Sulfur (S) is an attractive electrode material since it has large theoretical capacity of 1673 mAh g^-1. However, its ionic/electric insulating properties and large volume changes during battery reactions prevent practical use in the battery system. Addressing these issues, carbon replica (CR) was selected as the conductive framework in the composite electrode. The CR with highly ordered three-dimensional mesopores could enhance conductivity and accommodate the volume changes in the composite electrode. The S-CR composite electrodes were used to fabricate the all solid-state Li-S battery, and relationship between the structures of the S-CR and electrochemical properties was investigated.

 The CRs with various pore sizes were prepared using the parent silica nanospheres with a uniform-size from 8 to 100 nm [1]. The S was introduced into the CR structure by gas-phase deposition method in a weight ratio of 30:70 for S/CR with various temperature and duration times. Structures of the CR and S-CR were characterized using FE-SEM, BET, X-ray diffraction, TG/DTA and small angle X-ray scattering methods. Electrochemical properties of the all solid-state battery incorporating the S-CR composite, Li_3.25Ge_0.25P_0.75S₄ and Li-Al alloy as a cathode, a solid electrolyte and an anode, respectively, were examined. The cells were subjected to the galvanostatic charge discharge measurements at 25 °C.

Pore size dependency of the S-CR structure and electrochemical properties was evaluated. The silica spheres of the size, 8, 12, 14, 20, 40 and 100 nm were used as templates, and the carbon replicas synthesized using these templates were indexed as CR8, CR12, CR14, CR20, CP40 and CR100, respectively. FE-SEM observation confirmed that the distribution of the pores for each carbon replica was quite uniform with close-packed and spherical pores with expected size, resulting in a formation of a highly ordered porous carbon framework. These spherical pores are three-dimensionally connected with each other through small holes between them. In addition, no changes in shape of the pores and no aggregate of deposited S were observed, indicating the S is highly distributed at the surface of CRs. In the charge discharge test, S-CR12, S-CR40 and S-CR100 electrodes were examined. First discharge capacity increased with decreasing pore size, and cycle/rate capabilities were also enhanced. These results indicated that higher distribution of S with smaller sized in the composite could contribute to enhancement of the electrochemical properties.

Using the CR12, various deposition conditions were examined; the temperature and duration time were selected, at 150, 200 or 300 °C and for 1 or 2 h, respectively. TG analysis revealed that there are three kinds of S condition on the CR, (i) out side of pore, (ii) in side of pore and (iii) strongly interacted to the CR. The amount of each type and total of the S is summarized in Fig. 1(a). With increasing of the temperature and time, total amount of S decreased while the ratio of S at in side/out side increased. On the other hand, the interacted S significantly increased at 300 °C. These results indicate that the deposition conditions are related to the S condition on the CR. Charge discharge measurements were carried out using these electrodes.

Discharge capacities of the batteries at 1st and 10th cycle are illustrated together with two structural parameters, relative amount of the sulfur at the inside to the sulfur at the outside: S_inside/S_outside and amount of interacted sulfur: S_interacted in Fig. 1(b). Discharge capacities increased with increase of S_inside/S_outside for the S-CR12 prepared at 150 and 200°C, indicating the S_inside contributes to better battery performance. However, lower capacities were obtained for the S-CR12 prepared at 300°C although these electrodes show higher values of S_inside/S_outside. This tendency could be related to higher amount of the S_interacted, indicating the S_interacted deteriorates the battery performance. The electrochemical properties are intricately related to the S-CR structure; merged characteristics from the positive effects (S_inside) and negative effects (S_interacted) could determine the battery performance. Further optimization of the fabrication process could realize the high electronic conductivity and high sulfur utilization of the S-CR electrodes for all solid-state batteries.

 Acknowledgements

This work was supported by a Grant-in-Aid from the Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA- SPRING) of the Japan Science and Technology Agency (JST).

 [1] T. Yokoi, Y. Sakamoto, O. Terasaki, Y. Kubota, T. Okubo and T. Tatsumi, J. Am. Chem. Soc. 128 (2006) (42), p. 13664.