Understanding the stability of the solid electrolyte interphase formed by vinylene carbonate and fluoroethylene...

The formation of a stable solid electrolyte interphase (SEI) in sodium-ion batteries is a challenge which is usually solved by introducing film-forming electrolyte additives. The functions and decomposition of common additives like vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are not fully understood and yield different results in full- and half-cells. This study reveals that the electrochemical reduction of an electrolyte solution based on 1 M NaPF₆ dissolved in ethylene carbonate and diethyl carbonate (EC : DEC) with no additive yields a lower charge loss, while electrolytes containing 2 wt% VC or FEC additives suffer from higher charge consumption for the formation and reformation of SEI due to higher solubility. To solely investigate stability and dissolution of the SEI in the absence of other ageing mechanisms, a model cell consisting of a carbon-coated aluminium foil working electrode and Prussian white counter and reference electrodes was used. Additionally, the experiments show a detrimental effect from using sodium metal counter electrodes. This work sheds light on the insufficiency of VC and FEC electrolyte additives in forming a perfect SEI. However, further investigations are required to account for additional ageing mechanisms to provide a comprehensive understanding of the role of VC and FEC in practical sodium-ion batteries.