Fig. 3: Lithiation mechanism on an Al anode.

a The Gibbs free-energy curves in Al-Li binary system. The two-phase reaction of fcc-Al/AlLi_Li-poor (whose Li composition is C₂) occurs during lithiation. The chemical driving force of fcc-Al/AlLi reaction (~emf⁰) is about 0.38 eV (yellow line), but the value is enhanced when certain amount of strain energy is accumulated by mechanical deformation (pink line). Basically, the consideration of local equilibrium of fcc-Al/AlLi_Li-poor is sufficient, but around the surface that of AlLi_Li-rich/Al₂Li₃ is also needed during lithiation. b Schematic illustration of the lithiation processes of mechanically hard and soft Al foils; (left) Al4N-AR and (right) Al4N-HT. Stages (I, II, III, and IV) corresponds to the SEM images in Fig. 2a; see also Supplementary Fig. 3. The appropriate hardness of Al4N-AR enables the homogeneous lithiation. As the diffusivity in the AlLi phase possessing an ordered lattice is generally slower than that in the fcc Al phase, the formation of AlLi in the remaining Al matrix is kinetically preferential, whereas the already deformed Al matrix tends to be preferentially attacked by Li thermodynamically. c Schematic profiles of gradients of electric potential, composition, and chemical potentials of Li and Al to illustrate the unidirectional volume-expansion mechanism during lithiation on Al4N-AR. Insets show the SEM image and SIMS mapping of Li at the ion-beam-fabricated cross-section of a partially lithiated Al4N-AR; full field images are given in Supplementary Fig. 4. The chemical-potential gradients of Li and Al due to those of respective compositions drives the interdiffusion of Al and Li, whose maximum driving force is estimated to be about 0.25 eV.