon Monoxide and Nitrogen Oxide was not observed. Through experiments, we can infer that what the ancients called "Mono Sulfur, Di Nitrogen and three Charcoal" is only a rough proportion. The gunpowder composition of various firecrackers and pyrotechnics is not the same, and their gunpowder formula and proportion should be corporate secrets. This study uses a simple method to quickly quantify the main components, and it also makes it easier for us to understand the gunpowder components in cupolas and pyrotechnics, and the differences brought about by different ratio configurations. The results of this study have been published in the Journal of Chromatography A (2013, 1305, 230-233) with the title “Screening of inorganic gases released from firework-rockets by a gas chromatography/whistle-accelerometer method.” The detailed conditions of the experiment can be found in the paper. This research is supported by the Ministry of Science and Technology, grant number 101-2811-M-003-023.    Original source: Journal of Chromatography A (2013, 1305, 230-233)