Diffusional lithium trapping as a failure mechanism of aluminum foil anodes in lithium-ion batteries
Aluminum foils are an appealing anode for lithium-ion batteries due to their high capacity and low-cost, but their viability has been limited due to poor cyclability arising from pulverization and solid-electrolyte interphase growth. We show in this thesis that significant capacity degradation of aluminum foil anodes during electrochemical cycling also occurs due to diffusional lithium trapping. Scanning electron microscopy of cross-sectioned, cycled foils in the delithiated state reveals large regions of β-LiAl that are passivated by a surface layer of ⍺-Al, which has poor Li⁺ diffusivity. It is found that lithium diffusion occurs preferentially along the β-LiAl grain boundaries, so the grain structure after initial lithiation significantly affects the trapping behavior. Diffusional lithium trapping is exacerbated by both higher delithiation rates and higher areal capacity, presenting a challenge towards commercialization of aluminum foil anodes. We further demonstrate that diffusional trapping in aluminum foil anodes can be mitigated through alloy design, with the addition of 2 - 3 wt.% Li yielding improved first cycle efficiency, and the addition of 1 wt.% Si yielding improved cycle life. These results provide a mechanistic understanding of diffusional lithium trapping in aluminum foil anodes and highlight compositional design of alloys as a promising strategy to overcome it.