Study of the Most Relevant Aspects Related to Hard Carbons as Anode Materials for Na-ion Batteries, Compared with Li-ion Systems

We present here a comprehensive description and discussion of the possible use of hard carbons as anode materials for sodium-ion batteries. The study compared the behavior of hard carbon electrodes in selected polar-aprotic Na- and Li-salt solutions. We also examined the choice of the binders on the electrodes' performance. We explored storage mechanisms, cycling, stability, surface chemistry, and impedance behavior. Propylene carbonate (PC) was found to be a suitable solvent. The presence of two percent of monofluorinated ethylene carbonate (FEC) in PC-based Na-salt solutions exhibits a very positive effect on the behavior of hard carbon electrodes as Na-ion insertion anodes. Based on spectral studies, we attribute the effect to surface chemical aspects, which are determined by the presence of FEC. The intercalation mechanisms of ions into hard carbon includes insertion of ions between graphene layers at high potentials and adsorption into micropores at low potentials. For the bigger Na ions, the availability of intercalation sites is limited; thereby, the capacity of hard carbon electrodes in Na-salt solutions is less than half of their capacity in Li-salt solutions. Hard carbon electrodes in both types of solutions exhibit impressive stability during prolonged cycling. Impedance measurements revealed that the resistivity of the surface films formed on hard carbon electrodes in Na-ion solutions is roughly one order of magnitude higher than in Li-salt solutions. Hence, surface films comprising ionic Li compounds provide a better solid electrolyte interphase behavior than surface films comprising Na-ion compounds.