Electrolyte is one of the biggest contributors to the battery safety. In the state-of-the-art Li and Na-ion battery electrolytes, carbonate-based solutions and fluorine-containing salts are typically used. However, due to the flammability of the carbonates and the harmful environmental effects of fluorine, the safety risks related to these electrolytes are considerable. The use of fluorine-containing salts is often justified with their good ionic conductivities, good ionic dissociation and their ability to passivate the aluminium current collector. However, it has recently been shown that alternatives with similar properties exist¹, and that the use of fluorine-based anions is not as well-justified as it was before.

One of the most promising fluorine-free electrolytes for Na-ion batteries consists of sodium bis(oxalate)borate (NaBOB) salt dissolved in triethyl phosphate (TEP)². NaBOB, however, is not without issues as it tends to decompose on the negative electrode during the first cycle. Some of this can be prevented by e.g. using additives³ but overall the issue is not understood well. In this work, the effect of the passivating electrolyte additives and different formation protocols on the formation, electrolyte performance and hard carbon sodiation are investigated. A modified formation protocol starting with a high C-rate step in the beginning of the charge to bypass NaBOB decomposition is proved to be more effective in improving the initial Coulombic efficiency of 0.35 M NaBOB-TEP than the additives. When a conventional formation protocol is used, 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. However, the modified formation protocol bypasses the positive SEI formation capability of PES and DTD, preventing the traditional SEI formation. Meanwhile 1,4-butane sultone (BS), a less reactive additive, benefits from the changes in the formation step similarly to the baseline NaBOB-TEP without additives. In addition, the electrolyte salt and additives and are observed to affect the sodiation reaction potentials on the hard carbon.

The results indicate that when additives and modified formation protocols are used together, the investigation for the best-performing additive system should be done using the intended cycling protocol, because the protocol affects the additive performance.