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 -tetra͑ethylene glycol͒ dimethyl ether ͑TEGDME͒ was studied by means of scanning electron microscopy ͑SEM͒, X-ray diffraction ͑XRD͒, and wave dispersive spectroscopy ͑WDS͒. The discharge process of the lithium sulfur cell could be divided in the first discharge region ͑2.4-2.1 V͒ 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 V͒ 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.

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 ͑SEM͒ 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 ͑SEM͒ 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.The successful development of a lithium sulfur battery which has been regarded as one of the candidates for next generation battery requires extensive research on the electrochemical behaviors under various operation conditions. 1-7 We demonstrated previously that the Li 2 S is formed on the carbon matrix in the cathode. In addition, little Li 2 S remained uncharged on carbon matrix of the cathode even at 100% depth of charge. 8 In this paper, the performance changes of the lithium/sulfur ͑Li/S͒ battery and the morphological changes of the sulfur cathode with discharge rate, cathode thickness, and cycles are reported. From the practical point of view, information on the behaviors of the Li/S battery with the changes of these factors are highly important in design and further improvement of the Li/S battery. The changes in the morphology of the cathode with discharge rate and cycle number were detected by means of scanning electron microscopy ͑SEM͒ and wave dispersive spectroscopy ͑WDS͒ measurements. Based on the SEM and WDS analysis, we tried to find a structural factor of the sulfur cathode that affects the rate capability and cycle characteristics of the Li/S batteries. ExperimentalThe sulfur ͑purity: 99.8%, Aldrich͒ was ground by ball-mixing method to reduce the particle size to 3-8 m before use. The sulfur cathode was prepared by doctor blade-coating of the slurry of sulfur ͑purity: 99.98%, Aldrich͒ and super-P ͑MMM carbon͒, a conducting agent, on one side of aluminum current collector. The resulting cathode consists of 56.7 wt % of sulfur, 27.3 wt % of super-P, and 16 wt % of poly͑butadiene-co-styrene͒ based binder. The thickness of the cathode layer measured with a thickness gauge ͑Mitutoyo digimatic indicator 407-320͒ was varied as 15, 30, and 60 m. The capacity and density of the resulting cathodes are summarized in Table I. The Li metal foil ͑Teckraf͒ with thickness of 240 m was used as an anode. The lithium sulfur ͑Li/S͒ cells were fabricated by winding the cathode (250 ϫ 40 mm), Li metal anode (260 ϫ 45 mm), and separator ͑Celgard 3501, H...

The cycle life and capacity fading mechanisms of secondary sulfur electrodes prepared by two different electrode fabrication methods were studied in an effort to improve the energy density of the cathode. Cycle life improved substantially when the uniformity of carbon distribution around the sulfur particles improved and electrode density increased, improving the overall structural integrity of the carbon matrix. Capacity fading in a high-energy-density cathode is mainly due to structural failure by physical crack propagations of the electrode structure and subsequent formation of the electrochemically irreversible Li 2 S layer at cracked surfaces of carbon particles or masses. The capacity fading due to these mechanisms appears to be reduced substantially and significantly by improved structural integrity with uniform pore structure of the carbon matrix. A Li/S cell containing a cathode fabricated by the improved technique cycled over 400 cycles without any severe structural damage of the electrode structure.

The structure and the room temperature performances of sulfur cathodes composed of sulfur, carbon, and poly(ethylene 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. © 2002 The Electrochemical Society. All rights reserved.

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