Inhomogeneous Charge Distribution in Anodes due to Re-Intercalation of Li Metal into Graphite


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An inhomogeneous charge distribution can lead to an accelerated aging of the Lithium-ion Batteries [1]. One reason for the inhomogeneous charge distribution can be local Lithium deposition at the anode, which is often introduced by charging at high current rates or at low temperatures [1, 2, 3]. During rest time, deposited Lithium metal can spontaneously re-intercalate into the anode [2, 3] and lead to a locally higher charged state. However, there is a lack of knowledge on the involved mechanisms, although they are highly relevant.
In an effort to understand the re-distribution and relaxation processes of charge within graphite electrodes, we conducted model experiments with Lithium pressed on graphite surfaces. The mixed open circuit potential (OCP) of the graphite/Lithium electrode was measured in pouch cell half-cells during the re-intercalation. The charge distribution within the graphite electrode was investigated with Raman spectroscopy, glow-discharge optical-emission-spectroscopy (GD-OES), and optical microscopy (OM).
The electrolyte was systematically varied in experiments, which revealed new insights on the pathways of Lithium-ions (see Figure). For example, a solid-state reaction between Lithium metal and graphite is possible even without electrolyte, however, the reaction over ionic pathways is much faster if electrolyte and conductive salt are present. Conclusions are drawn on Post-Mortem methodology, inhomogeneity of aging mechanisms, and pre-lithiation of graphite electrodes.

[1] Forouzan et al. Journal of The Electrochemical Society 165 (2018) A2127.
[2] C. Uhlmann, J. Illig, M. Ender, R. Schuster, E. Ivers-Tiffée, J. Power Sources 279 (2015) 428.
[3] Lüders et al., Journal of Power Sources 342 (2017) 17.

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