Investigation of Aging Effects in Layered Lithium Transition Metal Oxides Using X-Ray and Plasma-Based Techniques
Authors: Evertz Marco , Winter Martin , Nowak Sascha 
Published: February 2020

The state-of-the-art Lithium Ion Battery (LIB) negative electrodes consist of graphite with a decent capacity 372 mAh g-1 and lithium transition metal oxides (LiMO2) as positive electrodes supplying capacities of around 150 mAh g-1 (M = Mn, Co, Ni, Al). One common material is the class of layered transition metal oxides with different benefits and drawbacks, e.g. increased capacities in the charged state and high thermal stability.[1] But, these materials suffer from fading capacities during continuous cycling, especially at elevated charge cut-off voltages exceeding 4.4 V, which cause is still a matter of discussion in literature.[2,3]

One postulated cause is assigned to the dissolution of transition metals - originating from the positive electrode – which claims to have a negative influence on the surface of the solid electrolyte interphase (SEI) as it leads to cracking of this. This would result in a loss of available active lithium as well as loss of active cathode material, resulting in steady capacity decrease.[2,3]

In order to examine transition metal deposition on negative electrodes and other cell components, a versatile and robust tool for the detection of transition metals is desirable. In detail, the requirements of low detection limits are of upmost importance while maintain sensitivity. Here, the x-ray fluorescence in the setup of total reflection (TXRF) is a powerful tool for the bulk detection of transition metals in carbonaceous anode materials as well as on lithium metal and in the electrolyte.[3] However, this technique suffers from the lack in lithium detection due to the low photon energy of the analyte which common detectors are incapable to measure, thus, another analytical tool is necessary. As plasma-based techniques (e.g. inductively coupled plasma-optical emission spectroscopy and mass spectrometry (ICP-OES and ICP-MS) or glow discharge-mass spectrometry (GD-MS)) are widely used in various research contexts, these analytical tools show excellent quantification limits for alkali-metals, hence, their application in the LIB context is promising.
In this work, several active layered transition metal oxides (NCM111, NCM532, NCM622, NCM811 and NCA) were cycled vs. Li/Li+ and investigated based on their transition metal dissolution. Therefore, TXRF was applied to quantify deposited content of transition metals in cycled lithium metal half cells. Furthermore, all other cell components – separator and electrolyte – were quantified on their transition metals content using the TXRF. Additionally, the cathodes were investigated to elucidate their degree of lithiation after certain stages of aging. Furthermore, the studies of active lithium losses were enhanced to carbonaceous anodes using depth-profile analysis to elucidate the influence of lithium losses on the LIB from an analytical point of view.
[1] R. Wagner, N. Preschitschek, S. Passerini, J. Leker, M. Winter, J. Appl. Electrochem., 2013, 43, 481-496.
[2] 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.
[3] M. Evertz, F. Horsthemke, J. Kasnatscheew, M. Börner, M. Winter, S. Nowak, J. Power Sources, 2016, 329, 364-371.