Preparation Conditions of Polypyrrole Film on Sulfur-Based Cathode to Prevent Polysulfide Dissolution in Lithium Secondary Batteries

To solve the problem of polysulfide dissolution into the electrolyte on a sulfur-based cathode, a novel S cathode structure by forming a film on the S-based electrode surface is proposed. In this paper, the S/Ketjenblack (S/KB) cathode, which is one of the typical S-based cathodes in use, is coated with an electropolymerized polypyrrole (PPy) film. The simple structure of coating the PPy film directly on the electrode surface makes no energy loss due to the shuttle phenomenon. Also it has an advantage of wide choice of electrolyte because the S electrode and the electrolyte are not directly in contact. A choice of an electrolyte suitable for the anode and allowance of high C-rate operation can be expected.

     Figures 1 (a) and (b) show the charge-discharge profiles of the S/KB cathode without and with the PPy film in 1.0 M LiTFSI DME/DOL (1:1 vol.%), respectively. As shown in Fig. 1 (a), on the S/KB cathode without the PPy film, the charge current continued to flow at ca. 2.4 V at the 2^ndcycle and never reached the upper voltage limit (3.0 V). Thus, further cycles could not continue. This overcharge mechanism is known as the shuttle phenomenon caused by the polysulfide dissolved in the electrolyte. On the other hand, the PPy coated S/KB (PPy-S/KB) cathode achieved over 300 cycles stably with over 97% coulombic efficiency during the all cycles even in the electrolyte which dissolve polysulfides easily. From farther investigation, the PPy film demonstrates ion-selectivity towards Li ion only, with the transport ratio of nearly unity in the organic solution. Thus the PPy-S/KB cathode was confirmed to have a superior ability for inhibiting polysulfide dissolution [1].

     The characteristics of a electropolymerization film is known to be greatly affected by the polymerization solvent, bath composition, polymerization potential, temperature and etc.. The main objective of this paper is to optimize a polymerization condition. Polymerization bath composition and polymerization potential were examined. Electrochemical properties of each PPy-S/KB cathode and characterizations of each PPy film were investigated. First, the effect of polymerization solvent were examined. Propylene carbonate (PC), 1,2-Dimethoxyethane (DME), trienthylene glycol dimethyl ether (Triglyme), which are known as popular solvent for LIB, and Ionic liquid 1-methyl-1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) were carried out for the experiment. In coin cell evaluation, PPy-S/KB cathode with Ionic liquid as the polymerization solvent showed superior battery characteristics compared to the other polymerization solvents. Note that the doping ratio of PPy film polymerized in the ionic liquid is known to be improved by a large amount of dopant anion present in the bath. Also, the polymerization potential of PPy film was changed from 4.0 V, a conventional polymerization onset potential, to 5.0 V. At the potential of 4.5 V, high discharge capacity of PPy-S/KB cathode and excellent Li ion permeability of PPy film were confirmed. To investigate the incorporating mechanism, six types of ionic liquid was examined. The case with pyrroridinium cation and TFSI anion showed the best coulombic efficiency and capacity retention. Detailed results and discussion will be presented. Investigating the structure of a Li ion permselective film may become a major contribution for Li/S batteries.

References

[1] Natsuki Nakamura, Tokihiko Yokoshima, Hiroki Nara, Toshiyuki Momma, Tetsuya Osaka, J. Power Sources 274(2015) 1263-1266.

 

Acknowledgements

     This work was partly supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA Spring)” from Japan Science and Technology Agency (JST), Japan.