The lithium ion battery (LIB) still suffers from capacity loss due to several aging mechanisms during cycling and storage. The electrolyte is not stable at the applied electrode potentials and forms passivating layers on the anode and cathode, the so-called solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI), respectively. These interphases consist of various inorganic lithium salts as well as organic carbonates and phosphates, whose quantitative speciation analysis remains challenging. Structural instability of the cathode active material at high cut-off voltages further leads to dissolution and redeposition of Ni, Co and Mn, which degrades the SEI and causes irreversible lithium loss at the anode. [1,2]

Elemental analysis, particularly inductively coupled plasma optical emission spectroscopy (ICP-OES), plays a critical role in unraveling these aging mechanisms. High demands are placed on the analytical system for simultaneous quantification of elements in different concentration ranges with high matrix loading. Direct elemental analysis of LIB materials by coupling electrothermal vaporization (ETV) to ICP-OES offers distinct advantages over conventional microwave-assisted digestion approaches. By eliminating the time-consuming and labor-intensive digestion step, analysis time can be significantly reduced, with less risk of cross-contamination and a potential gain in sensitivity and accuracy.[3] This study demonstrates the application of ETV-ICP-OES in the LIB context. Method development for quick element quantification as well as speciation analysis of phosporus-containing SEI was performed.

Aged anode samples from Li1Ni0.8Mn0.1Co0.1 NMC811||artificial graphite + 10-20 % SiOx and NMC622||artificial graphite cells were analyzed regarding their Li, Ni. Mn and Co content, as well as with temperature programming for SEI speciation. Method development for the analysis was performed on an ETV (ETV 4000c, Spectral Systems, DE), equipped with an autosampler (AD-50-III, Spectral Systems, DE), which was directly coupled to an ICP Spectrometer (ARCOS, Spectro Analytical Instruments, DE). Sulfur hexafluoride was used as modifier gas. External calibration of the instrument was performed using liquid standards, synthesized matrix-matched standards or certified reference materials, while the Ar 763.5 nm line was used for internal standardization. The analytical performance of the developed methods was evaluated by comparison to microwave-assisted ICP-OES and total reflection X-ray fluorescence (TXRF) measurements.

Acknowledgements
The authors would like to acknowledge the German Federal Ministry of Education and Research (BMBF) for funding the project “E-FloA” (03XP0349B).

[1] J. Xie, Y.-C. Lu, Nature Communications 2020, 11, 2499.
[2] S. Klein, S. van Wickeren, S. Röser, P. Bärmann, K. Borzutzki, B. Heidrich, M. Börner, M. Winter, T. Placke, J. Kasnatscheew, Adv. Energy Mater. 2021, 11, 2003738.
[3] L. Huang, D. Beauchemin in Sample Introduction Systems in ICPMS and ICPOES (Ed.: D. Beauchemin), Elsevier, Amsterdam, 2020, pp. 411–467.