The Carbon-Electrolyte Interface at High Cathodic Voltages

Carbon conductive additives are important constituents of the Li-ion battery cathodes, as the oxidic cathode materials used up to now are suffering from too low electronic conductivities. The next generation, high capacity cathodes are expected to operate at higher voltages than the current state-of-the-art cathodes, which will be challenging for the stability of the carbon conductive additive. It is already know that the anion PF₆^-intercalates in graphite at potentials above 4.2 V, and also that decomposition of electrolyte and exfoliation of graphite due to co-intercalation of electrolyte may occur [1,2]. At higher voltages all know electrolytes will oxidize, which in particular cause degradation of high surface area conductive additives, like carbon black [3].

The purpose of this study is to compare different carbon conductive additives with respect to their electrochemical performance at high cathodic voltages. In addition to conventional graphite powder KS6 and the carbon black SuperP Li from TIMCAL, a multilayer graphene powder was included in the study. The latter had a graphitic structure, however with a much broader particle size distribution than the KS6, and a surface area comparable to the carbon black. Materials were characterized with XRD, SEM, Raman and nitrogen adsorption combined with a DFT model for identification of surface area of defect, basal and edge planes [4]. Electrodes from these materials were then cycled galvanostatically in coin cells at various rates, and investigated in 3-electrode cells with cyclic voltammetry and impedance spectroscopy. The electrolytes used were 1M LiPF₆in 3:7 EC:DMC or 1:1 EC:DMC. In-situ X-ray diffraction experiments were conducted in order to investigate the intercalation reaction and structural damage to the materials. One electrolyte additive was also investigated.

Results show that intercalation of anions occured in both graphitic materials, as expected, but at lower capacities than previously observed for oxidation resistant electrolytes [1]. The intercalation potential of PF₆^- was slightly higher for multilayer graphene compared to KS6, and the intercalation also appeared to be more reversible. On the other hand, the multilayer graphene has a much higher irreversible capacity loss in the first cycle, indicating more severe electrolyte oxidation. The first cycle irreversible loss correlates well with the surface area of edge planes of the materials studied, thus the decomposition of electrolyte appear to be correlated to the intercalation process. The most oxidation resistant material, the carbon black, had a very low capacity for intercalated anions, which was attributed to the low crystallinity of this material. All materials showed stable performance upon galvanostatic cycling at 120 mAh/g, and the oxidation reaction was suppressed at the high rates compared to low rates. Apart from electrolyte oxidation products, no changes to the electrodes could be observed by SEM after cycling. In-situ XRD performed for the graphitic materials showed that the most crystalline materials, KS6, was most susceptible to structural damage, and that the damage was mainly introduced in the initial cycle up to 5.0 V. Upon repeated cycling, the capacity for anion intercalation was reduced due to formation of surface films, and for KS6, there was also a strong rate dependence of the intercalation process.

References

1. J.A. Seel and J.R. Dahn, J. Electrochem. Soc. 147(3) (2000) p 892.
2. W. Märkle, J.T. Coli, D. Görs, M.E. Spahr, P.Novak, Electrochimica Acta, 55 (2010) p. 2727.
3. J. Syzdek, M. Marcinek, R. Kostecki, J. Power Sources, 245(3) (2014) p. 739.
4. J. P. Olivier, M. Winter, J. Power Sources, (97-8) (2001) p. 151.