Origin of Heterogeneous Stripping of Lithium in Liquid Electrolytes

Lithium metal batteriessuffer from low cycle life. During discharge,parts of the lithium are not stripped reversibly and remain isolatedfrom the current collector. This isolated lithium is trapped in theinsulating remaining solid-electrolyte interphase (SEI) shell andcontributes to the capacity loss. However, a fundamental understandingof why isolated lithium forms and how it can be mitigated is lacking.In this article, we perform a combined theoretical and experimentalstudy to understand isolated lithium formation during stripping. Wederive a thermodynamic consistent model of lithium dissolution andfind that the interaction between lithium and SEI leads to locallypreferred stripping and isolated lithium formation. Based on a cryogenictransmission electron microscopy (cryo TEM) setup, we reveal thatthese local effects are particularly pronounced at kinks of lithiumwhiskers. We find that lithium stripping can be heterogeneous bothon a nanoscale and on a larger scale. Cryo TEM observations confirmour theoretical prediction that isolated lithium occurs less at higherstripping current densities. The origin of isolated lithium lies inlocal effects, such as heterogeneous SEI, stress fields, or the geometricshape of the deposits. We conclude that in order to mitigate isolatedlithium, a uniform lithium morphology during plating and a homogeneousSEI are indispensable.

Automated summary

Lithium metal batteries suffer from low cycle life due to the formation of isolated lithium during stripping. The interaction between lithium and solid-electrolyte interphase (SEI) leads to preferred stripping and isolated lithium formation. Cryogenic transmission electron microscopy (cryo TEM) observations confirm that isolated lithium occurs less at higher stripping current densities. To mitigate isolated lithium, a uniform lithium morphology during plating and a homogeneous SEI are indispensable.