Won-Jin Kwak

An Advanced Lithium-Air Battery Exploiting an Ionic Liquid-Based Electrolyte

A novel lithium-oxygen battery exploiting PYR14TFSI-LiTFSI as ionic liquid-based electrolyte medium is reported. The Li/PYR14TFSI-LiTFSI/O2 battery was fully characterized by electrochemical impedance spectroscopy, capacity-limited cycling, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The results of this extensive study demonstrate that this new Li/O2 cell is characterized by a stable electrode-electrolyte interface and a highly reversible charge-discharge cycling behavior. Most remarkably, the charge process (oxygen oxidation reaction) is characterized by a very low overvoltage, enhancing the energy efficiency to 82%, thus, addressing one of the most critical issues preventing the practical application of lithium-oxygen batteries.

A Mo2C/Carbon Nanotube Composite Cathode for Lithium–Oxygen Batteries with High Energy Efficiency and Long Cycle Life

Although lithium-oxygen batteries are attracting considerable attention because of the potential for an extremely high energy density, their practical use has been restricted owing to a low energy efficiency and poor cycle life compared to lithium-ion batteries. Here we present a nanostructured cathode based on molybdenum carbide nanoparticles (Mo2C) dispersed on carbon nanotubes, which dramatically increase the electrical efficiency up to 88% with a cycle life of more than 100 cycles. We found that the Mo2C nanoparticle catalysts contribute to the formation of well-dispersed lithium peroxide nanolayers (Li2O2) on the Mo2C/carbon nanotubes with a large contact area during the oxygen reduction reaction (ORR). This Li2O2 structure can be decomposed at low potential upon the oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the typical Li2O2 discharge products.

Understanding the behavior of Li–oxygen cells containing LiI

Mankind has been in an unending search for efficient sources of energy. The coupling of lithium and oxygen in aprotic solvents would seem to be a most promising direction for electrochemistry. Indeed, if successful, this system could compete with technologies such as the internal combustion engine and provide an energy density that would accommodate the demands of electric vehicles. All this promise has not yet reached fruition because of a plethora of practical barriers and challenges. These include solvent and electrode stability, pronounced overvoltage for oxygen evolution reactions, limited cycle life and rate capability. One of the approaches suggested to facilitate the oxygen evolution reactions and improve rate capability is the use of redox mediators such as iodine for the fast oxidation of lithium peroxide. In this paper we have examined LiI as an electrolyte and additive in Li oxygen cells with ethereal electrolyte solutions. At high concentrations of LiI, the presence of the salt promotes a side reaction that forms LiOH as a major product. In turn, the presence of oxygen facilitates the reduction of I3− to 3I− in these systems. At very low concentrations of LiI, oxygen is reduced to Li2O2. The iodine formed in the anodic reaction serves as a redox mediator for Li2O2 oxidation.

Li–O2 cells with LiBr as an electrolyte and a redox mediator

After many years of successful and disappointing results, the field of Li–O2 research seems to have reached an equilibrium state. The extensive knowledge that has accrued through advanced analytical studies enables us to delineate the weaknesses of the Li–O2 battery. It is now clear that the instability of the cell components toward extreme conditions existing during cell operation leads to early cell failure as well. One serious challenge is the high oxidation potential applied during the charge process. Redox-mediators may reduce the over-potential and, therefore, improve the efficiency and cyclability of Li–O2 cells. Their use in Li–O2 cells is mandatory. We have previously shown that LiI can indeed behave in such a manner; however, it also promotes the formation of side products during cell operation. We have, therefore, embarked on a comprehensive study of lithium halide salts as electrolytes for use in Li–O2 cells. We examine herein the effect of other components in the cell, such as solvents and contaminants, on the lithium halide salt activity. Based on the electrochemical behavior and the identity of the final cell products under various conditions, we can glean substantial information regarding the detailed operation mechanisms for each specific case. We have concluded that low concentration of LiBr in diglyme solution can improve the cell performance with fewer side effects than LiI. With LiBr, only the desired Li2O2 is formed during discharge. During charge, the bromine redox couple (Br−/Br3−) can reduce the oxidation potential to only 3.5 V. Higher efficiency and better cyclability of cells containing LiBr demonstrate that the electrolyte solution is the key to a successful Li–O2 battery.

Mechanistic Role of Li+ Dissociation Level in Aprotic Li–O2 Battery

The kinetics and thermodynamics of oxygen reduction reactions (ORR) in aprotic Li electrolyte were shown to be highly dependent on the surrounding chemical environment and electrochemical conditions. Numerous reports have demonstrated the importance of high donor number (DN) solvents for enhanced ORR, and attributed this phenomenon to the stabilizing interactions between the reduced oxygen species and the solvent molecules. We focus herein on the often overlooked effect of the Li salt used in the electrolyte solution. We show that the level of dissociation of the salt used plays a significant role in the ORR, even as important as the effect of the solvent DN. We clearly show that the salt used dictates the kinetics and thermodynamic of the ORR, and also enables control of the reduced Li2O2 morphology. By optimizing the salt composition, we have managed to demonstrate a superior ORR behavior in diglyme solutions, even when compared to the high DN DMSO solutions. Our work paves the way for optimization of various solvents with reasonable anodic and cathodic stabilities, which have so far been overlooked due to their relatively low DN.

A sustainable iron-based sodium ion battery of porous carbon–Fe3O4/Na2FeP2O7 with high performance

A type of porous carbon–Fe3O4 (e.g., PC–Fe3O4) composite with an industrially scalable production was introduced in the sodium ion battery application for the first time. The PC–Fe3O4 composite, consisting of highly dispersed Fe3O4 nanocrystals within the porous carbon with a relatively low weight percent of 45.5 wt%, could efficiently demonstrate high capacities of 225, 168, 127, 103, 98 and 90 mA h g−1 under the current densities of 50, 100, 200, 300, 400 and 500 mA g−1 with a good stability over 400 cycles. The utilization co-efficient of Fe3O4 nanocrystals was proven to be much higher than most of the Fe3O4 nanoparticles reported recently via the study of the capacity contribution of carbon originally. In addition, the robustness of electrode during the charge–discharge was well characterized by ex situ XRD and emission scanning electron microscopy (SEM). More importantly, a new concept of an elemental iron-based sodium ion battery of PC–Fe3O4/Na2FeP2O7 is presented. This is the first example to introduce an element-rich configuration in the sodium ion battery from the viewpoint of sustainability. The full battery demonstrated a superior capacity of 93 mA h g−1, high capacity retention of 93.3% over 100 cycles and work voltage around 2.28 V with the energy density of 203 W h kg−1. Such configuration of an iron-based sodium battery would be highly promising and sustainable owing to its low cost and high stability in grid storage.

Iron–cobalt bimetal decorated carbon nanotubes as cost-effective cathode catalysts for Li–O2 batteries

Despite the extremely high theoretical specific capacity of lithium oxygen (Li–O2) electrochemistry, low energy efficiency resulting from the large potential gap between the discharge and charge makes this system impractical. In this report, an iron cobalt bimetal decorated carbon nanotube (FeCo–CNT) composite was synthesized as a catalytic air cathode material for Li–O2 batteries. An Li–O2 battery using FeCo–CNT air electrodes exhibited higher efficiency (72.15%) than that of pristine CNTs (62.57%) as well as higher capacity (3600 mA h g−1vs. 1276 mA h g−1). Spectroscopic and electron microscopy analyses showed that the improved cell performances can be attributed to the catalytic effect of FeCo. As cost-effective non-noble metal catalysts, FeCo–CNTs demonstrated performance comparable to noble metal catalysts in Li–O2 systems.

Understanding problems of lithiated anodes in lithium oxygen full-cells

Lithium oxygen batteries are attractive battery systems which can provide high energy density for the next generation. However, even if many research studies have made progress for years, the studies about substitution of Li metal which has inherent limitations in terms of stability and long term cycling properties are terribly deficient. Herein, our group clearly demonstrates the ambiguous unsolved problems of lithium oxygen full-cells using an alternative anode for Li metal by XRD and SEM analysis. The amount of Li source in the alternative anode is limited compared to the quasi-infinite amount of Li source in Li metal. The returning lithium ions during charging form lithium hydroxide which passivates the anode by a side reaction with moisture in the electrolyte and from outside. This report will help to accelerate the development of lithium oxygen full-cells.

A physical pulverization strategy for preparing a highly active composite of CoOx and crushed graphite for lithium-oxygen batteries.

A new physical pulverization strategy has been developed to prepare a highly active composite of CoOx and crushed graphite (CG) for the cathode in lithium-oxygen batteries. The effect of CoOx loading on the charge potential in the oxygen evolution reaction (Li(2)O(2) →2 Li(+) +O(2) +2e(-)) was investigated in coin-cell tests. The CoOx (38.9 wt %)/CG composite showed a low charge potential of 3.92 V with a delivered capacity of 2 mAh cm(-2) under a current density of 0.2 mA cm(-2). The charge potential was 4.10 and 4.15 V at a capacity of 5 and 10 mAh cm(-2), respectively, with a current density of 0.5 mA cm(-2). The stability of the electrolyte and discharge product on the gas-diffusion layer after the cycling were preliminarily characterized by (1)H nuclear magnetic resonance spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The high activity of the composite was further analyzed by electrochemical impedance spectroscopy, cyclic voltammetry, and potential-step chronoamperometry. The results indicate that our near-dry milling method is an effective and green approach to preparing a nanocomposite cathode with high surface area and porosity, while using less solvent. Its relative simplicity compared with the traditional solution method could facilitate its widespread application in catalysis, energy storage, and materials science.

Feasibility of Full (Li-Ion)–O2 Cells Comprised of Hard Carbon Anodes

Aprotic Li-O2 battery is an exciting concept. The enormous theoretical energy density and cell assembly simplicity make this technology very appealing. Nevertheless, the instability of the cell components, such as cathode, anode, and electrolyte solution during cycling, does not allow this technology to be fully commercialized. One of the intrinsic challenges facing researchers is the use of lithium metal as an anode in Li-O2 cells. The high activity toward chemical moieties and lack of control of the dissolution/deposition processes of lithium metal makes this anode material unreliable. The safety issues accompanied by these processes intimidate battery manufacturers. The need for a reliable anode is crucial. In this work we have examined the replacement of metallic lithium anode in Li-O2 cells with lithiated hard carbon (HC) electrodes. HC anodes have many benefits that are suitable for oxygen reduction in the presence of solvated lithium cations. In contrast to lithium metal, the insertion of lithium cations into the carbon host is much more systematic and safe. In addition, with HC anodes we can use aprotic solvents such as glymes that are suitable for oxygen reduction applications. By contrast, lithium cations fail to intercalate reversibly into ordered carbon such as graphite and soft carbons using ethereal electrolyte solutions, due to detrimental co-intercalation of solvent molecules with Li ions into ordered carbon structures. The hard carbon electrodes were prelithiated prior to being used as anodes in the Li-O2 rechargeable battery systems. Full cells containing diglyme based solutions and a monolithic carbon cathode were measured by various electrochemical methods. To identify the products and surface films that were formed during cells operation, both the cathodes and anodes were examined ex situ by XRD, FTIR, and electron microscopy. The HC anodes were found to be a suitable material for (Li-ion)-O2 cell. Although there are still many challenges to tackle, this study offers a more practical direction for this promising battery technology and sets up a platform for further systematic optimization of its various components.