Lithium/sulfur redox couple has a theoretical specific capacity of 1,675 mAh/g, and theoretical specific energy of 2,600 Wh/kg, based on sulfur active material, assuming the complete reaction of lithium with sulfur to Li2S. Generally, utility of sulfur is advantageous as it has environmentally benign and cheap material. Therefore, there has been a much attention to scientist on the development of Li/S batteries due to its low cost and high capacity. Furthermore, it has been reported that Li/S cells have serious problems of low-active material utilization efficiency and poor cycle life due to the insulating nature of Li2S which leads to the loss of active material in form of soluble polysulfide reaction products. In order to commercialize the high capacity Li/S battery, both the sulfur utilization and cycle life should be improved.
Charge and discharge characteristics of lithium/sulfur cells operation at room temperature (30℃) are presented with a Triglyme (triethylene glycol dimethylehter) as a liquid electrolyte with lithium salt (LiTFSI). The initial discharge capacity of 20wt.% sulfur positive electrode is around its theoretical capacity of 1,650mAh/g-sulfur at 2.0V vs. Li/$Li^+$. However, it is clearly seen that while increasing cycle number the discharge capacity of elemental sulfur ($S_8$) cathode is decreased rapidly. Only after 30 cycles, most of its initial discharge capacity is lost. During the discharge of Li/S battery, a dense and non-uniform $Li_2S$ solid film were covered over the surface of sulfur cathode. These $Li_2S$ passivation layers determine the charge-discharge efficiency, cycle life and the rate capability of Li/S battery. In this work, the method for improving charge-discharge cycle life and other electrochemical properties of sulfur cathode was proposed.
In the part 1, in order to promote the reversible redox reaction of sulfur with lithium and provide higher electric conductivity, nickel metal powder is added into the sulfur positive electrode. During charge-discharge cycling, a new phase is formed and it has good cycle life property. In order to make this phase, we use the mechanical alloying. A homogeneous nickel sulfide (NiS) phase is synthesized after ball milling for 12 hours sulfur with high surface area nickel. The initial discharge capacity of nickel sulfide positive electrode is 590mAh/g-NiS, at 1.5V vs. Li/$Li^+$. The NiS powders synthesized by ball milling show an excellent cyclic property. It retained 65% of its initial capacity even after 100 cycles at 30℃. It can be suggested that nickel plays a good catalyst for the reversible redox reaction of sulfur with lithium and also good electric conductor the cathode in the form of nickel sulfide.
In th part 2, it was focused the effect of nano-sized polysulfide adsorbing material on the electrochemical performance of the sulfur cathode to increase the sulfur utilization with holding most of soluble polysulfides. The adsorbing materials prevent the dissolution of polysulfides from the cathode into the electrolyte and other cell components. It was investigated that the effect of Iron sulfate ($FeSO_4$) and Multi-walled carbon nanotudes (MWNTs) on the performance of the sulfur cathode with an aim to increase the sulfur utilization as provides good structural stability of the cathode and confines polysulfide in the cathode. Iron sulfate was effectively coated on sulfur active material by wetting method and it could improve the cycle life of sulfur cathode. However, the initial discharge capacity is decreased after $FeSO_4$ coating. The reason is that the Iron sulfate is not electric conductor. So, it should be required to adsorbing material which has high electric conductivity. Here with, MWNTs were synthesized by thermal CVD method. The use of multi-walled carbon nanotube as an adsorbing material in the cathode provides a reasonable solution in order to increase the retention of soluble polysulfide on cathode compared with other additives and to increase the electric conductivity of the sulfur cathode. It was found that the assembly of the relatively long and thin carbon nanotubes could furnish three-dimensional network with regular pores in sulfur cathode, The high retention of the polysulfide in the cathodes based on carbon nanotubes would also contribute to high sulfur utilization. The Sulfur/MWNTs electrode retains high specific capacity at high discharge rate : this behavior is also highly accordance with the fact to the electronically conducting network developed from the MWNTs which are more effective than that of acetylene black. In addition, very interesting property is that the Sulfur/MWNTs electrode exhibit less apparent capacity loss upon cycling.
Li/S전지는 1980년대에 이스라엘의 Peled, 미국의 Rauh연구팀에서는 THF, TOL등의 유기전해질을 이용하고 유황을 Li-S화합물형태로 전해질에 용해시킨 리튬/유황전지에 대한 연구를 수행하였다. 그러나 유황은 전기부도체이며, 반응성이 강해 전해질선택이 매우 어려워서 전지개발에 매우 큰 어려움이 있었다. 최근들어, 유황을 직접 리튬전지용 양극재료로 사용하는 연구가 활발히 진행중인데, 기존의 리튬코발트산화물이나 FeS, organo-sulfur전극에 비하여 3배 이상 에너지밀도가 크다. 그러나, 충방전 싸이클 수명이 30회 이내로써 전극수명이 낮은 단점이 있다. 따라서, 매우 높은 방전용량(1,675mAh/g-sulfu)r을 가지는 고용량 Li/S 전지의 전극수명을 향상시킨다면, 차세대 고용량, 고성능 이차전지를 개발할 수 있다.
