Understanding the Reductive Decomposition of Highly Concentrated Li Salt/Sulfolane Electrolytes during Li Deposition and Dissolution

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.