Effect of Hexafluorophosphate and Fluoroethylene Carbonate on Electrochemical Performance and the Surface Layer of Hard Carbon for Sodium-Ion Batteries

Electrochemical sodium insertion for hard carbon is examined in a cyclic alkylene carbonate based solution containing a NaClO4 or NaPF6 salt with a fluoroethylene carbonate (FEC) additive to study electrolyte dependency for sodium-ion batteries. NaPF6-based electrolytes provide superior reversibility and cyclability of sodium insertion into hard carbon compared with NaClO4-based ones. The FEC-derived passivation film improves capacity retention because of better passivation with a thinner surface layer, as revealed by hard and soft X-ray photoelectron spectroscopy (PES). The use of both the NaPF6 salt and FEC additive results in a synergetic effect on passivation for the hard-carbon electrode, leading to enhanced cycle per-formance. Hard-carbon electrodes with polyvinylidene difluoride binder in propylene carbonate based electrolytes containing NaPF6 and FEC demonstrate excellent capacity retention with a reversible capacity of about 250 mAhg(-1). The difference in capacity retention for the electrolytes is expected to originate as a consequence of the difference in the surface interphase layer formed on the hard-carbon electrodes. Surface analyses with PES and time-of-flight secondary ion mass spectrometry reveal differences in surface and passivation chemistry which depend on the salts, solvents, and FEC additives used for the hard-carbon negative electrodes.