Matrix-matched standards for the quantification of elemental lithium ion battery degradation products deposited on carbonaceous negative electrodes using pulsed-glow discharge-sector field-mass spectrometry

Abstract

In this work an external calibration approach for glow discharge-sector field-mass spectrometry (GD-SF-MS) using matrix-matched self-prepared carbonaceous standards for elemental battery degradation products of LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) positive electrodes like lithium, manganese, cobalt and nickel is adapted. Firstly, the standards were prepared using graphite mixed with increasing contents of NCM111 which was coated on a thin copper foil as a current collector. The homogeneous distribution of NCM111 in the standards was proven via SEM/EDX images and the bulk homogeneity of the electrode sheets was validated via ICP-OES. Afterwards, sufficient linearity could be obtained in a calibration range of 1 mg g⁻¹ to 28 mg g⁻¹ for ⁷Li with respect to the active material mass. Additionally, the matrix-matched relative sensitivity factors (RSFs) of each element could be calculated. Limits of detection (LODs) ranging from 80 μg g⁻¹ (⁷Li) up to 393 μg g⁻¹ (⁵⁸Ni) could be achieved at low (R > 300) and medium (R > 4000) resolutions for the Element GD, respectively. Secondly, we adapted the matrix-matched RSF values in order to investigate cycled electrodes by monitoring the ⁷Li signal as well as common isotopes from lithium ion batteries – such as ³¹P and ¹⁹F, originating from the conducting salt – and transition metals to conduct depth-resolved analysis. The concentration of transition metals in all of the cycled electrodes was below the LOD of the GD-SF-MS method which was investigated in a previous study, showing a maximum bulk deposition of transition metals of 4.5 mg g⁻¹. As expected, an accumulation of ⁷Li in the first few minutes (=surface layers) of sputtering was observed in the cycled carbonaceous negative electrodes followed by a decreasing ⁷Li signal with ongoing sputtering indicating the presence of a solid electrolyte interphase (SEI) passivation layer.