Origin of heterogeneous stripping of lithium in liquid electrolytes

TL;DR

This study combines theory and experiments to understand why isolated lithium forms during stripping in lithium metal batteries, revealing local effects like heterogeneous SEI and deposit shape as key factors, and suggests uniform lithium morphology and SEI as mitigation strategies.

Contribution

The paper provides a fundamental understanding of isolated lithium formation by integrating a thermodynamic model with cryo TEM observations, highlighting local effects as critical factors.

Findings

Isolated lithium formation is influenced by local effects such as heterogeneous SEI and deposit shape.

Higher stripping current densities reduce isolated lithium occurrence.

Heterogeneous SEI and non-uniform lithium morphology are key to isolated lithium formation.

Abstract

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 (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 is indispensable.