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Curiously Absent Knock-On Damage of Lithium Metal at Cryogenic Temperatures

Damage from the electron beam in a transmission electron microscope can be grouped into two categories: knock-on damage and sputtering (electron-nuclear scattering) and radiolysis (electron-electron scattering). The nuclear scattering is nearly elastic at small angles (the electron being so much lighter than the nucleus) and radiolysis is inelastic. There exists an easily calculable knock-on damage threshold below which crystals such as graphite and silicon are effectively stable under the electron beam, regardless of temperature. (The beam current determines the rate at which atoms are sputtered off the surface, a small effect when the number of volume atoms to surface atoms is large.) Carbon allotropes vary considerably in this threshold, but in the case of graphite we see stability below a high tension of roughly 120 kV [1, 2], and silicon below 200 kV [2]. With a 300 kV accelerating voltage, graphite suffers from knock-on damage; before and after such damage is shown in Figure 1. Cryogenic temperatures offer no protection here (the energy transfer to the nucleus is much larger than any thermal energy) and the walls are visibly deformed after a few minutes of exposure to a differential fluence of 200 e/Å 2 /µs. A summary of calculated knock-on damage thresholds is shown in the Figure 1 inset. The knock-on damage mechanism seems curiously absent in lithium metal, which has a damage threshold of approximately 30 kV (assuming a conservative activation/displacement energy of 9 eV [3]). Lithium metal dendrites electrodeposited in a battery coin cell [4] can be readily imaged after cryo-transfer techniques that prevent oxidation of the air-sensitive