Grain size and hardness: Key parameters for enhancing the chemo-mechanical stability of lithiated aluminum foil anodes

Abstract

Aluminum (Al) has gained attention as a potential anode material for lithium-ion batteries (LIBs) due to several advantageous properties such as a relatively low volume expansion of approximately 97% during lithiation, a large charge capacity of 993 mAh/g, a low potential of 0.36 V vs. Li/Li+, high electronic conductivity, and its abundance in the Earth's crust. Furthermore, Al is readily available as foil, eliminating the need for binders, conductive additives, and expensive copper (Cu) current collectors. However, one of the challenges associated with Al as a LIB anode is the mechanical cracking of the lithiated β-LiAl phase, leading to rapid capacity fading. This paper introduces an effective strategy to address this issue by suppressing plastic deformation and pulverization of the Al anode during the lithiation process through grain refinement and strengthening of aluminum foil. The study demonstrates that cold-rolled low-purity (99.5%) purity Al foil anode exhibits higher resistance against plastic deformation during lithiation compared to cold-rolled high-purity (99.99%) Al foil. The superior chemo-mechanical stability of the cold-rolled 99.5% Al foil can be attributed to its higher hardness (90 Hv) and smaller grain size (1.7 µm) in comparison to the cold-rolled 99.99% Al foil (30 Hv and 73 µm). These microstructural and mechanical features play a key role in maintaining high chemo-mechanical stability in facing significant volumetric expansion during the phase transformation from Al to β-LiAl, while also facilitating the formation of a uniform lamellar β-LiAl layer on the Al substrate. This paper also presents a model that establishes a relationship between chronoamperometric current and plastic strain, enabling real-time estimation of invisible strain by monitoring the current during the lithiation process. This approach provides a means to assess the structural integrity of the anode during battery operation.