Hee-Tak Kim

Rechargeable Lithium Sulfur Battery I. Structural Change of Sulfur Cathode During Discharge and Charge

In this paper, the structural change of the sulfur cathode during the electrochemical reaction of a lithium sulfur battery employing 0.5 M LiCF 3 SO 3 -tetra(ethylene glycol) dimethyl ether (TEGDME) was studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and wave dispersive spectroscopy (WDS). The discharge process of the lithium sulfur cell could be divided in the first discharge region (2.4-2.1 V) where the reduction of elemental sulfur to form soluble polysulfides and further reduction of the soluble polysulfide occur, and the second discharge region (2.1-1.5 V) where the soluble polysulfides are reduced to form a nonuniform Li 2 S solid film covered over the carbon matrix. It was also found that the charge of lithium sulfur cell leads to the conversion from Li 2 S to the soluble polysulfide, resulting in the removal of Li 2 S layer formed on carbon matrix. However, the oxidization of the soluble polysulfide to solid sulfur hardly occurs and few Li 2 S are left on carbon matrix even at 100% depth of charge.

Rechargeable Lithium Sulfur Battery II. Rate Capability and Cycle Characteristics

This paper reports on the investigation of rate capability and cycle characteristics of a lithium sulfur battery. The second discharge region where solid Li 2 S is formed on the surface of the carbon matrix in the cathode was highly sensitive to cathode thickness and discharge rate. The scanning electron microscope (SEM) observation suggests that thick Li 2 S layer formed at the surface of the cathode causes the diminution of the second discharge region at high discharge rate. Upon repeated cycle, the delocalization of the surface Li 2 S layer happened, however, the irreversible Li 2 S gradually increased with cycle as evidence by (SEM) and wave dispersive spectroscopy measurements, causing capacity fade. The formation of the irreversible Li 2 S was more significant for higher rates of discharge. It is believed that the destruction of the carbon matrix by stress generated during the localized deposition of Li 2 S is responsible for the formation of irreversible Li 2 S.

Ionic conduction in plasticized PVC/PMMA blend polymer electrolytes

The new plasticized polymer electrolyte composed of the blend of poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) as a host polymer, the mixture of ethylene carbonate and propylene carbonate as a plasticizer, and LiCF3SO3 as a salt was studied. The effect of the PMMAPVC blend ratio and the plasticizer content on the ionic conductions in these electrolytes were investigated. The electrolyte films revealed a phase separated morphology due to immiscibility of the PVC with the plasticizer; the PVC-rich phase and the plasticizer-rich phase were produced during the film casting. The mechanical property was significantly enhanced by the incorporation of PVC into the electrolyte system. The ionic conductivity decreased with increasing the PVCPMMA ratio and increased with increasing the plasticizer content. These behaviors were explained in terms of the morphology of the film. Since the plasticizer-rich phase contains much more plasticizer than the PVC-rich phase, the ions preferentially move through plasticizer-rich phase. Due to the slow ionic transport through the PVC-rich phase, the conductivity decreased with increasing PVCPMMA ratio.

Binary electrolyte based on tetra(ethylene glycol) dimethyl ether and 1,3-dioxolane for lithium-sulfur battery

An electrolyte based on a mixture of tetra(ethylene glycol) dimethyl ether (TEGDME) and 1,3-dioxolane (DOXL) is studied for a use in lithium–sulfur battery. The maximum ionic conductivity is found at the intermediate mixing ratio of TEGDME:DOXL ¼ 30:70, because TEGDME readily solvates LiCF3SO3 and DOXL effectively reduces the viscosity of the electrolyte medium. The lithium–sulfur battery based on the binary electrolyte shows two discernable voltage plateaux at around 2.4 and 2.1 V, which correspond to the formation of soluble polysulfides and of solid reduction products, respectively. The UV spectral analysis for TEGDME-based and DOXL-based electrolytes suggests that the shorter polysulfide is favourably formed for DOXL-based electrolyte in the upper voltage plateau at around 2.4 V. The lower voltage plateau at around 2.1 V is highly dependent on the TEGDME:DOXL ratio. The sulfur utilization in the lower voltage plateau region can be correlated with the viscosity of the electrolyte, but with the ionic conductivity. The low polysulfide diffusion for the electrolyte with high viscosity causes significant passivation at the surface of the positive electrode and results in low sulfur utilization. # 2002 Elsevier Science B.V. All rights reserved.

Structural Factors of Sulfur Cathodes with Poly(ethylene oxide) Binder for Performance of Rechargeable Lithium Sulfur Batteries

The structure and the room temperature performances of sulfur cathodes composed of sulfur, carbon, and polyethylene oxide) (PEO) binder were studied with varying preparation method and binder content. Two different methods, ball mixing and mechanical stirring, were employed for preparation of slurry to obtain morphologically different cathodes. The cathodes prepared with mechanical stirring (SC cathodes) revealed more porous structure than those with ball mixing (BC cathodes). Scanning electron microscopy observations showed that sulfur particles were covered with dense and thick PEO film in the BC cathodes, whereas sulfur particles were bonded with porous PEO film in the SC cathodes. The SC-based cells exhibited much higher discharge capacity yielding 75% of sulfur utilization than the BC-based cells. The difference of discharge behaviors with different mixing methods indicates that the porosity of the cathode and the morphology of PEO binder are highly important factors for favorable electrochemical performance of lithium sulfur batteries. The cycle life of the SC-based cell was improved with the increase of binder content and with the roll pressing due to the increase of adhesiveness and cohesiveness of the sulfur cathode.

Tuning selectivity of electrochemical reactions by atomically dispersed platinum catalyst

Maximum atom efficiency as well as distinct chemoselectivity is expected for electrocatalysis on atomically dispersed (or single site) metal centres, but its realization remains challenging so far, because carbon, as the most widely used electrocatalyst support, cannot effectively stabilize them. Here we report that a sulfur-doped zeolite-templated carbon, simultaneously exhibiting large sulfur content (17 wt% S), as well as a unique carbon structure (that is, highly curved three-dimensional networks of graphene nanoribbons), can stabilize a relatively high loading of platinum (5 wt%) in the form of highly dispersed species including site isolated atoms. In the oxygen reduction reaction, this catalyst does not follow a conventional four-electron pathway producing H2O, but selectively produces H2O2 even over extended times without significant degradation of the activity. Thus, this approach constitutes a potentially promising route for producing important fine chemical H2O2, and also offers opportunities for tuning the selectivity of other electrochemical reactions on various metal catalysts.

Sustainable Redox Mediation for Lithium–Oxygen Batteries by a Composite Protective Layer on the Lithium‐Metal Anode

A synergic combination of a soluble -redox mediator and a protected Li metal -electrode to prevent the self-discharge of the redox mediator is realized by -exploiting a 2,2,6,6-tetramethylpiperidinyl 1-oxyl (TEMPO) redox mediator and an Al2 O3 /PVdF-HFP composite -protective layer (CPL). Stabilization of Li metal by simple CPL coating is effective at -suppressing the chemical reduction of the oxidized TEMPO and opens up the possibility of sustainable redox mediation for robust cycling of Li-O2 batteries.

Directly grown Co3O4 nanowire arrays on Ni-foam: structural effects of carbon-free and binder-free cathodes for lithium–oxygen batteries

The problem of carbon and binder decomposition, degrading the performance levels of the cathodes used in lithium–oxygen (Li–O2) batteries, remains unsolved. For this reason, using carbon and binder-free cathodes may be an ideal approach to remedy this problem. Here, we have developed a carbon free- and binder-free cathode for Li–O2 batteries based on vertically grown Co3O4 nanowire (NW) arrays on Ni-foam and demonstrated the suppression of this type of decomposition. The highly organized texture and high catalytic activity of the cathode provide high capacity with a reduced overvoltage. Interestingly, the charge voltage profile changed with the discharge rate, which is associated with the variation in the phase and local distribution of the discharge products. At a low discharge rate, a morphology resembling the pointed tip of a brush was observed, indicating that the crystalline discharge products were concentrated on the skin of the cathode, causing the deformation of the NW array. At a high discharge rate, a more uniform distribution of the quasi-amorphous discharge products was favored, resulting in a relatively stable voltage profile with cycling. These findings suggest the importance of the electrode structure and discharge product distribution during the design of carbon- and binder-free cathodes.

Phase-separated polymer electrolyte based on poly(vinyl chloride)/poly(ethyl methacrylate) blend

The characteristics of polymer electrolytes based on a poly(vinyl chloride) (PVC)/poly(ethyl methacrylate) (PEMA) blend are reported. The PVC/PEMA based polymer electrolyte consists of an electrolyte-rich phase that acts as a conducting channel and a polymer-rich phase that provides mechanical strength. The dual phase was simply developed by a single-step coating process. The mechanical strength of the PVC/PEMA based polymer electrolyte was found to be much higher than that of a previously reported PVC/PMMA-based polymer electrolyte (poly(methyl methacrylate), PMMA) at the same PVC content, and even comparable with that of the PVC-based polymer electrolyte. The blended polymer electrolytes showed ionic conductivity of higher than 10 � 3 Sc m � 1 and electrochemical stability up to at least 4.3 V. A prototype battery, which consists of a LiCoO2 cathode, a MCMB anode, and PVC/PEMA-based polymer electrolyte, gives 92% of the initial capacity at 100 cycles upon repeated charge–discharge at the 1 C rate. # 2002 Elsevier Science B.V. All rights reserved.

Effect of binary conductive agents in LiCoO2 cathode on performances of lithium ion polymer battery

Abstract This paper reports the effect of a binary conductive agent consisting of two kinds of carbon particles with different sizes in a LiCoO 2 cathode on the performance of a lithium ion polymer battery. Super-P (30 nm) and Lonza-KS6 (6 μm) were selected as the components for the binary conductive agents. The total amount and the ratio of the two components of the binary conductive agents were varied. At a fixed Lonza/Super-P ratio of 1:1, the cathode with the higher conductive agent content showed a lower electrical resistance and a lower density, which resulted in better rate and cycling characteristics. However, this advantage was somewhat offset by the decrease of energy density. At a fixed binary conductive agent content of 8 wt.%, the electrode density increased and the surface electrode resistance decreased with increasing Super-P content. In spite of the decreased ionic diffusion rate, the charge transfer resistance was decreased and the rate capability was increased with increasing Super-P content, which indicated that the formation of effective electronic conduction paths was highly important. The SEM microscopic observation and measurement of planar density of LiCoO 2 indicate that the addition of a small amount of Lonza-KS6 provides a uniform dispersion of LiCoO 2 particles in the cathode. As a consequence, the cell with the binary conductive agent at an appropriate Lonza/Super-P ratio had a better cycle life than those with single conductive agents.