Abstract text (incl. figure legends and references)

Zdravko Kochovski¹, Yaolin Xu^1,2, Yan Lu^1,3

¹ Department of Electrochemical Energy Storage (CE-AEES), Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

² Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, 02139 USA

³ Institute of Chemistry, Potsdam University, Karl-Liebknecht-Str. 24–25, 14476 Potsdam, Germany

INTRODUCTION

As current batteries are nearing their theoretical limits, future generation high-energy-density battery chemistries demand a fundamental understanding of their operation and failure pathways at the atomic level. However, most energy materials and most notably lithium (Li) are sensitive and unstable under the electron beam in conventional TEM. This problem has been addressed in recent years by the application of Cryo-EM methods, borrowed from structural biology [1].

OBJECTIVES

A major obstacle hindering the implementation of Li-metal batteries is the limited understanding of the Li nucleation and growth mechanisms. Furthermore, the solid-electrolyte interphase (SEI) is essential for the reversibility of the lithium Li-metal electrode, but the lack of in-depth understanding of its structure and unclear formation/evolution mechanisms have significantly hampered its rational design. We applied Cryo-EM and Cryo-ET to unravel the morphology and inner structure of Li deposits and to investigate the morphological evolution of Li deposition and the interface confined SEI.

MATERIALS & METHODS

We have used a JEM2100 Cryo-TEM equipped with a bottom mounted 4k TVIPS F416 camera and a Gatan 914 cryo-transfer holder. We have used a cryo-transfer procedure where the TEM grids containing the Li deposits are transferred to the TEM column without exposure to the ambient environment. Cryo-EM micrographs and tilt series have been acquired using SerialEM. Tilt series have been processed with IMOD and segmentation has been done in Amira.

RESULTS

In one work, Cryo-EM revealed two distinct types of Li deposits, depending on the current density: Li-balls, which were found to be primarily amorphous and Li-whiskers, which were found to be highly crystalline [2]. Additionally, their solid electrolyte interface (SEI) layers showed a difference in structure and composition, correlated to the underlying deposition mechanism (Fig. 1). In another work (accepted manuscript), we used Cryo-EM to reveal the morphological evolution of the SEI layer during Li plating, showing a thick (~100 nm) and wrinkled SEI layers forming in the initial stage, which progressively stretched and thinned to up to 7nm after 24 min of Li deposition (Fig 2a-c). Cryo-EM and Cryo-ET also revealed in detail the morphologies of the Li deposits (Fig2d-i).

CONCLUSION

Cryo-EM and Cryo-ET, which are methods well established in the field of structural biology, could be well adapted to the study of sensitive energy materials.

REFERENCES

[1] Y. Li et al., Chem, 4 (2018), pp. 2250-2252

[2] K. Dong et al., ACS Energy Lett. 6 (2021), pp. 1719-1728

FIGURE LEGENDS

Fig. 1 (left) Cryo-EM micrographs of Li-balls and Li-whiskers; (right) Schematic illustrations of the SEI layer structures of Li-balls and Li-whiskers. Adapted from [2].

Fig. 2 Morphological evolution of Li deposition and interface-confined SEI. ( a-c) Cryo-EM images of SEI layers on 150 s, 12 min, and 24 min Li deposits; (d-f) Cryo-EM micrographs; (g-i) Cryo-ET iso-surface segmentations.