We focus on in-operando structural evolution of model Li-ion battery anodes, which are comprised of multi-bilayer Ni/NiO films with active NiO layers sandwiched between buffer Ni layers. The films were deposited on sapphire via pulsed laser deposition (PLD). The morphological changes accompanying lithiation were tracked via in-situ and ex-situ X-ray reflectivity (XRR) and cross-sectional transmission electron microscopy in a series of films produced by systematically varying the thicknesses of active and buffer layers (20 Å to 200 Å) and the number of periodic bilayers (1 to 5). The key results of these combined studies are:

1. Complete lithiation of all the active NiO layers occurs only when the thickness of the buffer Ni layers is less than ~50 Å. For fully lithiated films, the active NiO layers doubled in thickness while being converted to Li₂O layers and the reduced Ni from the NiO migrated to the bordering Ni buffer layers. This uniaxial lithiation process of multi-bilayer anodes started at the top and progressed toward the bottom of the stack. The expansion was accompanied initially by lateral inhomogeneities, which diminished over time leading to laterally uniform lithiated Li₂O/Ni multilayers.
2. Ni layers with a thickness >50 Å present a kinetic barrier for the diffusion of lithium ions. This results in a partial lithiation of Ni/NiO multilayer anodes such that only the top NiO layer is lithiated.
3. XRR shows that for the 5-bilayer Ni/NiO films, the electron density of the fully lithiated layers is a constant (no density fluctuations). This attests that the reduced Ni is rather uniformly distributed inside the Li₂O matrix after the completion of the conversion reaction. This contrasts the situation for 2-bilayer Ni/NiO films. Here, low-density regions are observed inside the fully lithiated active layers near the Ni interlayers.
4. For the fully lithiated Ni/NiO 5-bilayer films, the thickness of the Ni layers increases by 6-7 Å (roughly two atomic diameters of Ni), suggesting segregation of reduced Ni at the Ni/lithiated NiO layers. Consistent with this, the thickness of the Ni layer increases by 3-4 Å when only the top NiO layer is lithiated (for Ni thickness >50 Å, see 2 above).

In my presentation, I will discuss the mechanisms of the observed structural transformations. These are the first systematic studies of vertically digitized multilayer anodes that precisely determine the relationships between the morphological changes and the film architecture during lithiation.

This research was supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.