Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A

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Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A Post Mortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES) Niloofar Ghanbari, Thomas Waldmann, Michael Kasper, Peter Axmann, and Margret Wohlfahrt-Mehrens J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07117 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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The Journal of Physical Chemistry

Inhomogeneous Degradation of Graphite Anodes in Li-ion Cells: A Post Mortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES) Niloofar Ghanbari, Thomas Waldmann*, Michael Kasper, Peter Axmann, Margret Wohlfahrt-Mehrens

ZSW – Zentrum für Sonnenenergie- und Wasserstoff-Forschung BadenWürttemberg, Helmholtzstrasse 8, D-89081 Ulm, Germany

*Corresponding author contact: Dr. Thomas Waldmann ZSW – Zentrum für Sonnenenergie- und Wasserstoff-Forschung, BadenWürttemberg Helmholtzstrasse 8 D-89081 Ulm, Germany [email protected] Tel: +49(0)731-9530-212 Fax: +49(0)731-9530-666

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Abstract Durability and performance of Li-ion cells are impaired by undesirable side reactions, observed as capacity decrease and resistance increase during their usage. This degradation is caused by aging mechanisms on the material level including surface film formation, especially in the case of graphite-based anodes. The present study evaluates the applicability of glow discharge optical emission spectroscopy (GDOES) as a powerful tool to study aging-induced film formation on graphite anodes of Li-ion cells, including deposition of metallic Li. The technique provides depth-resolved information on elemental distribution in the samples from the anode surface to the current collector (through-plane resolution). Additionally, conducting GD-OES depth profiling at different positions of an aged graphite anode reveals differences in surface film growth across the anode plane (in-plane resolution). After verification of the GD-OES method by well-established analytical techniques, aged anodes from commercial state-of-the-art Li-ion cells are analyzed. The results show through-plane and in-plane inhomogeneity in surface film growth: local island-like Li deposition is revealed for 16Ah pouch cells cycled at 45°C and high charging current density while a more homogeneous Li plating gradient is found for cycling 26650-type cells at 20°C.


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1. Introduction The human-induced climate change due to increasing CO2 emission,1–3 can be counteracted by moving towards zero emission transport systems.4,5 High gravimetric and volumetric energy density of Li-ion cells has made them a promising candidate for zero/low emission means of transport.4,5 However, unwanted side reactions occurring during the use of Li-ion cells limit their life-time.6,7 Understanding the root causes of aging mechanisms is the key to improve lifetime and sustainability of Li-ion cells while complying with the demands of the automotive industry. State-of-the-art Li-ion cells utilize anodes composed of graphite particles, which are prone to chemical reactions with electrolyte leading to growth of the solid electrolyte interphase (SEI) during usage.6–12 Another less known aging mechanism is deposition of metallic Li as a parallel reaction to Li intercalation into graphite. Due to the comparably high reactivity of metallic Li with electrolyte, Li deposition leads to rapid capacity loss9,13–18 and can impair cell safety.19,20 Li deposition is so far often referred to as ‘Li plating’, however, the term ‘metal plating’ is only correct in case of a homogeneously deposited metal film. For example, Jalkanen et al. and Bauer et al. have recently shown that Li deposition can also be more inhomogeneous and localized.21,22 Li deposition can be detected by negative anode potentials vs. Li/Li+ in 3-electrode full cells with an additional reference electrode.9,17,18,20,23 However, localized and/or small amounts of Li deposition are likely not to be detected in the electrochemical data. Li deposition conditions are reported to be fulfilled when charging at low temperatures

in

combination

24,22,23,9,14,13,18,20

densities).

with

too

high

C-rates

(charging

current

The temperature dependency of aging mechanisms9,17 in

combination with temperature gradients inside Li-ion cells25–29 or with local inhomogeneities12,21,30 suggest non-uniform aging inside Li-ion cells, which is not well understood. Since the aging phenomena are taking place on the material level, the method of choice to investigate them is disassembly of Li-ion cells and subsequent analysis of the cell components by physico-chemical methods. This process is known as ‘PostMortem analysis.7–9,13,30–33 Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) are well known methods for morphological and compositional characterization of electrode surfaces in Post-Mortem analysis.9,12,30 However, EDX is not sensitive to light elements such as Li and H, which are



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important in the analysis of aged Li-ion cells. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is sensitive to Li but provides an average elemental concentration in a few cm2 scraped off an electrode; hence, no surface information is obtained. X-ray photoelectron spectroscopy (XPS) in combination with Ar+ sputtering has the drawback of very slow removal of material from the sample, resulting in depth profiling in the sub µm range. In contrast, glow discharge optical emission spectroscopy (GD-OES) provides fast (~1µm/min) depth-resolved analysis of metal samples.34 Depth profiling in GD-OES is achieved through sample sputtering using plasma (usually Ar+ ions) and the simultaneous detection of the removed elements by an optical emission spectrometer. Applying the GD-OES technique on porous electrodes from Li-ion cells, recently Saito and Rahman35 and Takahara et al.36 measured depth profiles across the whole depth from the sample surface to the current collector (through-plane resolution). Further research by Takahara et al.10,37 and in our group8 focused on quantification of the Li distribution in graphite anodes with SEI growth. First results on graphite anodes with additional Li plating in our lab showed that it is possible to detect metallic Li on graphite anodes by GD-OES33 which is usually not possible by other methods. The present study focuses on Li quantification in aged graphite anodes to differentiate between SEI growth and Li deposition/plating. In addition to the depthresolved data acquisition, in-plane aging inhomogeneity in graphite anodes of largeformat cells are studied through conducting GD-OES measurements at multiple positions of the same aged anodes. Such investigations have, to the best of our knowledge, not been reported before. In order to yield a sound basis for further experiments, the present study firstly validates the GD-OES method by comparison with other physico-chemical analysis methods and by exclusion of measurement artifacts. Secondly, we define the terms ‘anode surface’ and ‘anode bulk’. Thirdly, applying the GD-OES method to Post-Mortem analysis of commercial Li-ion cells, we show that different aging phenomena including SEI growth and Li deposition/plating can be present inside such cells.

2. Materials and Methods 2.1 Analysis methods


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The GD-OES analysis was carried out using a GDA750 device (Spectruma). A method was developed in pulsed radio frequency (rf) mode with 550V and 2hPa gas pressure. A mixture of 1% H2 with Ar was chosen as discharge gas, which was reported to provide better measurement sensitivity and depth resolution for analyzing porous carbon-based coatings.38,39 For comparison to our previous studies and to secure reproducibility,8,33 also pure Ar was used. The analyzed sample area had a diameter of 2.5mm. The resulting crater depth was measured mechanically using a profilometer (Taylor Hobson, Form Talysurf 50). Roughness values were extracted from the width of Gaussian curves fitted to the profilometer data as implemented in the software provided by the manufacturer. GD-OES calibration was performed using a set of reference coatings prepared by systematic variation of LiH2PO4 in water-based graphite slurries (0-10 mass-%). The reference coatings exhibit a similar porosity and chemical composition with the aged graphite anodes, which is necessary for a matrix specific calibration. The calibrated method was evaluated through correlation of the quantified GD-OES results with ICP-OES analysis (Arcos, Spectro) of the samples with known composition and is shown in Figure 1. The linearity with a regression factor of R2=0.963 verifies the calibration. The observed elements and the emission lines used for this study were H (121nm), O (130nm), C (156nm), and Li (670nm). GD-OES results were further complemented by SEM/EDX (LEO 1530VP Gemini) and laser-scanning microscopy (LSM) (Keyence VK-X200). We note that due to our experimental setup, samples were in contact with air for these methods. For GDOES, we found in this study that air traces within the pores of anode samples can lead to invalid measurements due to changes of the sputtering rate. Therefore, all GD-OES were kept under Ar atmosphere (MBraun, O2

Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A

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