Measuring Local Atomic Structure Variations through the Depth of Ultrathin (<20 nm) ALD Aluminum Oxide: Implications for Lithium-Ion Batteries

Understanding the atomic structure of ultrathin (<20
nm) atomic
layer deposition (ALD) coatings is critical to establish structure–property
relationships and accelerate the application of ALD films to stabilize
battery interfaces. Previous studies have measured the atomic structure
of nanoscale ALD films using cryogenic electron diffraction with a
large (∼200 nm) beam diameter. However, for ultrathin ALD coatings,
these measurements provide only ensemble average structural information
and cannot be used to directly measure differences in atomic structure
through the depth of the ALD film. In this study, we localize the
electron beam to a small (∼5 nm) spot size using cryogenic
scanning transmission electron microscope (STEM), and we collect electron
diffraction data at multiple points along the depth of a 12 nm thick
ALD AlOx film deposited onto a carbon
nanotube (CNT) substrate without a contribution from the substrate.
We couple these diffraction measurements with pair distribution function
(PDF) analysis and iterative reverse Monte Carlo-molecular statics
(RMC-MS) modeling to compare atomic structure metrics at different
positions in the film depth. We interpret the modeling results considering
the three-dimensional (3D) concentric cylindrical sample geometry
of a CNT with uniform AlOx coating. These
measurements confirm a two-phase bulk/interface structural model proposed
previously for ALD AlOx and indicate that
the interfacial layer at the CNT–AlOx interface is 2.5 nm thick5 times larger than previously
reported. This report demonstrates direct measurement of atomic structural
variations across nanoscale material interfaces that is of broad interest
for electrochemical applications and will help inform the use of ALD
coatings to stabilize lithium-ion battery interfaces.