599 research outputs found
New and Improved Algorithms for Unordered Tree Inclusion
No full textThe tree inclusion problem is, given two node-labeled trees P and T (the "pattern tree" and the "text tree"), to locate every minimal subtree in T (if any) that can be obtained by applying a sequence of node insertion operations to P. Although the ordered tree inclusion problem is solvable in polynomial time, the unordered tree inclusion problem is NP-hard. The currently fastest algorithm for the latter is from 1995 and runs in O(poly(m,n) * 2^{2d}) = O^*(2^{2d}) time, where m and n are the sizes of the pattern and text trees, respectively, and d is the maximum outdegree of the pattern tree. Here, we develop a new algorithm that improves the exponent 2d to d by considering a particular type of ancestor-descendant relationships and applying dynamic programming, thus reducing the time complexity to O^*(2^d). We then study restricted variants of the unordered tree inclusion problem where the number of occurrences of different node labels and/or the input trees' heights are bounded. We show that although the problem remains NP-hard in many such cases, it can be solved in polynomial time for c = 2 and in O^*(1.8^d) time for c = 3 if the leaves of P are distinctly labeled and each label occurs at most c times in T. We also present a randomized O^*(1.883^d)-time algorithm for the case that the heights of P and T are one and two, respectively
Development of novel assembly approaches for superior electrode materials of lithium-ion batteries
No full textRechargeable lithium ion batteries have attracted tremendous attention as “green” technology for electric vehicles and smart grids. In addition, the demand for flexible and high energy batteries has increased exponentially due to the growing need for smartphones and bio-devices over last few years. The conventional inorganic cathode materials (e.g., LiCoO2 and LiFePO4) for Lithium ion batteries are not flexible; they are also restricted by their low theoretical specific capacity. To satisfy the emerging large-scale applications of energy storage, new generation batteries should have high power and energy densities, and a long cycle life. In near term, new inorganic cathode and anode materials are developing to increase their capacity. In long term, the next generation batteries were proposed to be made from inexpensive renewable and/or recyclable resources via low energy consumption processes for energy sustainability with minimal environmental footprint. However, issues such as low electronic conductivity, large volume change during the charge/discharge cycles and dissolution of the active materials, commonly existed in these new electrode materials. These problems not only decrease their energy and power density, charging/discharging rate, but also lead to poor cycling performance, which largely hampered their practical applications. In this thesis, novel assembly approaches were developed to address some of the critical issues for the next generation battery devices with optimum electrochemical performance. Chapter 1 will include a general overview of basic but important information of current rechargeable Lithium ion battery technology, the requirement for the next generation Lithium ion battery for sustainable energy storage and their current issues and challenges. In addition, the structures, physical properties, methods of fabrications and applications of graphene, an important carbon material was employed for assembly with electrode materials in this thesis, will also be discussed. In Chapter 2, a simple, efficient and scalable assembly method was introduced for the controllable fabrication of nano-structured electrochemical active organic material for sustainable energy storage. Croconic acid disodium salt (CADS) as a sustainable organic electrode example to investigate the size effect on the battery performance of organic electrodes. CADS organic wires with different diameters were fabricated through a facile synthetic route using anti-solvent crystallization method. Cracks and pulverization were observed for micromter size CADS and its relative low capacity retention rate revealed that lithiation induced strain was also contributed to the limited cycling performance for organic electrode materials. The CADS nanowire exhibits much better electrochemical performance than its crystal bulk material and microwire counterpart. CADS nanowire with a diameter of 150 nm delivers a reversible capability of 177 mAh g-1 at a current density of 0.2 C, and retains capacity of 170 mAh g-1 after 110 charge/discharge cycles. The nanowire structure also remarkably enhances the kinetics of croconic acid disodium salt. The CADS nanowire retains 50% of the 0.1 C capacity even when the current density increases to 6 C. In contrast, the crystal bulk and microwire material completely lose their capacities when the current density merely increases to 2 C. Such a high rate performance of CADS nanowire is attributed to its short ion diffusion pathway and large surface area, which enable fast ion and electron transport in the electrode. In Chapter 3, we successfully developed a one-step, bottom up method for direct conversion of H2S to sulfur@graphene core-shell composite with various shapes (nanoparticles, nanosheets, and nano-wires), which can be used as cathode materials for the next generation Li batteries. This method employed graphene quantum dots as novel catalytic soft templates, taking advantage of their unique amphiphilicity and catalytic characteristics. We found, for the first time, the graphene quantum dots undergo micelle formation in aqueous solution (various solvents). The size and shape of the graphene micelle can be easily adjusted by changing the solution condition (ionic strength, dielectric constant) and it determines the size and shape of the resulted sulfur@graphene core-shell composite material. Our sulfur source, H2S, a major air pollutant, was directly converted as sulfur based cathode material, which opens up a potential route toward effective pollution control. We developed a general route to fabricate graphene based free standing, carbon black and binder free, flexible electrodes for high energy lithium ion battery in Chapter 4. Various particles (element sulfur, element Tin, Tin oxide and Li1.2Mn0.5Ni0.3Co0.3O2) were wrapped by graphene oxide through a simple solution phase assembly approach, no special interaction was needed. The as-prepared composite can be easily fabricated as free standing, flexible film and directly used as anode/cathode after recover graphene’s conductivity through thermal annealing. A free standing, flexible electrode of SnO2@Graphene was fabricated and used as an example for high energy lithium ion battery. The inter-connected graphene network functions as a conductive buffer matrix for the volume expansion of SnO2 during charge and discharge. A high specific capacity of 726 mAh g-1 (calculated by the total mass) and area specific capacity of 2.2mAh/cm2 was retained after 50 cycles for SnO2@Graphene composite anode at the current density of 500mA g-1. The assembly method developed in this study is general, robust and easy to apply on other functional materials rather than battery material, which opens up an easy path to fabricate flexible devices. In the Chapter 5, we report a new approach to intentionally induce phase transition of Li-excess layered cathode materials for high-performance lithium ion batteries. In high contrast to the limited layered-to-spinel phase transformation that occurred during in-situ electrochemical cycles, we hereby completely convert a Li-excess layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (LMNCO) to a Li4Mn5O12-type spinel product via ex-situ ion-exchanges and a post-annealing process. Such a layered-to-spinel phase conversion is examined using in-situ X-ray diffraction (XRD) and in-situ high-resolution transmission electron microscopy (HRTEM). It is found that generation of sufficient lithium ion vacancies within the Li-excess layered oxide plays a critical role for realizing a complete phase transition. The newly-formed spinel material exhibits initial discharge capacities of 313.6, 267.2, 204.0 and 126.3 mAh g-1 when cycled at 0.1, 0.5, 1 and 5 C (1 C = 250 mA g-1), respectively, and can retain a specific capacity of 197.5 mAh g-1 at 1 C after 100 electrochemical cycles, demonstrating remarkably improved rate capability and cycling stability in comparison with the original Li-excess layered cathode materials. This work sheds light on fundamental understanding of phase transitions within Li-excess layered oxides. It also provides a novel route for tailoring electrochemical performance of Li-excess layered cathode materials for high-capacity lithium ion battery. In chapter 6, a facile surfactant-free sonication-induced route is developed to prepare colloidal nanocrystals of lithium-excess transition metal oxide. The sonication process plays a critical role in forming LMNCO nanocrystals in ethanol (ethanol molecules marked as EtOHs) and inducing the interaction between LMNCO and ethanol molecules. The formation mechanism of LMNCO-EtOHs supramolecules in the colloidal dispersion system is proposed and examined by theoretical simulation and Zeta potential measurement. It is suggested that the as-formed supramolecule is composed of numerous ethanol molecules capping at the surface of LMNCO nanocrystal core via hydrogen bonding. Such chemisorption gives rise to dielectric polarization of the absorbed ethanol molecules, resulting in a negative surface charge of LMNCO colloids. Additionally, diverse superstructures are resulted from self-assembly of LMNCO colloids during evaporation of ethanol. Such self-assembly behaviors of colloidal LMNCO nanocrystals are then investigated by tuning the solvent evaporation condition. The assembled LMNCO architecture also exhibits remarkably improved capacity and cycleability compared to original LMNCO particles, demonstrating a very promising cathode material for next-generation lithium-ion batteries. This work thus provides new insight into the formation and self-assembly of multiple-element complex inorganic colloids in common and surfactant-free solvents for enhanced performance in device applications.Ph.D.Includes bibliographical referencesby Ruiming Huan
Graphene: Microwave Enabled One‐Pot, One‐Step Fabrication and Nitrogen Doping of Holey Graphene Oxide for Catalytic Applications (Small 27/2015)
No full textPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112226/1/smll201570159.pd
Study the effects of mechanical activation on Li-N-H systems with H-1 and Li-6 solid-state NMR
No full textTo gain insight into the effects of mechanical activation (MA) on the hydrogen desorption of the lithium amide (LiNH2) and lithium hydride (LiH) mixture. LiNH2 and LiH + LiNH2 were mechanically activated by high-energy ball milling. The formed products were studied with in situ H-1 and Li-6 nuclear magic angle spinning (MAS) magnetic resonance (NMR) spectroscopy from ambient temperature to 180 degrees C. Up-field chemical shift was observed in Li-6 MAS NMR spectra with increased milling time, indicating that average local electronic structure around Li nuclei was modified during MA. H-1 MAS NMR was used to dynamically probe ammonia release from the activated LiNH2 at temperature as low as 50 degrees C. In the case of activated LiH + LiNH2 mixtures, the H-1 MAS NMR results implied that MA enhanced the dehydrogenation reaction of LiNH2 + LiH = Li2NH + H-2. (c) 2007 Elsevier B.V All rights reservedclose201
Spray-drying synthesis and characterization of Li4Ti5O12 anode material for lithium ion batteries
No full textAbstractSpherical and nanostructured Li</jats:p
Phase transitions and related electrochemical performances of Li-Rich layered cathode materials for high-energy lithium ion batteries
No full textThe present work systematically probes and tracks the phase transition of Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (marked as LMNCO) by using an ex-situ chemical activation that is realized through ion-exchange and post-annealing processes, in order to understand related electrochemical performances of Li-rich cathode materials for advanced lithium-ion batteries. Ion exchanges of H+-Li+ and subsequent TBA+-H+ (TBA: tetrabutylammonium) in LMNCO are carried out, resulting in its layered-to-spinel phase transition after optimal heat treatments. The resultant compound shows a Li4Mn5O12-type spinel structure. This converted spinel cathode material can deliver discharge capacities higher than 300 mAh/g at 0.1 C and 200 mAh/g at 1 C (1 C = 250 mA/g), respectively, and also exhibits better cycling stability and rate capability in comparison with pristine layered LMNCO and other derivatives. This work offers a feasible route to study all changes of morphologies, crystal structures, chemical compositions, surface areas and related electrochemical lithium storage behaviors during phase transitions of Li-rich layered cathode materials, and thus provides insights on optimizing electrochemical performances for high-energy and high-power lithium ion batteries
CHEMFLUOR-VAE: REVERSE DESIGN OF ORGANIC FLUOROPHORES BASED ON EXPERIMENTAL OPTICAL PROPERTIES AND VARIATIONAL AUTOENCODER
No full textOrganic fluorescent molecules with desired optical properties attracted great attention, while the rational design was hindered by unclear structure properties relationship and the lack of rapid/affordable prediction methods. With the introduction of statistics-based methods in the prediction of photophysical properties for organic dyes, reverse design of fluorophores without traversing chemical space is still challenged by the features used for current methodologies.
In this work, we construct a self-referencing embedded strings (SELFIES)-based variational autoencoder (VAE) and a prediction model, which uses the latent space as the input, for the organic fluorophores, in the absence of joint training. The VAE can reproduce the structure of midsize organic dyes with acceptable accuracy. A tree-based prediction model based on Gradient Boosted Regression Trees (GBRT) can estimate the optical properties of organic dyes with a MAE 0.134 eV for emission energy and an accuracy of 0.81 for photoluminescence quantum yield (PLQY), which is comparable with the state-of-the-art quantum-mechanical based approach, time-dependent density-functional theory (TD-DFT). The feasibility of our approach in reverse design is proved by preliminary attempts at skeleton optimization and validated by first-principles calculations. New experimental synthesized molecules demonstrated the accuracy of our prediction model. Meanwhile, due to the continuous values in the latent space, this VAE-based methodology makes gradient optimization become possible for large organic materials. Combined, our statistical learning methodology opens a new venue for the design of organic fluorophore, can also be extended to the field of organic solar cell (photo conversion efficiency, PCE) and organic field-effect transistor (conductivity)
生物情報ネットワークのグラフ理論に基づく解析法
No full text京都大学新制・課程博士博士(情報学)甲第24730号情博第818号新制||情||138(附属図書館)京都大学大学院情報学研究科知能情報学専攻(主査)教授 阿久津 達也, 教授 山本 章博, 教授 岡部 寿男学位規則第4条第1項該当Doctor of InformaticsKyoto UniversityDFA
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