In-Situ Scanning Tunneling Microscopy and Electrochemical Quartz Crystal Microbalance Studies of the SEI Formation on Graphite Electrodes

Electric vehicles and portable electronic devices depend on high performance energy storage technology like rechargeable Li ion batteries, whose successful operation requires high energy- and power-densities just as low cost, reliability, safety and long lifetime.^1,2 The energy and power density of batteries, as well as their costs are mainly determined by the electrode materials, whereas safety, lifetime and efficiency are strongly affected by the properties of the Solid Electrolyte Interphase (SEI).³ In the past three decades research tried to deepen the understanding of the SEI by a large variety of experimental methods, ranging from basic electrochemistry over microscopic techniques to spectroscopic tools⁴. Questions regarding its formation mechanism, its morphological appearance and its chemical composition have been tackled, but there is still a lack of understanding on the molecular level.

In the mid-1990s M. Inaba et al. carried out a series of experiments about the SEI-formation on model graphite electrodes and the co-intercalation of Li⁺-ions together with their solvation shell  using Electrochemical Scanning Tunneling Microscopy (EC-STM).⁵ EC-STM is a very powerful technique for surface studies, since it offers the possibility to in-situ image molecular processes at the electrode/electrolyte interface. In the present study, the SEI-formation was examined in depth by in-situ EC-STM on highly oriented pyrolytic graphite (HOPG) model electrodes both in commercial Li⁺-ion battery electrolytes as well as alternative electrolyte systems (see Figure 1). In addition, the Electrochemical Quartz Crystal Microbalance technique (EQCM) as another in-situ method gave further insights into the SEI-formation. Both in-situ techniques provided valuable information about the electrochemical characteristics of this system, leading to better understanding of the formation mechanism and the morphological appearance of the SEI.

Figure 1. In-Situ EC-STM study of the SEI-formation on HOPG in 1M LiPF₆ in EC/DMC electrolyte: The surface is imaged while the potential (line) is scanned from 2.0 V vs. Li/Li⁺ to 1 V, held at this potential, and scanned back to 2 V. The y-axis of the STM image is converted to a time scale to correlate changes in morphology to the applied potential

¹M. Armand, J.-M. Tarascon, Nature, 451 (2008) 652

²J. B. Goodenough, J. Solid State Electrochem, 16 (2012) 2019

³P. B. Balbuena, Y. Wang, Imperial College Press (2004), ISBN 1-86094-362-4

⁴H. Tavassol, J.W. Buthker, G.A. Ferguson, L.A. Curtiss, A.A. Gewirth, J. Electrochem. Soc., 159 (2012) A730-A738.

⁵M. Inaba, Z. Siroma, A. Funabiki, Z. Ogumi, T. Abe, Y. Mizutani, M. Asano, Langmuir, 19 (1996) 1535