Characterization of Electrolyte Reduction Reactions on Freshly Plated Lithium Using Electrochemical Heat Flow Calorimetry
Authors: Berg Clara Marie , Reuter Lennart , Link Lara Paulina , Gasteiger Hubert Andreas 
Published: December 2024

Electrochemical heat flow calorimetry can be used to measure the evolved heat during the operation of a lithium-ion battery cell. The data obtained are useful for the optimization of thermal management systems on the cell and system level, but they can also be used to detect and characterize parasitic reactions, such as unwanted reactions of the electrolyte. The heat evolved from a reversibly operated lithium-ion cell can be described as the sum of irreversible heat, resulting from cell polarization, and reversible heat, related to entropy changes.1 In order to quantitatively measure the contribution of a parasitic heat flow, which is usually small in comparison, the irreversible and reversible heat flow must be known to a high accuracy. Two main approaches have been reported in the literature to obtain such defined measurement conditions.2, 3 The first method requires the integration of the heat flow over a full charge/discharge cycle.2 This cancels the reversible heat flow contributions and enables an estimation of the total irreversible heat from the difference of the charge and discharge energies. The second method analyzes the heat flow during a voltage hold after the cell has reached a steady-state condition and exclusively exhibits parasitic processes.3 Both of these approaches allow for an estimation of the mean reaction enthalpies per mole of electrons when the parasitic heat is correlated to the irreversible capacity or to the parasitic current, respectively.
In the present work, we introduce an approach to characterize the heat of the electrolyte reduction reactions on freshly plated lithium. For this, we measure the heat flow during overlithiation of an artificial graphite electrode in a half-cell, using our previously reported setup.4 Figure 1 shows the voltage profile and the measured heat flow for an exemplary experiment, where the graphite electrode is first fully lithiated and then overlithiated to induce significant lithium plating before the electrode is fully delithiated again. Both the voltage and heat flow curves show characteristic features indicating lithium plating (LP) and lithium stripping (LS).5 During continuous lithium plating we expect a significant contribution of a parasitic heat flow from the electrolyte reduction reactions on the newly exposed lithium surface. The here analyzed conditions enable a straightforward separation of this parasitic heat flow from the reversible and irreversible heat flow contributions from lithium plating: since lithium is being plated on already deposited lithium on the graphite electrode surfaces and simultaneously being stripped from the lithium metal counter electrode, the system can be considered a quasi-symmetrical cell. This means that the reversible heat flow contributions cancel out and that the irreversible heat flow from cell polarization can be calculated from the offset of the measured cell voltage from 0 V. In the present study, we analyzed the parasitic heat flow during lithium plating, changing different measurement parameters (e.g., degree of overlithiation, plating current, electrolyte) to gain insights into the interplay of lithium plating and electrolyte reactions. By a correlation to the irreversible capacity losses, the mean reaction enthalpy per mole of electrons can be estimated. Finally, the results were correlated to electrolyte reduction reactions postulated by gassing analysis from online electrochemical mass spectrometry (OEMS) measurements. The gas analysis allows for comparing electrolyte reduction pathways at larger potentials (~0.8 V vs. Li+/Li) during the formation of a pristine graphite surface as compared to electrolyte decomposition onto plated lithium at potentials around 0 V vs. Li+/Li.

Acknowledgements

We gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) for its financial support within the AQua HysKaDi project (03XP0321B).

References

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5. L. E. Downie, L. J. Krause, J. C. Burns, L. D. Jensen, V. L. Chevrier and J. R. Dahn, J. Electrochem. Soc., 160(4), A588-A594 (2013).

Figure 1