Recently, Li/S secondary battery has been developed as highly attractive power source with high energy density because of the high theoretical specific capacity of sulfur cathode material (approximately 1,675 mAh/g). However, since Li/S battery has poor cyclic durability, it is still far from commercialization. In order to improve cycle life, it is most necessary to elucidate the detailed capacity fading mechanism of sulfur cathode material. In this work, the capacity fading mechanism was investigated in view of the dissolution of sulfur active material into an electrolyte. In general, during discharging process, sulfur cathode shows two plateau regions. The first and second plateau at 2.4V(vs. Li/$Li^+$) and 2.0V(vs. Li/$Li^+$) represent the polysulfides$($Li_2S_x)$ and $Li_2S$ formation reaction, respectively. To investigate more active step for sulfur dissolution in two representative reactions and the influence of electrolyte, cycling test was performed in the divided potential regions employing two different electrolytes, which are EC:DEC(1:1) containing 1M LiPF6 with high ionic conductivity$(10^{-2}Scm^{-1})$ and triglyme containing 0.1M LiTFSI. The discharge capacity of cell cycled at 2.4V plateau was degraded more rapidly compared to the cell cycled at 2.0V plateau, regardless of the electrolyte type. During cycling at 2.4V plateau, when EC:DEC electrolyte used, the discharge capacity dropped to zero at 50th cycle. On contrary, the cell using triglyme electrolyte retained 77% of the initial capacity. In the cycling at 2.0V plateau, 70 and 92% of initial capacity were maintained in cells using EC:DEC and triglyme electrolyte, respectively. The electrolyte analyses during cycling indicate that more sulfur at 2.4V plateau was reacted and dissolved into electrolyte than at 2.0V plateau. While, over high voltage plateau(2.4V), it was observed that 24 and 3.3wt% of sulfur were dissolved into EC:DEC and triglyme electrolyte, respectively. 6.6 and 1.1wt% of sulfur were dissolved in EC:DEC and triglyme electrolyte respectively during cycling at low voltage(2.0V) plateau. Therefore, it can be concluded that the degradation mechanism is the severe dissolution of sulfur active material into liquid electrolyte and the dissolution occurs more vigorously in the 2.4V plateau region. It is confirmed that triglyme electrolyte is more favorable for suppression of sulfur dissolution than EC:DEC electrolyte. The reason for this is considered that the unique structure of triglyme electrolyte. The triglyme electrolyte has similar molecular structure to PEO(Polyethyleneoxide) electrolyte which has rigid backbone of EO(ethyleneoxide) molecule and has low reactivity with polysulfides.
