Designing Ionic Pathways to Improve Overall Kinetics of Lithium ion Batteries

Department of Energy Engineering(Battery Science and Technology)Lithium-ion batteries (LIBs) have been considered as the most attractive energy storage systems. They have been used in a wide range of applications from portable devices to large-scale stationary energy storage systems (ESSs) due to their merits superior to other energy storage systems such as leadacid, Na-S, and redox flow batteries. They have higher cell voltages at > 3.6 V, higher efficiencies at >90 % and longer cycle life when compared with their alternatives. However, more research and development activities would be required before large-scale LIBs are widely spread for automotive and stationary fields. Power densities as well as energy densities should be improved higher with cost reduction. Many efforts have been devoted to not only discovering high-performance active materials but also enhancing ionic and electric conductivities. In order to increase ionic conductivities, smaller particles in a nanoscale level are favored to give larger surface area and shorter ionic diffusion length. Electric conductivities are enhanced by coating active materials with conductive materials such as metallic and carbonaceous compounds. The main topic of this thesis is how to design ionic pathways for improving overall kinetics of LIBs even if electric conductivity enhancement is involved. As the simplest strategy which can be used as a startup, nanosizing in the primary particle level was tried for LiMnPO4 olivine(LMP) to overcome its poor electric and ionic conductivities. By confining Mn3(PO4)2 precipitation on surface of a precursor seed of Li3PO4, the size of LMP particles is limited to less than 100 nm for a smaller dimension. Larger active area and shorter ionic transport length resulting from the nanosizing improved kinetic properties of LMP as a cathode material for LIB cells. When compared with LMP particles synthesized by a conventional co-precipitation method, the performances are recognized to be considerably enhanced. As the next strategy, the primary-particle-level nanosizing was evolved to the secondary-structure level of morphology control. Hollow structures with porous shells were designed for a conversionreaction-based anode material Fe3O4. The structure was chosen because hollow particles benefit from larger surface area on which active materials meet electrolyte, shorter pathways for lithium ions to pass through and voids within hollow shell providing buffer space during lithiation. The hollow structure was proved more beneficial in terms of electrochemical performances when compared with its nonhollow counterpart. Hollow void of ~80 nm diameter accommodated volume expansion during lithiation while the porous shell structure allowed lithium ions move through in a facile manner and enhanced accessibility to surface of the active materials. As the third strategy of morphological control following primary- and secondary-structure levels, higher level structures were designed for another conversion-reaction-based anode materials, Co3O4. Two different morphologies of Co3O4 (plate-like and rod-like) were achieved through pseudomorphic conversion, depending on macroscopic morphologies of parent metal-organic-frameworks (MOFs). Both Co3O4 nanostructures were composed of almost identical 10 nm-sized primary nanocrystals. These Co3O4 nanomaterials were utilized as an electrode in lithium ion batteries (LIBs), and their electrochemical properties were comparatively investigated. It was revealed that the different cyclability and rate capability are attributed to their different microstructures. The pseudo-monolithic integration of primary and secondary structures at higher level was the governing factor, which determined the electrochemical performances of the Co3O4 electrode. In addition to the morphology controls in nanoscales, crystallographic parameters of graphite as an anode material were controlled for the same purpose of improving ionic conduction or transport during faradaic reactions. To widen the ionic pathways inside active materials, the d-spacing of graphite increased from 0.3359 nm to 0.3395 nm by oxidizing natural graphite under a mild condition. Oxygencontaining functional groups were developed not only at edges but also on planes of graphite. Subsequent thermal reduction of the oxidized graphite eliminated a portion of the functional groups, but did not change d-spacing significantly. The enlargement of d-spacing reduced kinetic hindrance of lithium ion movement within the expanded graphite by reserving more space for the ionic transport route. In addition, the activation energy of lithium ion intercalation in expanded graphite are reduced by surface charge polarization of graphite induced by hydrogen bonds between oxygen atoms of carbonates in electrolytes and hydrogen atoms of surface functional groups. The expanded graphite showed higher delithiation capacities especially at high currents. By designing ionic pathways of electroactive materials, overall kinetics was enhanced, resulting in a much better improved electrochemical storage system.ope

Clustering of Nodes in Layered-Tree Topology for Wireless Sensor Networks

Wireless sensor network is composed of a large number of sensor nodes of limited energy resource. The node clustering approach can improve the scalability and lifetime of wireless sensor network. In this paper we propose a novel node clustering protocol based on layered-tree topology for self-organizing distributed wireless sensor networks. It decides optimal number of clusters by employing a new approach for setting threshold value, including the probability of optimum number of cluster-heads and residual energy of the nodes. We also introduce a new scheme for layered-tree construction in each cluster. As a result, the proposed scheme can significantly improve the energy efficiency of the network and increase its lifetime. Computer simulation shows that the proposed scheme effectively reduces and balances the energy consumption of the nodes, and thus significantly extends the network lifetime compared to the existing schemes

The Korean Emissions Trading Scheme: Focusing on Accounting Issues

The purpose of this study is to examine the accounting standard-setting process in relation to emissions rights and related liabilities in the Korean context in order to provide a better understanding of accounting issues under an emissions trading scheme (ETS). Using an interpretive inductive approach, this study comprises semi-structured, face-to-face interviews and analysis of relevant documents. Interviews were carried out with a wide range of key players, including accounting standard setters (Korean Accounting Standards Board, International Accounting Standards Board, and Autorité des Normes Comptables), accounting experts, industry and government. This study identifies how problematic accounting issues on emissions rights and related liabilities have been addressed by accounting standard setters. The key accounting issues under ETS are linked mainly with free allowances. It is found that accounting standard setters attempt to establish the most appropriate accounting standard under the given circumstances reflecting a variety of considerations, and that the most common elements affecting the development of accounting standards for ETS are the legal and economic context, the existing accounting framework, and preceding models and practices. Nevertheless, these factors affect the development of accounting standards for ETS in different ways. Accordingly, the primary accounting issues on which each standard setter concentrates vary depending on different circumstances and considerations. This study investigates the accounting standard-setting process for emissions rights by Korean accounting standard setters, from the agenda-setting stage to the final publication of the standard. The findings reinforce the importance of political factors in the standard-setting process, including stakeholders’ participation in the process, prominent stakeholders, and the motivation, methods and timing of lobbying activities. In particular, the findings have important implications for the effectiveness of lobbying. Overall, the findings confirm that accounting standards are likely to be the political outcome of interactions between the accounting standard setter and stakeholders. The findings highlight desirable factors for accounting models of emissions rights. Desirability or appropriateness of standard is judged by the extent to which stakeholders in institutional environments consider the promulgation to be legitimate or authoritative. Therefore, accounting standard setters must make greater efforts to encourage stakeholders to participate in the standard-setting process in order to ensure institutional legitimacy. The originality of this study lies in its empirical research on accounting issues for ETS from a practical point of view. In particular, in its timely and detailed investigation of Korean accounting standard setters, this study provides a broader understanding of the accounting standard-setting process in the Korean context. The study also advances legitimacy theory by offering a framework particularly applicable to accounting standard setting process, which also incorporates stakeholder theory research. The study finds support from the framework and further contributes to the related literature by reviewing legitimacy conflicts. From an accounting policy point of view, the findings have implications for both national and international standard setters and provide guidance on how to achieve high-quality accounting standards with a high degree of compliance

Transparent conductive oxide films mixed with gallium oxide nanoparticle/single-walled carbon nanotube layer for deep ultraviolet light-emitting diodes

We propose a transparent conductive oxide electrode scheme of gallium oxide nanoparticle mixed with a single-walled carbon nanotube (Ga(2)O(3) NP/SWNT) layer for deep ultraviolet light-emitting diodes using spin and dipping methods. We investigated the electrical, optical and morphological properties of the Ga(2)O(3) NP/SWNT layers by increasing the thickness of SWNTs via multiple dipping processes. Compared with the undoped Ga(2)O(3) films (current level 9.9 × 10(-9) A @ 1 V, transmittance 68% @ 280 nm), the current level flowing in the Ga(2)O(3) NP/SWNT increased by approximately 4 × 10(5) times and the transmittance improved by 9% after 15 times dip-coating (current level 4 × 10(-4) A at 1 V; transmittance 77.0% at 280 nm). These improvements result from both native high transparency of Ga(2)O(3) NPs and high conductivity and effective current spreading of SWNTs

Comparison of electrical and thermal performances of glazed and unglazed

Photovoltaic-thermal (PVT) collectors combine photovoltaic modules and solar thermal collectors, forming a single device that receives solar radiation and produces electricity and heat simultaneously. PVT collectors can produce more energy per unit surface area than side-by-side PV modules and solar thermal collectors. There are two types of liquid-type flat-plate PVT collectors, depending on the existence of glass cover over PV module: glass-covered (glazed) PVT collectors, which produce relatively more thermal energy but have lower electrical yield, and uncovered (unglazed) PVT collectors, which have relatively lower thermal energy with somewhat higher electrical performance. In this paper, the experimental performance of two types of liquid-type PVT collectors, glazed and unglazed, was analyzed. The electrical and thermal performances of the PVT collectors were measured in outdoor conditions, and the results were compared. The results show that the thermal efficiency of the glazed PVT collector is higher than that of the unglazed PVT collector, but the unglazed collector had higher electrical efficiency than the glazed collector. The overall energy performance of the collectors was compared by combining the values of the average thermal and electrical efficiency