Fluoroethylene Carbonate as an Additive for Sodium-Ion Batteries: Effect on the Sodium Cathode

Driven by the wide-scale implementation of intermittent renewable energy generating technologies, such as wind and solar, sodiumion batteries have recently attracted attention as an inexpensive energy storage system due to the abundance, low cost, and relatively low redox potential of sodium. However, in comparison with lithium-ion batteries, which are known for long cycle life, sodium-ion batteries usually suffer from significant capacity fading during long-term cycling due to the large volume expansion/contraction of the electrode active materials caused by insertion/extraction of the large sodium ion. In recent years, intense effort has been focused on the search for high performance electrode materials and electrolytes to improve the cyclability of sodium-ion batteries, and some progress has been achieved. The incorporation of additives into the electrolyte is a simple and efficient method of improving the cycle stability of sodium-ion batteries. Fluoroethylene carbonate (FEC) is generally considered to be a suitable additive for the formation of the anode solid electrolyte interphase (SEI), due to a relatively low-lying lowest unoccupied molecular orbital (LUMO). However, it is suggested that FEC it will not be oxidized on the cathode since it also has a relatively low highest occupied molecular orbital (HOMO). In this study, we investigated the effect of FEC as an additive on the cycle life of a sodium-ion battery with a P2-NaxCo0.7Mn0.3O2 (x approximate to 1) layered sodium transition metal oxide as the cathode active material, a sodium metal foil anode, a glass fiber separator, and an electrolyte composed of NaCIO4 and a varying mass content of FEC dissolved in propylene carbonate (PC). We analyzed the effect of the FEC additive on the morphology and chemical composition of the separator and cathode electrode surface using scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS), and studied the evolution of the crystalline structure of the cathode active material during charge and discharge using in situ X-ray diffraction (XRD). We found that an appropriate amount of FEC additive significantly suppressed the decomposition of the PC solvent, and assisted the formation of a NaF-rich protective layer on the cathode surface, which helped to maintain the structural stability of the cathode material, thereby improving the cycle stability of the sodium-ion battery. Density functional theory (DFT) calculations showed that FEC coordinates more readily with the CIO4- anion on the cathode surface than does the PC solvent. This drives the formation of the NaF-rich protective layer on the cathode surface. We believe these results could provide inspiration in the design of electrolyte additives for protection of the sodium cathode during cycling, thus improving the cycling performance of sodium-ion batteries.