Lithiation of multilayer Ni/NiO electrodes: criticality of nickel layer thicknesses on conversion reaction kinetics

X-ray reflectivity and transmission electron microscopy (TEM) were used to characterize the morphological changes in thin film electrodes with alternating Ni and NiO layers during lithiation as a function of the Ni buffer layer thickness. Complete lithiation of the active NiO layers occurs only when the thickness of the Ni/NiO bilayers are less than 75 Å - a threshold value that is determined by the sum of the Ni quantity in the Ni/NiO bilayer of the multilayer stack. Thicker Ni/NiO bilayers present a kinetic barrier for lithium ion diffusion inside the stack resulting in partial lithiation of the multilayer electrodes in which only the top NiO layer lithiates. Lithiation of NiO layers in a multilayer stack also leads to an interface-specific reaction that is observed to increase the thicknesses of adjacent Ni layers by 3-4 Å and is associated with the formation of a low-density Li₂O layer, corresponding to an interfacially-driven phase separation of the NiO. Rate dependent cyclic voltammetry studies reveal a linear relation between the peak current and scan rate suggesting that the lithiation kinetics are controlled by charge transfer resistance at the liquid-solid interface.