Potential Understanding of Li Deposition/Lithium Metal Interfaces Via NMR/MRI
Authors: Hsieh Yi-Chen , Küpers Verena , Winter Martin , Brunklaus Gunther 
Published: February 2020

Owing to its high theoretical capacity and low reduction potential, lithium metal represents an ideal candidate anode material for application in lithium ion batteries and anode-free batteries. Its widespread usage, however, is hampered by the tendency to form inhomogeneous lithium deposits upon charging/discharging in lithium ion batteries, which may pose safety risks. The conditions and strategies for protecting lithium metal anodes or preventing/controlling the morphology of the lithium deposits for the lifetime of a battery are still not fully elucidated, despite some recent progress [1,2]. By using nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI), we can obtain quantitative and molecular understanding of how lithium microstructure grows in a working battery.

In this work, Li|Li and Li|Cu cells were assembled and studied by in-situ 7Li-NMR, in order to derive insights into the formation of microstructures/morphologies of Li deposits in a working battery. Notably, the either linear or exponential growth of Li deposits in case of Li|Li and Li|Cu cells, respectively, indicated that the actual cell setup significantly impacts the Li plating. An exponential increase in Li deposits upon cycling of Li|Cu cells reflects an apparent 1st order reaction, following , where a corresponding k value of 6.3*10-5 s-1 was derived from fitting, in good agreement with previous work on Li dissolution kinetics in case of pre-lithiation[3]. The similarity of the obtained k values strongly suggests that the kinetic reactions of lithium dissolution and deposition in a working battery are predictable. Furthermore, through better understanding of the kinetic growth of Li deposits in Li|Cu cells, the properties and lifetime of anode-free batteries may be also improved.

Besides, not only the impact of cell system designs will be discussed, but also the influence of the applied current densities, which may affect the results. At lower current densities, the 7Li NMR peak reflecting lithium deposition grows relatively homogenous compared to the corresponding signal at higher current densities. The obtained kinetic data may explain the formation of inhomogeneous lithium deposits at higher current densities while in addition, the electrolyte formulation is a crucial factor in the cell system. Different electrolyte additives may tailor the actual morphology of the lithium deposits and their microstructures, as monitored via different 7Li chemical shifts in in-situ NMR/MRI measurements. In summary, we demonstrate that in-situ NMR/MRI constitutes a promising technique to provide insight into the molecular mechanisms of reactions occurring at electrolyte-electrode interfaces in lithium ion batteries.

[1] Chang, Hee Jung, et al. "Investigating Li microstructure formation on Li anodes for lithium batteries by in situ 6Li/7Li NMR and SEM." The Journal of Physical Chemistry C 119.29 (2015): 16443-16451.

[2] Chang, Hee Jung, et al. "Correlating microstructural lithium metal growth with electrolyte salt depletion in lithium batteries using 7Li MRI." Journal of the American Chemical Society 137.48 (2015): 15209-15216.

[3] Holtstiege, Florian, et al. "New insights into pre-lithiation kinetics of graphite anodes via nuclear magnetic resonance spectroscopy." Journal of Power Sources 378 (2018): 522-526.