Lithium plating in commercial lithium-ion cells: An extensive study combining electrical methods with post-mortem analysis

The identification of lithium plating represents a fundamental aspect in the development of fast charging algorithms. This extensive study correlates established electrical detection methods with results of a post-mortem investigation. Commercial cells were therefore charged under a variety of operating conditions, including variations in temperature, current, cooling, clamping pressure and state-of-charge. Indicators of Li plating were analyzed using electrical stripping and voltage relaxation measurements. The post-mortem analysis included the digital image processing of over 5000 flatbed scans of anode electrode surfaces. To analyze the Li plating morphology and validate that the observed macroscopic structures are lithium plating, laser-scanning and scanning electron microscopy measurements combined with energy dispersive X-ray (EDX) analysis were performed. The use of a windowless EDX detector enabled the measurement of Li K$\alpha$ X-rays at 54 eV, thereby confirming the presence of lithium metal in the grown structures. The structural analysis of the surface deposits revealed, for the first time in a commercial cell, a novel polyhedral Li plating morphology. These results contribute to the field of Li plating detection, as it is essential to understand the plating process and detect metallic lithium at an early stage in order to optimize battery lifetime and increase safety for the target application.