Flaw detection in the coating process of lithium-ion battery electrodes with acoustic guided waves

• Fehlererkennung im Beschichtungsprozess von Lithium-Ionen-Batterie-Elektroden mit geführten akustischen Wellen

Gitis, Alexander; Sauer, Dirk Uwe (Thesis advisor); Kwade, Arno (Thesis advisor)

1st ed.. - Aachen : Institut für Stromrichtertechnik und Elektrische Antriebe ISEA (2017)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge des ISEA 101
Page(s)/Article-Nr.: 1 Online-Ressource (iv, 109 Seiten) : Illustrationen

Dissertation, RWTH Aachen University, 2017

Abstract

The energy requirements of current technologies, such as smartphones, drones, and electric vehicles, have forced lithium-ion cell manufactures to relentlessly increase the energy density of their cells. As a consequence, safety margins have been exhausted and cells operate near the edge of stability. In this configuration, minor inhomogeneities and flaws induced during the production process strongly affect cell performance and durability. Consequently, safety risks can emerge under demanding circumstances. The production of lithium-ion cells, especially of electrodes thereof, is a complex process. It requires continuous monitoring of process parameters and of output quality. Particularly, the detection and classification of adhesion losses at the coating-foil interface suffer under inefficient and high-cost monitoring systems. In this dissertation, a novel non-destructive evaluation (NDE) method is developed based on ultrasonic Lamb waves and horizontally polarized shear waves to overcome this shortcoming. In the proposed methodology, metal foils serving as substrates in lithium-ion battery (LIB) electrodes are used as wave guides for measuring ultrasonic signals. The theory of Lamb waves and horizontally polarized shear waves is discussed and extended for singe-layered and multi-layered structures. In particular, the excitation and reception of acoustic guided waves (AGW) are investigated theoretically and experimentally. The presence of inhomogeneities, such as flaws in the foil, adhesion flaws at the coating-foil interface, and gaps in the coating material, changes boundary conditions and influences guided wave propagation parameters. The correlation between flaw types and changes in the received signal amplitude, frequency, energy, and propagation velocity is investigated experimentally and later utilized for flaw identification and classification. The excitation and reception of both guided wave types are performed with optimized ultrasonic wedge transducers, which are positioned on both uncoated side areas of the foil. A mechanical construction was developed and manufactured to ensure valid acoustic contact at low contact pressure conditions, which in turn guarantees an undisturbed sliding of the foil under the sensors. The utilization of little or no couplant media also prevents any coating corruption.Advantages of multi-frequency wave excitation through the utilization of thickness and planar oscillation in piezoelectric ceramic PZT wedge transducers are discussed as well as theoretically and experimentally investigated. Flaw identification in blank foils is performed with the high frequency component of the PZT transducer oscillations. In this way, the high spatial sensitivity of short wavelengths can be exploited. The results prove that even 2 mm long scratches and cracks can be detected with this approach by analyzing the signal’s peak amplitude. In general, the sensing signals are highly damped and distorted by the coating material. Therefore, the low frequency component of the PZT transducer oscillations must be used to excite and to receive appropriate sensing amplitudes. The experimental findings show that signal changes depend on the coating material absorption rate. In coatings with lower attenuation, the received signal decreases, and in those with higher attenuation, the received signal increases in the presence of flaws. The slope and end-value of square signal integrals are identified as the preferred measurands for adhesion and gap flaw detection and classification in electrodes. Overall, the proof of concept for the proposed guided-wave-based monitoring system was successfully provided, and this supports the suitableness of the developed methodology as well as the mechanical setup.