Lithium metal batteries (LMBs) are attractive for applications requiring high specific energy. There are still some hurdles to overcome on the way to the commercial application of LMBs. The main problems lie in the cycle stability of LMBs and safety concerns. This is due to heterogeneous lithium deposition, which may damage the separator and cause internal shorts. A stable solid-electrolyte interphase (SEI) is needed to enable homogeneous deposition. The SEI, which forms from electrolyte decomposition, plays a key role during cycling as it determines the morphology during deposition and the reversibility during stripping[1]. Thus, the main challenge is to find a suitable electrolyte formulation that enables a good SEI and, consequently, homogeneous plating and stripping. We focus on the underlying failure mechanisms of LMBs in conventional carbonate-based electrolytes and investigate the role of the SEI in order to rationalize better SEI properties.
In carbonate-based electrolytes, Li whiskers can form during plating, which entangle and grow into mossy lithium. This form of heterogeneous deposition is detrimental as it leads to low cycling reversibility[2]. We computationally investigate both lithium whisker formation and dissolution with the special focus on the role of the SEI.
Lithium experiences large volume expansion during plating. This can cause cracking of the covering SEI. This allows the very soft lithium to relief stress by creep flow to the cracks. The underlying stress relaxation mechanism is atom transport to weak spots in the SEI, where lithium extrudes in the form of one-dimensional whiskers[3]. We simulate the stress-induced lithium extrusion through cracks in the SEI on the basis of power-law creep.
The inherent heterogeneities lead to preferential stripping spots during lithium whisker stripping. Instabilities occur, which disconnect isolated lithium from the current collector[2]. We find that isolated lithium formation can be mitigated at higher stripping current densities. For practical LMBs, the formation of isolated lithium has to be avoided, which highlights the importance of mitigating lithium whisker formation and a stable SEI. A stronger SEI is formed in highly concentrated electrolytes, where the SEI is inorganic-rich, and less whisker formation is observed.

[1] B. Horstmann et al., Energy Environ. Sci. 2021, 14(10), 5289-5314.
[2] M. Werres et al., ACS Nano 2023, 17(11), 10218-10228.
[3] M. Werres et al., in preparation