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Department of Energy Engineering (Battery Science and Technology)Aprotic electrolyte based lithium-oxygen batteries are of considerable interest due to its ultrahigh theoretical specific energy density (1675 mAh per gram of oxygen) against the present lithium-ion battery. In spite of the attractiveness of its high theoretical capacity, there is a number of drawbacks such as instability of electrochemical reaction of electrode and electrolytes. In order to overcome these parasitic reactions, significant efforts have been devoted to developing the key materials such as carbon-free air cathodes and high concentrated electrolytes. However, the CO2 evolution during the charging process and low ionic conductivity limit the ideal electrochemical reaction in aprotic electrolytes. In this thesis, we applied the molten electrolyte based on nitrate-based electrolyte (Li/Na/K/Cs/Ca-NO3). The molten electrolyte, which has a eutectic point of 65???, has the advantages of high stability and high-temperature operation, thereby preventing detrimental solvent byproducts in lithium-oxygen batteries. We examined the Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR) on operating temperature using in situ pressure drop and gas analyses, Differential Electrochemical Mass Spectrometry (DEMS). Our results demonstrated that the Li2O2, a discharge product, formed a stable hexagonal morphology in the lithium-oxygen battery upon discharge process by scanning electron microscopy and X-ray diffraction techniques. Also, it leads to improved oxygen mobility at high temperature since a molten salt was used as the electrolyte in lithium-oxygen batteries. In addition, we found that kinetics are improved with increasing operating temperature in molten salt electrolyte cells.ope

Biodegradable ??-Chitin as a binder material for Li-O2 Battery

High capacity Li-O2 batteries are of interest due to its exceptional theoretical energy density (3840 mAh/g) against the present Li-ion technology. To make Li-O2 cell more attractive secondary battery as future energy storage device, there are still number of drawbacks such as parasitic reaction on carbon cathodes, aprotic electrolyte decomposition, high over potential and low OER/ORR ratio. Although significant effort has been devote to develop the key materials such as a highly porous gold electrode, separators and organic electrolytes. Cathode binders are rarely studied because of their limited diversity. Here, we demonstrated the new type of biodegradable chitin material as a binder component for Li-O2 battery. The result clearly exhibited that the carbon cathode with chitin binder shows decent chemical stability toward oxygen species and exceptional electrochemical performances in Li-O2 battery

Chemically impregnated NiO catalyst for molten electrolyte based Gas-tank-free Li-O2 battery

Closed Li-O-2 batteries consisting of electrolytes derived from molten salt have emerged as attractive energy storage cells because of their unique oxygen-supply mechanism to form a stable Li2O discharge product without requiring an oxygen-gas-reservoir. However, the formation of stable Li2O discharge product increases the overpotential during the charging process, which compromises the cell performance because of the resulting parasitic reaction. In this study, we demonstrate a potent approach to reversibly operate an oxygen-gas-reservoir-free Li-O-2 battery by using chemically impregnated nickel oxide (NiO) nanoparticles as a catalyst on the carbon electrode. The efficient bottom-up process for decorating NiO on a carbon material in binary molten electrolyte enables not only to significantly reduce the loading level of the catalyst but also to enhance the electrochemical performance with preventing the detrimental parasitic reaction in the oxygen-gas-reservoir-free Li-O-2 cell. In particular, using the in situ gas analysis with electrochemical measurements, the 20 wt% NiO added to the carbon cathode is sufficient to reduce the charging potential without generation of parasitic gas evolution

Reversible Li-Oxygen battery based on Liquid Crystal Electrolyte

Next generation, high-capacity secondary batteries such as advanced Li-ion, Li-S, and alkali anode based oxygen (metal-oxygen) batteries have been successfully proposed from many different research groups. Among the high capacity secondary battery candidates, metal-oxygen batteries are of interest not only due to their ultra-high capacity based on Li metal anode with light oxygen cathode but also due to their compatibility with battery powered transportation system such as hybrid/electric vehicle. However, previous reports implied that metal-oxygen batteries showed critical limitation because of irreversible electrochemical process during the battery operation. One of the most urgent problems is that metal-oxygen battery formed detrimental Li dendrite on the surface during the discharge-charge process that significantly erodes the cell performance. Here, we investigated newly designed electrolyte using the liquid crystal materials for metal-oxygen battery. This well aligned liquid crystal electrolyte shows retardation of rapid growth of Li dendrite on metal surface, giving rise to a longer cycle life with high Coulombic efficiency in symmetry cell architecture. Furthermore, we successfully demonstrated Liquid crystal electrolyte based Li-O2 battery with OER/ORR ratio of over than 80 % without CO2 evolution

Uniaxial Alignment of poly(vinylidenefluoride-co-trifluoroethylene) Nano Fibers for Ultra High Sensitive Pressure Sensor

Organic ferroelectric materials have interested in flexible electronics because of their useful piezo- and pyroelectric response upon the applied pressure and temperature, respectively. Among the ferroelectric polymers, poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE) exhibit excellent bistable polarization property which allowed us to apply various different types of flexible electronic devices such as non-destructive memory devices, energy harvesting cells and pressure sensors. Specifically for pressure sensor, PVDF-TrFE nano fiber structure shows advantage for more reliable sensing margin with promising endurance in the lab scale characterization than spin coated thin films. However, fabricating the PVDF-TrFE nano fiber remains as a challenging task. Here, we investigated highly aligned PVDF-TrFE array of nanofiber by using rotary jet spinning (RJS) method. The strong elongation force during the jet spinning process provides nano meter scale PVDF-TrFE fibers with global crystal orientation. As consequences a pressure sensor, our PVDF-TrFE nano fiber exhibited decent sensing margin in the bending test and achieved long endurance stability without further process

Stable and Dense Sodium Metal Deposition Using a Fluorination Solvent Containing Electrolyte with a 1 M Salt Concentration

Extensive academic and industrial efforts have been dedicated to developing battery-based energy storage technologies with high energy density, low cost, long cycle life, high energy efficiency, and ease of deployment in daily life. In particular, for large-scale electrical energy storage, attention has shifted to sodium (Na) metal batteries owing to the highest specific capacity and the lowest redox potential of metallic Na and the natural abundance of Na resources. Nevertheless, the exceedingly high electrochemical and chemical reactivity of Na metal electrodes toward organic liquid electrolytes and severe Na dendrite formation limit their commercialization. Since most Na deposition occurs at the interface between the anode (or the current collector) and the electrolyte, the discovery of a stable electrolyte is essential for utilizing Na metal anodes in practical applications. In this study, we demonstrate that fluorination solvent containing electrolyte dramatically enhances the reversibility of Na plating and stripping reactions in Na/Cu cells, realizes dense Na deposition, and leads to improved cycling stability at high current densities. By examining the detailed mechanism of the novel electrolyte system, we have found that the interfacial layer contains NaF, which has a high shear modulus and enhances the mechanical integrity of the interlayer by the attractive interaction between the F- ions and Na+ ions of ionic compounds such as Na2CO3 and sodium alkylcarbonates (NaO2CO-R-), resulting in the formation of mechanically strong and ion-permeable interlayers that suppress uncontrolled Na metal plating. The discovery of this work represents a step forward in the electrolyte design of Na metal anodes

Fluoroethylene Carbonate-Based Electrolyte with 1 M Sodium Bis(fluorosulfonyl)imide Enables High-Performance Sodium Metal Electrodes

Sodium (Na) metal anodes with stable electro-chemical cycling have attracted widespread attention because of their highest specific capacity and lowest potential among anode materials for Na batteries. The main challenges associated with Na metal anodes are dendritic formation and the low density of deposited Na during electrochemical plating. Here, we demonstrate a fluoroethylene carbonate (FEC)-based electrolyte with 1 M sodium bis(fluorosulfonyl)imide (NaFSI) salt for the stable and dense deposition of the Na metal during electrochemical cycling. The novel electrolyte combination developed here circumvents the dendritic Na deposition that is one of the primary concerns for battery safety and constructs the uniform ionic interlayer achieving highly reversible Na plating/stripping reactions. The FEC NaFSI constructs the mechanically strong and ion-permeable interlayer containing NaF and ionic compounds such as Na2CO3 and sodium alkylcarbonates

Hierarchical Chitin Fibers with Aligned Nanofibrillar Architectures: A Nonwoven-Mat Separator for Lithium Metal Batteries

Here, we introduce regenerated fibers of chitin (Chiber), the second most abundant biopolymer after cellulose, and propose its utility as a nonwoven fiber separator for lithium metal batteries (LMBs) that exhibits an excellent electrolyte-uptaking capability and Li-dendrite-mitigating performance. Chiber is produced by a centrifugal jet-spinning technique, which allows a simple and fast production of Chibers consisting of hierarchically aligned self-assembled chitin nanofibers. Following the scrutinization on the Chiber-Li-ion interaction via computational methods, we demonstrate the potential of Chiber as a nonwoven mat-type separator by monitoring it in Li-O2 and Na-O2 cells.clos

An Asymmetric Boron-Centered Ionic Additive Enables High-Energy Density Lithium-Ion Batteries

To cater for drastically growing demands for batteries with high energy density and long-term cycle life in transportation sector and large-scale energy storage systems, advancement of energy storage technologies is urgently required. Because ultimate energy density of batteries can be realized through developing high-capacity electrode materials, structural optimization of electrode materials that are capable of storing high energy has been intensively addressed. In this study, we demonstrate the significant roles of artificially constructed surface on high-capacity Li-rich cathodes preventing mechanical fracture of the cathode particles, mitigating the exposure of inner surface of the cathode toward electrolytes by intergranular cracking, and alleviating voltage decay of the cathode by phase transformation and vulnerable interface. Newly synthesized asymmetric boron-centered ionic additive with partially fluorinated malonate bonded to a central boron core is employed as a functional additive ensuring interfacial stability of high-capacity SGC anodes and Li-rich cathodes. The action of the protective surface layer created by asymmetric boron-centered ionic additive on intergranular cracking of Li-rich cathode particles synthesized via co-precipitation is explored using field emission scanning electron microscopy (FE-SEM). The unique feature of asymmetric boron-centered ionic additive -induced interfacial structure scavenging the oxygen species derived by Li2MnO3 and mitigating undesirable phase transformation of Li-rich cathodes are elucidated through in situ differential electrochemical mass spectrometry (DEMS) and high-resolution transmission electron microscopy (HR-TEM) analyses, respectively. Furthermore, we examine the chemistry of the surface of both electrodes evolved by asymmetric boron-centered ionic additive using ex situ X-ray photoelectron spectroscopy (XPS) and demonstrate the possible mechanisms of asymmetric boron-centered ionic additive constructing stable electrolyte-electrode interface

Unsymmetrical fluorinated malonatoborate as an amphoteric additive for high-energy-density lithium- ion batteries

High-capacity Si-embedded anodes and Li-rich cathodes are considered key compartments for post lithium-ion batteries with high energy densities. However, the significant volume changes of Si and the irreversible phase transformation of Li-rich cathodes prevent their practical application. Here we report lithium fluoromalonato(difluoro)borate (LiFMDFB) as an unusual dual-function additive to resolve these structural instability issues of the electrodes. This molecularly engineered borate additive protects the Li-rich cathode by generating a stable cathode electrolyte interphase (CEI) while simultaneously tuning the fluoroethylene carbonate (FEC)-oriented solid electrolyte interphase (SEI) on the Si-graphite composite (SGC) anode. The complementary electrolyte design utilizing both LiFMDFB and FEC exhibited an improved capacity retention of 85%, a high Coulombic efficiency of ???99.5%, and an excellent energy density of ???400 W h kg−1 in Li-rich/SGC full cells at a practical mass loading after 100 cycles. This dual-function additive approach offers a way to develop electrolyte additives to build a more favorable SEI for high-capacity electrodes