Understanding the Interaction of the Carbonates and Binder in Na-Ion Batteries: A Combined Bulk and Surface Study

Organic carbonates are the predominant electrolyte base used in the best Li- and Na-ion battery systems. However, to date, their precise interactions with the electrode are not fully understood. Here, we elucidate the role of a prototypical electrolyte additive, fluoroethylene carbonate (FEC), in Na-ion batteries and illustrate its influence on the solid electrolyte interphase (SEI) composition. Through the use of bulk and surface analytical techniques, we find that early FEC decomposition forms a surface-passivating constituent (Na2CO3), reduces the rate of decomposition of the propylene carbonate electrolyte, and supports the preservation of the polyvinylidene difluoride (PVDF) binder. Furthermore, we observe that NaF is formed by the decomposing PVDF binder when no additive is present, resulting in the early degradation of battery performance. This new understanding of the SEI offers greater chemical insight into the selection of binder and electrolyte combinations, allowing better targeted use of these battery constituents.