Beyond Volume Variation: Anisotropic and Protrusive Lithiation in Bismuth Nanowire

Materials storing energy via an alloying reaction are promising anode candidates in rechargeable lithium-ion batteries (LIBs) due to their much higher energy density than the current graphite anode. Until now, the volumetric expansion of such electrode particles during lithiation has been considered as solely responsible for cycling-induced structural failure. In this work, we report different structural failure mechanisms using single-crystalline bismuth nanowires as the alloying-based anode. The Li-Bi alloying process exhibits a two-step transition, that is, Bi-Li₁Bi and Li₁Bi-Li₃Bi. Interestingly, the Bi-Li₁Bi phase transition occurs not only in the bulk Bi nanowire but also on the particle surface showing its characteristic behavior. The bulk alloying kinetics favors a Bi-(012)-facilitated anisotropic lithiation, whose mechanism and energetics are further studied using the density functional theory calculations. More importantly, the protrusion of Li₁Bi nanograins as a result of anisotropic Li-Bi alloying is found to dominate the surface morphology of Bi particles. The growth kinetics of Li₁Bi protrusions is understood atomically with the identification of two different controlling mechanisms, that is, the dislocation-assisted strain relaxation at the Bi/Li₁Bi interface and the short-range migration of Bi supporting the off-Bi growth of Li₁Bi. As loosely rooted to the bulk substrate and easily peeled off and detached into the electrolyte, these nanoscale protrusions developed during battery cycling are believed to be an important factor responsible for the capacity decay of such alloying-based anodes at the electrode level.