Sang-Eun Cheon

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.

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.

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.