New Insights into SEI Formation in Lithium Ion Batteries: Inhomogeneous Distribution of Irreversible Charge Losses Across Graphite Electrodes

The development of lithium ion batteries with long cycle life requires a good control of degradation mechanisms and irreversible charge losses (ICL). The most prominent example in this respect is the use of electrolyte additives such as vinylene carbonate (VC) to improve the physical properties of the Solid Electrolyte Interphase (SEI) [1].

Until now, the SEI was considered to be formed homogeneously throughout the thickness of the electrode. Unfortunately, standard analytical techniques are either surface sensitive or they probe the entire averaged properties of the electrode and hence cannot proof otherwise. Porous electrodes, however, are characterized by a distribution of charge-transfer currents across the electrode due to a mass-transport limitation of educts diffusing from outside the electrode [2]. A multi-layered working electrode (MWE) was therefore developed to resolve electrochemical reactions in this vertical dimension. It was observed that the intercalation of lithium ions into graphite proceeds stage-by-stage and layer-by-layer from top to bottom [3].

For the reduction of electrolyte constituents in small quantities such as additives or contaminants, however, the case is different. The surface layer of the MWE suffers from a higher irreversible reduction current due to an additional influx of reactants from the separator. Since these reactants are fully depleted from the electrolyte during first charge, graphite particles close to the electrode surface effectively act as “electrochemical sieve” for contaminants and additives. X-ray photoelectron spectroscopy (XPS) studies confirm that this sieving effect results in a different SEI on the electrode surface as compared to the electrode bulk. The results presented here are to the best of our knowledge the first direct experimental proof that ICL and SEI are not evenly distributed across graphite electrodes.

[1]        P. Verma, P. Maire, P. Novák; Electrochim. Acta 2010, 55, 6332–6341.

[2]        J. Euler and W. Nonnenmacher; Electrochim. Acta, 1960, 2, 268.

[3]        S. Klink, W. Schuhmann, F. La Mantia; ChemSusChem, 2014, submitted