Operando NMR Study of the Kinetics of Lithium Plating on Graphite-Based Negative Electrode

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Improving the durability and safety of lithium-ion batteries (LIBs) is essential for large-scale green mobility. However, under specific conditions during charging, metallic lithium can deposit on the negative graphite electrode, which reduces cycle life and safety1. After deposition, reactive plated lithium can take various paths: reinsertion into graphite, disconnection leading to dead metallic lithium or chemical oxidation with electrolyte components. To understand these reactions and their kinetics, operando nuclear magnetic resonance (NMR) spectroscopy is a very interesting tool as it provides real-time access to the chemical shift of lithium of the electrolyte/SEI, lithium (de)intercalated in graphite, and lithium plating2. Nevertheless, characterization on cells with electrochemical performances close to commercial ones remains challenging. In this work, we develop a dedicated pouch-cell design and the associated instrumentation to follow the dynamic of lithium using 7Li nuclear magnetic resonance (NMR) spectroscopy. These developments provide 7Li NMR spectra every 3 min recorded in a static mode, offering a good temporal resolution over the charging process. The complex dynamics of lithium is investigated during various charging scenarios within LiNi0.8Mn0.1Co0.1O2 (NMC 811)/graphite pouch cells with industry-standard electrodes (areal loading around 2.5 mAh/cm²). Despite the static NMR conditions, distinct signals from electrolyte, LiC x phases and metallic lithium deposition are visualized and quantified, offering insights on the competition between lithiation, plating and re-intercalation processes for various temperature and charging rates. Interestingly, the lithium deposition occurs mainly during the constant-current (CC) regime, while lithium re-intercalation, oxidation or disconnection are observed during the subsequent constant-voltage (CV) phase (See Figure 1), highlighting the interest of reducing the current during fast-charge scenario. Quantification of the deposited lithium metal shows that at temperature of 0°C and charge rate equal to 2C more than 60% of the exchanged capacity during the CC phase can be attributed to plated lithium. Furthermore, these experimental results were used to validate a multi-physics porous electrode model that includes an irreversible lithium deposition term in the lithium plating/stripping model3. The temporal resolution of the experiment provides insights on the evolution of the plating/stripping kinetics with temperature. [1] T. Waldmann, B.-I. Hogg and M. Wohlfahrt-Mehrens, Li plating as unwanted side reaction in commercial Li-ion cells – A review, J. Power Sources, 384, 107-124 (2018) [2] K. Marker, C. Xu, and C.P. Grey, Operando NMR of NMC811/Graphite Lithium-Ion Batteries: Structure, Dynamics, and Lithium Metal Deposition, J. Am. Chem. Soc, 142, 17447−17456 (2020) [3] C. von Lüders, J. Keil, M. Webersberger and A. Jossen, Modeling of lithium plating and lithium stripping in lithium-ion batteries, J. Power Sources, 414, 41-47 (2019) Figure 1

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