Correlating
Microstructural Lithium Metal Growth with
Electrolyte Salt Depletion in Lithium Batteries Using <sup>7</sup>Li MRI
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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 <sup>7</sup>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