Laminated Cell Properties of Lithium Sulfur Battery Using Polypyrrole Modified High Sulfur Loading Cathode with Comparing Al Foil and 3D Foam Current Collectors
Authors: Nakamura Natsuki , Yokoshima Tokihiko , Nara Hiroki , Mikuriya Hitoshi , Shiosaki Ayahito , Ahn Seongki , Momma Toshiyuki , Osaka Tetsuya 
Published: December 2020

In order to solve the problem of polysulfide dissolution into the electrolyte on sulfur-based cathodes, we have proposed a novel method to coat a sulfur cathode with Li+ permselective polypyrrole (PPy) by oxidative electropolymerization. The PPy coated S/KB (PPy-S/KB) cathode demonstrated relatively high specific capacity for 300 cycles stably with coulomb efficiency more than 97% in a coin cell [PPy-S/KB(S loading = 0.5 mg cm2) // 1.0 M LiTFSI DME/DOL (1:1 vol.) // Li] [1,2]. However, to increase energy density further, it is essential to increase sulfur-loading in the cathode. As increasing of sulfur-loading, lower PPy resistance is required to respond high current density.
In this study, sulfur cathodes with sulfur-loadings of 1 mg cm-2, 2.5 mg cm-2, 5 mg cm-2 with Al foil current collectors and sulfur-loadings of 5 mg cm-2, 10 mg cm-2 and 15 mg cm-2 with 3D Al foam current collectors were prepared. The S/KB composite slurry was prepared by mixing 96.5 wt. of the S/KB composite (S/KB=6/4 by wt.) and 3.5 wt. of CMC/SBR. The slurry was applied to Al foil or Al foam to have an arbitrary sulfur-loading. PPy was coated on the cathode by using lithium-glyme equimolar ratio complex (LiTFSI: triglyme (1: 1, mol) + LiTFSI: monoglyme (1: 2, mol) (1: 1, vol.)) as a polymerization bath and the same technique as in our previous report [1,2]. Stacked laminated cells consist of 20*20 mm2 S/KB cathode with/without PPy, 1M LiTFSI DME/DOL (1/1 vol.) as electrolyte and Li foil as anode were prepared (E/S was 5-35).
Fig. 1 shows the relationship between the sulfur-loading and the discharge capacity density using Al foil or 3D Al foam. Although the discharge capacity density decreased when sulfur-loading was increased to 5 mg cm-2 with Al foil (E/S=7), a high discharge capacity density of 1190 mAh/g-sulfur was maintained even when S loading was increased to 15 mg cm-2 with 3D Al foam (E/S=5). In the case of Al foil current collector, the discharge capacity density decreased with an increase in the sulfur loading would be caused by decreased sulfur utilization with longer paths of Li ions. On the other hand, since 3D Al foam has a better path for Li ions and electrons than a normal coated electrode, a high sulfur utilization was maintained even with a high sulfur-loading. Fig. 2 shows a charge/discharge curve of sulfur-loading of 10 mg cm2 with PPy coating at 0.01 C. Overcharging due to polysulfide dissolution was not observed even in DME/DOL electrolyte by the PPy coating, and a high discharge capacity of 1248 mAh/g-sulfur was achieved (E/S=9), while without PPy coationg, polysulfide dissolution and less discharge capacity were observed. It was confirmed that the PPy coating suppressed the dissolution of sulfur into the electrolyte and increased the utilization of sulfur, thereby slightly increasing the discharge capacity as compared to that without PPy coating. We will report the rate characteristics and cycle characteristics of laminated cells with various sulfur-loadings, PPy film thicknesses, and conductive additives.

References:

[1] N. Nakamura, T. Yokoshima, H. Nara, T. Momma, and T. Osaka, "Suppression of polysulfide dissolution by polypyrrole modification of sulfur-based cathodes in lithium secondary batteries", J. Power Sources, 274, 1263-1266 (2015).
[2] N. Nakamura, Y. Wu, T. Yokoshima, H. Nara, T. Momma, and T. Osaka, "Film Properties of Electropolymerized Polypyrrole for a Sulfur/Ketjenblack Cathode in Lithium Secondary Batteries", J. Electrochem. Soc.,
163 (5) A683-A689 (2016).
[3] H. Nara, T. Yokoshima, H. Mikuriya, S. Tsuda, T. Momma, and T. Osaka, “The potential for the creation of a high areal capacity lithium-sulfur battery using a metal foam current collector”, J. Electrochem. Soc., 164, A5026–A5030 (2017).

Figure 1