Yuwon Park

An amorphous red phosphorus/carbon composite as a promising anode material for sodium ion batteries.

An amorphous red phosphorus/carbon composite is obtained through a facile and simple ball milling process, and its electrochemical performance as an anode material for Na ion batteries is evaluated. The composite shows excellent electrochemical performance including a high specific capacity of 1890 mA h g(-1), negligible capacity fading over 30 cycles, an ideal redox potential (0.4 V vs. Na/Na(+)), and an excellent rate performance, thus making it a promising candidate for Na ion batteries.

Charge carriers in rechargeable batteries: Na ions vs. Li ions

We discuss the similarities and dissimilarities of sodium- and lithium-ion batteries in terms of negative and positive electrodes. Compared to the comprehensive body of work on lithium-ion batteries, research on sodium-ion batteries is still at the germination stage. Since both sodium and lithium are alkali metals, they share similar chemical properties including ionicity, electronegativity and electrochemical reactivity. They accordingly have comparable synthetic protocols and electrochemical performances, which indicates that sodium-ion batteries can be successfully developed based on previously applied approaches or methods in the lithium counterpart. The electrode materials in Li-ion batteries provide the best library for research on Na-ion batteries because many Na-ion insertion hosts have their roots in Li-ion insertion hosts. However, the larger size and different bonding characteristics of sodium ions influence the thermodynamic and/or kinetic properties of sodium-ion batteries, which leads to unexpected behaviour in electrochemical performance and reaction mechanism, compared to lithium-ion batteries. This perspective provides a comparative overview of the major developments in the area of positive and negative electrode materials in both Li-ion and Na-ion batteries in the past decade. Highlighted are concepts in solid state chemistry and electrochemistry that have provided new opportunities for tailored design that can be extended to many different electrode materials for sodium-ion batteries.

Sodium Terephthalate as an Organic Anode Material for Sodium Ion Batteries

Disodium terephthalate and its various derivatives are synthesized via simple acid-base chemistry for anode materials in Na ion batteries. They show excellent electrochemical performance, including little capacity fading over 90 cycles, ideal redox potential, and excellent rate performance, making them promising candidates for Na ion batteries.

A hybrid genetic algorithm/dynamic programming approach to optimal long-term generation expansion planning

Abstract This paper describes an application of a hybrid approach to optimal generation expansion planning (GEP) using a refined genetic algorithm (GA) and the tunnel-based dynamic programming (DP). Long-term GEP is concerned with a highly constrained non-linear discrete dynamic optimization problem that can only be fully solved by complete enumeration, a process which is computationally infeasible in a real-world GEP problem. For this reason, commercial packages have searched a reduced solution space or applied mathematical programming algorithms, which result in being stuck in a local optimum. This paper proposes a hybrid approach combining a refined GA with the tunnel-based DP, a method employed in the Wien Automatic System Planning Package (WASP). The main advantage of this approach lies in the GAs capability to find the global optimum and the tunnel-based DPs high performance to get a local optimum. The framework developed can simultaneously overcome the “curse of dimensionality” and a local optimal trap inherent in the conventional mathematical programming approaches. The suggested method has been successfully applied to two test systems with 15 existing power plants, five types of candidate plant and a 14-year planning period, and a practical long-term system with a 24-year planning period.

Structural Effect on Electrochemical Performance of Ordered Porous Carbon Electrodes for Na-Ion Batteries

Ordered meso- or macro-porous carbons (OMCs) were applied as anodes in Na ion battery (NIB) systems. Three different block copolymers (BCPs) enabled us to control the pore sizes (6, 33, and 60 nm) while maintaining the same 2-D hexagonal structure. To exclude other effects, the factors including precursors, particle sizes, and degrees of graphitization were controlled. The structures of OMCs were characterized by nitrogen physisorption, Raman spectroscopy, X-ray analyses (XRD and SAXS), and microscopies (TEM and SEM). With a galvanostatic charge/discharge, we confirmed that OMC electrode with medium pore size (OMC-33) exhibited a higher reversible capacity of 134 mA h g(-1) (at 20th cycle) and faster rate capability (61% retention, current densities from 50 to 5000 mA g(-1)) than those of OMC-6, and OMC-60 electrodes. The high performance of OMC-33 is attributed to the combined effects of pore size and wall thickness which was supported by charge/discharge and electrochemical impedance spectroscopy (EIS) analyses.

Polymer-derived carbon nanofiber network supported SnO2 nanocrystals: a superior lithium secondary battery material

In the present study, new three-dimensional tin dioxide–carbon (SnO2–C) composites as anode materials for lithium-ion batteries are achieved via a simple hydrothermal route and subsequent calcination process using polypyrrole-based carbon networks as the support and conductive buffering layer. The structure and morphology of the novel composites are characterized by powder X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction techniques. The resulting carbon networks composed of highly flexible, hollow, and end-opening nanofibers with diameters of approximately 40–70 nm and lengths of several microns are homogenously coated by nano-crystalline SnO2 (ca. 3–5 nm in size). The electrochemical performance of the mentioned SnO2–C (15.1% carbon) is investigated by cyclic voltammetry and discharge–charge cycling on half-cells in the potential range of 0.005–2 V at 25 °C. Galvanostatic cycling shows a stable and high charge capacity (598.3 mA h g−1) at a current density of 100 mA g−1 over 50 cycles with a low capacity fading of about 0.7% per cycle. By increasing the rate after 5 cycles in steps from 50 to 300 mA g−1 up to 30 cycles, a high reversible capacity (657.9 mA h g−1) is retained. Much improved lithium ion storage properties in terms of capacity, rate capability, and cycling stability may benefit from both the buffering action of conductive carbon networks and the size effect of SnO2 nanocrystals.

Abnormal Excess Capacity of Conjugated Dicarboxylates in Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are considered to be key energy storage systems needed to secure reliable, sustainable, and clean energy sources. Redox-active organic compounds have been proposed as interesting candidates for electrode materials for the next-generation LIBs because of their flexible molecular design, recyclability, and low production cost. Despite wide interest, a molecular-level understanding of the electrochemical lithiations/delithiations of those materials remains rudimentary. We synthesized a set of π-conjugated dicarboxylates and discovered unprecedented excess capacities for inverse-Wurster-type nonfused aromatic compounds (dilithium terephthalate and dilithium thiophene-2,5-dicarboxylate). Molecular structural investigations based on solid-state CP/MAS (13)C NMR combined with the stable isotope labeling method and ex situ X-ray diffraction were carried out to elucidate the origin of the excess reversible capacity. Interestingly, an open-chain-type dilithium muconate did not show an analogous behavior, signifying the key role played by the cyclic moiety in the electrochemical reaction.

A photo-cross-linkable polymeric binder for silicon anodes in lithium ion batteries

A new photo-cross-linkable poly(acrylic acid) (PAA) polymer functionalized with a photoreactive benzophenone group (PAA-BP) was synthesized and examined as a binder for Si-based anodes. Upon UV irradiation, the PAA-BP binders formed an irreversible cross-linked structure through the formation of a new three-dimensional C–C bond network between the benzophenone moiety and the PAA backbone. The photo-cross-linked PAA-BP binder demonstrated a marginal volume expansion (38%) after full lithiation, compared to conventional binders and this resulted in an improved cycle performance of the Si anode over 100 cycles with a high reversible capacity of ca. 1600 mA h g−1. We attributed this phenomenon to the enhanced mechanical properties of the photo-cross-linked PAA-BP binder, which were evaluated using nanoindentation and swelling measurements.

Application of expert system to power system restoration in local control center

A method of restoration in power systems is developed by using sensitivity analysis. Heuristic rules are used in the plan to restore the blackout area that is recognized by the network topology and on/off status of switches (circuit breakers, line switches). The rules are categorized as flow-control rules, energization rules and line switching rules. A set of network switching changes not only the network topology but also the state of power systems. The switching for alleviating overloads is determined by analyzing the line power variations using sensitivities with respect to line switching and load shedding. This paper deals with each feeder power in a substation as a unit power, so the sum of load shedding can be reduced by the methodology proposed in this study. The representation of the power system components has hierarchical data structures for the real time recognition of network topology.

Discrete-time adaptive sliding mode power system stabilizer with only input/output measurements

Abstract In this paper a discrete-time adaptive sliding mode control method is newly developed and applied to the power system stabilization problem. A controllable canonical form of state space realization is constructed using the parameters identified by the on-line recursive least squares method and the system state is estimated from the input/output measurements and the simple state transformations. The identified parameters and the estimated state are then used by the discrete-time sliding mode control, which is suitable for the digital equipment. The most important advantage of the proposed power system stabilizer (PSS) is that it is able to maintain its regulating performance with a slower sampling period than that of the conventional sliding mode PSS because it is developed in a pure discrete-time domain. Another advantage of the proposed PSS is that it needs neither a mathematical model of the power system nor the full-state measurements because they are identified through on-line identifications. Several computer simulations for the linear power system are performed to verify the performance of the proposed PSS. In the computer simulations for various circumstances which are probable in a power system are considered, such as transitions of the active and reactive powers, change of parameters of the synchronous machine, line-to-ground faults and measurement noise. As a result, a new power system stabilizer which can operate in a wide range of operating conditions and can overcome various disturbances and measurement noises is proposed.