Synergy or interference? The effect of electrolyte additives and formation protocols combined in sodium-ion batteries

Abstract

Sodium bis(oxalato)borate in triethyl phosphate (NaBOB–TEP) is a promising fluorine-free non-flammable electrolyte for sodium-ion batteries, though it suffers from irreversible capacity losses during the first cycle due to the reduction of NaBOB. Here, the effects of the passivating electrolyte additives and different formation protocols on the electrolyte performance are investigated. The normal formation protocol is based on slow C-rate while the skip protocol has a high C-rate step until 3 V to bypass the NaBOB decomposition. The skip protocol is proved to be more effective in improving the initial coulombic efficiency of 0.35 M NaBOB–TEP than the additives but in the long-term cycling the effect is evened. The cyclic voltammetry results show that during normal formation the additives 1,3,2-dioxathiolane 2,2,-dioxide (DTD) and prop-1-ene-1,3-sultone (PES) are reduced before NaBOB which stabilizes the solid electrolyte interphase (SEI) and the cell cycling. On the other side, the skip protocol bypasses the positive SEI formation capability of PES and DTD, preventing the traditional SEI formation. Meanwhile, the less reactive 1,4-butane sultone (BS) additive that is not able no prevent NaBOB decomposition on its own, benefits from the changes in the formation step similarly to the baseline electrolyte, and this combined with a small amount of sulphur in the SEI from the BS decomposition, results in the best cycling performance when the skip protocol is used. The results indicate that if additives and skip protocol are used together, the investigation for the best-performing additive should be done using the intended cycling protocol, because the protocol affects the additive performance as well.