Understanding the Reductive Decomposition of Highly Concentrated Li Salt/Sulfolane Electrolytes during Li Deposition and Dissolution
Yosuke Ugata, Ryoichi Tatara, Toshihiko Mandai, Kazuhide Ueno, Masayoshi Watanabe, Kaoru Dokko
Abstract
Li deposition and dissolution in highly concentrated electrolytes consisting of sulfolane (SL) and two amide-type Li salts, LiN(SO2CF3)2 (LiTFSA) and LiN(SO2F)2 (LiFSA), were investigated. The chronopotentiometry test of Li/Cu cells containing these two electrolytes demonstrated that the reversibility of Li deposition/dissolution and the cycling performance were better in the LiFSA/SL electrolyte than in the LiTFSA/SL electrolyte. Gas analysis with electrochemical mass spectroscopy revealed that the SL molecules were reduced to form tetrahydrothiophene (THT) and butane in the LiTFSA/SL electrolyte during Li deposition. In contrast, these side reactions were significantly suppressed in the LiFSA/SL electrolyte. The X-ray photoelectron spectroscopy analysis for the deposited Li in the LiTFSA/SL electrolyte suggests that Li2O and sulfurous compounds were formed on the Li surface by the reductive decomposition of SL. For the LiFSA/SL electrolyte, the LiF-rich passivation layer derived from the FSA anion could effectively suppress further decomposition of SL, resulting in highly reversible Li deposition and dissolution.