Morphological Evolution of Multilayer Ni/NiO Thin Film Electrodes during Lithiation

Oxide conversion reactions in lithium ion batteries are challenged by substantial irreversibility associated with significant volume change during the phase separation of an oxide into lithia and metal species (e.g., NiO + 2Li⁺ + 2e^– → Ni + Li₂O). We demonstrate that the confinement of nanometer-scale NiO layers within a Ni/NiO multilayer electrode can direct lithium transport and reactivity, leading to coherent expansion of the multilayer. The morphological changes accompanying lithiation were tracked in real-time by in-operando X-ray reflectivity (XRR) and ex-situ cross-sectional transmission electron microscopy on well-defined periodic Ni/NiO multilayers grown by pulsed-laser deposition. Comparison of pristine and lithiated structures reveals that the nm-thick nickel layers help initiate the conversion process at the interface and then provide an architecture that confines the lithiation to the individual oxide layers. XRR data reveal that the lithiation process starts at the top and progressed through the electrode stack, layer by layer resulting in a purely vertical expansion. Longer term cycling showed significant reversible capacity (∼800 mA h g^–1 after ∼100 cycles), which we attribute to a combination of the intrinsic bulk lithiation capacity of the NiO and additional interfacial lithiation capacity. These observations provide new insight into the role of metal/metal oxide interfaces in controlling lithium ion conversion reactions by defining the relationships between morphological changes and film architecture during reaction.