Novel Perceptions of Lithium Ion Batteries: Isotopic Labeling for Insights into Lithium Losses and Solid Electrolyte Interphase Formation By Means of Plasma-Based Techniques

Abstract

Since their introduction in the early 90’s, the lithium ion battery (LIB) has to face challenging demands for mobile and stationary applications, respectively. In state-of-the-art LIBs, carbonaceous negative electrodes and lithium transition metal oxide (e.g. LiNi1/3Co1/3Mn1/3O2 (NCM111)) positive electrodes are used delivering outstanding energy densities. However, the cell suffers from performance losses - especially at elevated charging cut-off voltages exceeding 4.4 V - which cause is still a matter of discussion in literature.1,2 One postulated degradation mechanism is assigned to the passivation layer on the carbonaceous negative electrode, which is called the solid electrolyte interphase (SEI). It is formed during the first cycle due to the high reductive potential affecting the electrolyte and consuming active lithium. Nonetheless, it is still unknown which lithium source, either electrolytic or cathodic lithium, is electrochemically lost during this SEI formation. In order to quantify the lithium loss in both positive and negative electrodes, plasma-based analytical techniques are well suited for these investigations. In this work, inductively coupled plasma-optical emission spectroscopy (ICP-OES) is used to investigate the lithium loss in charge/discharge aged layered NCM-based cathode materials as well as the lithium loss in carbonaceous anodes using glow discharge-sector field-mass spectrometry (GD-SF-MS). Additionally, investigations of 6Li-enriched cathode material or electrolyte solutions (6LiPF6) were conducted using inductively coupled plasma-mass spectrometry (ICP-MS) and GD-SF-MS for bulk and depth-resolved quantification of lithium losses in charge/discharge aged LIBs. Therefore, a NCM111 material is synthesized using 6Li-enriched components as source material as well as an isotope labelled conducting salt and cycled in a full-cell set-up to elucidate the origin of lithium losses. With this approach, it is possible to distinguish between lithium losses either from the conducting salt or the cathode material. Furthermore, the origin of the lithium, which forms the SEI, can be traced via this approach. The electrode-near SEI consists of lithium originating from the electrolyte, while a mixed interlayer of lithium from both electrolyte and electrode is formed afterwards. Eventually, lithium in the electrolyte-near SEI originates from the cathode material (see Figure). 1 J. Vetter, P. Novák, M.R. Wagner, C. Veit, K.-C. Möller, J.O. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, J. Power Sources, 2005 , 147, 269-281. 2 J. Kasnatscheew, M. Evertz, B. Streipert, R. Wagner, R. Klöpsch, B. Vortmann, H. Hahn, S. Nowak, M. Amereller, A. C. Gentschev, P. Lamp and M. Winter, Phys. Chem. Chem. Phys., 2016, 18 , 3956-3965. Figure 1