Lithium metal batteries
suffer from low cycle life. During discharge,
parts of the lithium are not stripped reversibly and remain isolated
from the current collector. This isolated lithium is trapped in the
insulating remaining solid-electrolyte interphase (SEI) shell and
contributes to the capacity loss. However, a fundamental understanding
of why isolated lithium forms and how it can be mitigated is lacking.
In this article, we perform a combined theoretical and experimental
study to understand isolated lithium formation during stripping. We
derive a thermodynamic consistent model of lithium dissolution and
find that the interaction between lithium and SEI leads to locally
preferred stripping and isolated lithium formation. Based on a cryogenic
transmission electron microscopy (cryo TEM) setup, we reveal that
these local effects are particularly pronounced at kinks of lithium
whiskers. We find that lithium stripping can be heterogeneous both
on a nanoscale and on a larger scale. Cryo TEM observations confirm
our theoretical prediction that isolated lithium occurs less at higher
stripping current densities. The origin of isolated lithium lies in
local effects, such as heterogeneous SEI, stress fields, or the geometric
shape of the deposits. We conclude that in order to mitigate isolated
lithium, a uniform lithium morphology during plating and a homogeneous
SEI are indispensable.