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

Abstract

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 performance. 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 mAh g−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.

Hard Carbons for Sodium‐Ion Battery Anodes: Synthetic Strategies, Material Properties, and Storage Mechanisms

“…The thickness of the SEI can exist in the nanometer range. [105] This result indicates that FEC aids in the facile formation of the SEI without affecting the nature of charge-discharge curves. [102][103][104] Interestingly,i norganic compounds are in immediate contact with the carbon surface, whereas organic components are closet ot he electrolyte side.…”

Electrolytes, SEI Formation, and Binders: A Review of Nonelectrode Factors for Sodium‐Ion Battery Anodes

“…Since the publication of Komaba's paper detailing the advantages of a FEC electrolyte additive, many papers have followed suit, also adding low wt.% of FEC in their electrolytes and detailing its effects . However, it seems that the beneficial effects of FEC addition are most pronounced in alloying and conversion type carbon composites as opposed to pure carbon materials .…”

Electrode Materials for Sodium-Ion Batteries: Considerations on Crystal Structures and Sodium Storage Mechanisms

“…The direct use of sodium metal as an anode material could cause many problems such as poor cycling performance, easy short-circuits due to the low melting point of Na (97.7 °C), high chemical reactivity and dendritic growth during charge and discharge [110,[112][113][114]. Therefore, many new materials such as carbonaceous compounds, alloy composites, metal oxides/sulphides, organic compounds containing carbonyl or C=N groups and phosphorus-based materials have been extensively investigated as anode materials for sodium-ion batteries [115,116].…”