Correlating Microstructural Lithium Metal Growth with Electrolyte Salt Depletion in Lithium Batteries Using ⁷Li MRI

Abstract

Lithium dendrite growth in lithium ion and lithium rechargeable batteries is associated with severe safety concerns. To overcome these problems, a fundamental understanding of the growth mechanism of dendrites under working conditions is needed. In this work, in situ (7)Li magnetic resonance (MRI) is performed on both the electrolyte and lithium metal electrodes in symmetric lithium cells, allowing the behavior of the electrolyte concentration gradient to be studied and correlated with the type and rate of microstructure growth on the Li metal electrode. For this purpose, chemical shift (CS) imaging of the metal electrodes is a particularly sensitive diagnostic method, enabling a clear distinction to be made between different types of microstructural growth occurring at the electrode surface and the eventual dendrite growth between the electrodes. The CS imaging shows that mossy types of microstructure grow close to the surface of the anode from the beginning of charge in every cell studied, while dendritic growth is triggered much later. Simple metrics have been developed to interpret the MRI data sets and to compare results from a series of cells charged at different current densities. The results show that at high charge rates, there is a strong correlation between the onset time of dendrite growth and the local depletion of the electrolyte at the surface of the electrode observed both experimentally and predicted theoretical (via the Sand's time model). A separate mechanism of dendrite growth is observed at low currents, which is not governed by salt depletion in the bulk liquid electrolyte. The MRI approach presented here allows the rate and nature of a process that occurs in the solid electrode to be correlated with the concentrations of components in the electrolyte.

Visualizing electromagnetic fields in metals by MRI

“…[12][13][14]17,[22][23][24] Interestingly, the 3d CSI sheds new light on previous bulk metal NMR studies. For instance, in an earlier study, 12 a similar shift difference between NMR peaks was observed at and ⊥ orientations (relative to B 0 ) of the major faces of a thinner metal strip, by carrying out two separate experiments.…”

“…Chief among them are the rapidly decaying B 1 inside the metal (Fig.1), and orientation of the bulk metal surface relative to incident B 1 . [9][10][11] Recent studies [12][13][14][15][16][17][18] have overcome these difficulties to demonstrate and highlight bulk metal NMR and MRI, albeit, primarily applied to batteries and electrochemical cells (section S2).…”

Real-time 3D imaging of microstructure growth in battery cells using indirect MRI

“…NMR measurements are sensitive to dendrite growth and are also able to resolve different lithium microstructure morphologies through chemical shift measurements (6)(7)(8)(9). MRI approaches provide additional spatial information, allowing specific structural changes to be localized, thereby enhancing the utility of the methods and simplifying interpretation (7,10,11).…”

“…NMR measurements are sensitive to dendrite growth and are also able to resolve different lithium microstructure morphologies through chemical shift measurements (6)(7)(8)(9). MRI approaches provide additional spatial information, allowing specific structural changes to be localized, thereby enhancing the utility of the methods and simplifying interpretation (7,10,11). Previous work relied on the measurement of 7 Li and 6 Li signals, where the identification of dendrites could be achieved via changes in the intensities and frequencies of the Li-metal signals due to skin-depth effects and susceptibility shifts, respectively (6-10, 12).…”

“…MRI approaches provide additional spatial information, allowing specific structural changes to be localized, thereby enhancing the utility of the methods and simplifying interpretation (7,10,11). Previous work relied on the measurement of 7 Li and 6 Li signals, where the identification of dendrites could be achieved via changes in the intensities and frequencies of the Li-metal signals due to skin-depth effects and susceptibility shifts, respectively (6-10, 12). The main limitations of resolution and imaging time arise from the inherent insensitivity of the lithium isotopes and relaxation phenomena.…”