Impact of the Gibbs–Thomson Effect on Lithium Solubility in Silver Used as an Alloy Lithium-Ion Battery Anode

Because the multiple phase transformations occurring in high-capacity Li-ion battery alloy anodes through solid-state diffusion are commonly hindered by the relatively sluggish Li-ion diffusion kinetics, nanostructured alloy anodes, such as nanoporous metals, have been widely used to reduce the diffusion distance and subsequently enhance the cycling performance. However, although the size-dependent Li solubility limits that dictate the (de)lithiation-induced solid–solid phase transformations have been studied for various nanostructured intercalation cathode materials, investigations of this structure–property relationship for nanostructured alloy anodes are limited. In this work, bulk and nanoporous Ag anodes are investigated as model alloy anodes to determine the impact of the structure size on the (de)lithiation phase transformation pathways that occur during cycling. First, we simulated the open-circuit voltage (OCV) profile of a Ag anode using a CALPHAD-based approach, allowing the profile features to be linked to the underlying solid solution and reconstitution phase transformations expected to occur during cycling by the Ag–Li equilibrium binary phase diagram. By comparing experimental voltage profiles with this calculated OCV profile and the X-ray diffraction patterns of the lithiated anodes, we determine that Li-rich phases are formed near the surface of the Ag active material, and while the bulk alloy anode follows the phase transformation pathway expected by the simulated OCV profile, the nanoporous Ag anode pathway includes the formation of an unpredicted, metastable β solid solution phase. From this information, we develop models for the phase morphology of the lithiated bulk and nanoporous anodes. This work demonstrates that due to the Gibbs–Thomson effect, nanostructured alloy anodes can proceed through a different phase transformation pathway compared to their bulk counterparts, and, as a result, these materials may form nonequilibrium phases and complex phase morphologies that can contribute toward increased interfacial energy and stress that can result in early failure.