X-Ray Microtomography Analysis of Li-Sulfur Batteries with a Block Copolymer Electrolyte
Résumé
Li-ion batteries are the dominant solution for many applications, from small electronic devices up to hybrid and fully electric vehicles.[1] However, this technology is now mature and new chemistries are needed to reach high specific energy while ensuring safety. Despite a relatively low operating voltage, sulfur (S8) is an attractive cathode active material due to its high theoretical specific capacity; a factor of six higher than traditional LiCoO2 cathodes.[2] To realize high energy density systems, the S8 cathode must be paired with a Li metal anode. Many challenges remain to be addressed to effectively couple a Li metal with a S8 cathode such as the low S8 utilization, the strong volume change of active material upon cycling, the insulating nature of S8 and of the final product Li2S, the solubility of polysulfides in the electrolyte.[3] This issues lead to poor capacity retention, limited cycle life, and low Coulombic efficiency. To address the polysulfide dissolution problem one approach consists in confining S8 within mesoporous and nanoporous carbon.[4] However, conventional liquid electrolytes are not stable against Li metal. A solution is then to use a solid polymer electrolytes, such as polystyrene-b-poly(ethylene oxide) (SEO) block copolymer doped with LiTFSI salt, known to stabilize Li metal.[5]
The goal of this study is to understand the behavior of Li-S8 batteries comprising a SEO electrolyte. The cathode is a composite made of carbon, SEO electrolyte and sulfur-impregnated carbon nanospheres.[6] The cells were cycled and exhibited significant capacity fade. We thus used hard X-ray microtomography to determine the reason for this capacity fade. Figure 1 represents tomography images prior and after cycling. In addition to polysulfide dissolution, our observations indicate that the battery failure in our system is also due to strong changes at the Li/SEO interface.
References
[1] J. B. Goodenough, K.-S. Park, J. Am. Chem. Soc., 135 (2013) 1167 (2013).
[2] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater., 11 (2012) 19.
[3] S. S. Zhang, J. Power Sources, 231, 153 (2013).
[4] X. Fan, W. Sun, F. Meng, A. Xing, J. Liu, Green Energy Environ., 3 (2018) 2.
[5] D. T. Hallinan, S. A. Mullin, G. M. Stone, and N. P. Balsara, J. Electrochem. Soc., 160, (2013) A464.
[6] D. Devaux, I. Villaluenga, X. Jiang, Y. H. Chang, D. Y. Parkinson, N. P. Balsara, J. Electrochem. Soc., 167 (2020) 060506