Authors: Oana Cojocaru-Mirédin, Yucheng Zhou, André Weber, Alexandre Mussi, Dagmar Gerthsen, Bai-Xiang Xu

It has been argued that the Li7La3Zr2O12/Li hetero-interface in all-solid-state batteries is prone to decomposition and degradation during synthesis and cycling, leading to the formation of Li dendrites and their propagation inside the Li7La3Zr2O12 bulk. However, the exact formation mechanism of these dendrites, as well as their chemical composition, remains not fully understood until now due to the difficulty of quantifying the Li concentration within the battery materials.

Therefore, in this work, we employed atom probe tomography in conjunction with advanced transmission electron microscopy to investigate the Li7La3Zr2O12 bulk in the vicinity of the Li7La3Zr2O12/Li heterointerface, a region prone to crack formation and propagation. We discovered that numerous Li-nanodendrites are present inside the LLZO bulk close to the Li/LLZO interface and that these nanodendrites appear similar to cracks being filled by Li. Therefore, this study raises a possible dilemma of causality between the crack formation and Li segregation in LLZO grains.

Interestingly, advanced microscopy investigations prove the existence of a high density of dislocations within LLZO for some grains. Moreover, the finite element modeling suggests that the dislocations’ cores can act as nucleation sites for strong Li segregation, leading to an increase in hydrostatic stress. This implies that this strong Li segregation at the dislocation cores and the resultant high hydrostatic stress might be the cause for the crack formation. Subsequently, Li can be further accumulated at the cracks, forming the Li-nanodendrites.

It is without doubt that the presence of such microscopic Li-rich nanodendrites in the as-deposited state will lead to the growth and propagation of the well-known macroscopic Li dendrites during cycling.