Fig. 2: Lithiation–delithiation processes of Al4N-AR (purity 99.99%, as-rolled Al) foil and Al4N-HT (purity 99.99%, well-annealed Al) foil.

a Morphology changes (I, II, II, and IV) on the front side (facing the Li electrode) of Al4N-AR during lithiation and delithiation, and EBSD pattern of the initial state. Al4N-AR shows a homogenous lithiation and maintains a stubborn Al-matrix layer with a good structure stability. b Cross-section of a bare Al4N-AR foil. c Cross-section of a 53%-lithiated Al4N-AR foil. A very flat interface is observed between lithiated phase and Al matrix. The lithiated layer becomes twice thick, suggesting that the volume expansion occurs along the direction normal to the Al foil (i.e., one-dimensionally). d XRD profiles obtained in the reflection geometry for the Al4N-AR foil before and after lithiation. e Cross-section of an Al4N-AR foil that has undergone 47% lithiation and subsequent 26% delithiation. Al matrix is self-organized to form a columnar structure during delithiation. f Morphology changes (I, II, III, and IV) on the front side of Al4N-HT during lithiation and delithiation, and EBSD pattern of the initial state. The AlLi grains are inhomogeneously formed, resulting in the pulverization during delithiation. g Cross-section of a 26% lithiated Al4N-HT foil, where the AlLi phase penetrates the foil accompanied with plastic deformation. h Backside view of an Al4N-HT foil that has undergone a 53% lithiation and delithiation cycle. The foil is pulverized during delithiation. Scale bars in SEM images are 20 μm for a–f and 100 μm for g, h. EBSD patterns of Al matrices inset in a and f indicate a marked grain growth in the recrystallization of Al4N-HT. Scale bars in EBSD patterns are 30 μm and 1 mm for Al4N-AR and Al4N-HT, respectively. The percentage figures of lithiation are displayed based on the total capacity of the Al foil.