High-Energy Laminated-Type Li-S Batteries Using High-Sulfur Loading Positive Electrode on Aluminum Foam
Authors: Mikuriya Hitoshi , Yokoshima Tokihiko , Jeong Moongook , Nara Hiroki , Momma Toshiyuki , Osaka Tetsuya 
Published: February 2020

[Introduction]
Interesting in Li-S batteries has been accelerated because it is expected to realize Li ion batteries delivering high energy density. Moreover, its poor cycle stability caused by severe dissolution of sulfur active material in organic carbonate electrolyte has been much improved by introducing novel materials such as Li ion selective separators [1] and Li salt solvate ionic liquid as insoluble electrolyte [2]. Hence, many researches have recently performed to enhance specific energy density. Although, Al foil, in general, has been widely used as a cathode current collector so far, it has not easy with increase in the loading amount of active material, in particularly S/KB composite. When the S/KB loading amount increases on the conventional Al foil, the cathode is suffered from difficulty in Li ion path into bulk active material and large internal resistance with reduced electrical conductivity. To solve such drawbacks, 3D mesh structured metallic foams have been researched and used as a current collector, which reported stable cycle performances with even 10 times larger sulfur loading amount than conventional Al foil [3]. In this report, we enlarged sulfur loading amount by introducing the 3D mesh structured Al foam as a current collector of cathode. Furthermore, optimization of the solvate ionic liquid was also investigated. Herein, it is suggested development on high energy density laminated Li-S battery delivering 200 wh kg-1.
[Experimental]
S/KB composite was obtained by mixing sulfur and KB in the weight ratio of 5:5, followed by heating at 155 degree C for 12 hours under N2 atmosphere. Cathode slurry was prepared in the weight ratio of S/KB (1/1) : KB : PVdF = 60 : 30 : 10. The sulfur amount of 1 – 12 mg cm-2 was loaded on 3D mesh structured Al foam (1mm in thickness). Laminated cells with 70 x 70 mm in size were prepared by assembling the prepared S/KB cathode, Li metal anode, separator (UPORE) and solvate ionic liquid (either G3 : LiTFSI : HFE = 1 : 1 : 4 or G1 + G3 : LiTFSI : HFE = 1 : 1 : 4). Cycle performances were tested between 1.5 V – 3.3 V with 16.7 mA g-1-S after 2 cycles of activation process.
[Results and discussion]
Both areal and specific discharge capacity of the laminated Li-S batteries are shown in Fig. 1a dependent for sulfur loading amount on 3D mesh structured Al foam, i.e., 1.3, 6.2 and 8.6 mg cm-2. Same values of specific discharge capacity delivering ca. 1400 mAh g-1-S were obtained from the different three samples. It revealed that sulfur was efficiently utilized compared to the previous report. This result assumed that loading S/KB on the 3D mesh structured Al foam enhanced Li ion and electron path in comparison to the conventional Al foil. Moreover, since the areal discharge capacity increased linearly with increase in sulfur loading amount, it is considered that stable Li ion and electron path would be maintained within this sulfur loading range.The laminated Li-S battery containing 11.5 mg cm-2 of sulfur loading was successfully prepared (see its picture in Fig.1b) and cycled with solvate ionic liquid (G1 + G3 : LiTFSI : HFE = 1 : 1 : 4). Fig.2 shows its charge-discharge potential profile. It attained specific discharge capacity delivering 1400 mA h g-1-S and areal discharge capacity delivering 15 mAh cm-2. Furthermore, the laminated Li-S battery achieved 735 mA h of cell capacity with a pair of electrode 70 x 70 mm in size.
[Conclusion]
We developed Li-S batteries composed of S/KB cathode with 3D mesh structured Al foam current collector, Li metal anode and Li salt solvate ionic liquid as electrolyte. The Li-S battery containing 11.5 mg cm-2 of sulfur achieved high specific and areal discharge capacity delivering 1400 mA h g-1-S and 15 mA cm-2, respectively. These results suppose that laminated batteries with high energy density delivering 200 Wh kg-1would be realized by efficiently laminating the electrode material and optimizing Li anode.

[Acknowledgement]
This work is supported by Advanced Low Carbon Technology Research and Development Program (JST-ALCA) Special Priority Research Area "Next-generation Rechargeable Battery".
[References]
[1]N. Nakamura, Y. Wu, T. Yokoshima, H. Nara, T. Momma, T. Osaka, J.Electrochem. Soc., 163, A683 (2016).
[2]K. Yoshida, M. Nakamura, Y. Kazue, N. Tachikawa, S. Tsuzuki, S. Seki, K.Dokko, and M. Watanabe, J. Am. Chem. Soc., 133, 13121 (2011).
[3]G. Zhou et al., Nano Energy(2015) 11, 356–365.

Figure 1