Anisotropic Swelling Governed by Orientation-Dependent Interfacial Na Diffusion in Single-Crystalline Sb

The
anisotropic volume expansion of anode
materials produces locally inhomogeneous residual stresses, which
frequently induce fracture of the anode materials and reduce battery
capacity and cycle life. Much of our understanding of the anisotropic
swelling behavior of anode materials is based on electron microscopy
and macroscopic structural analysis techniques, which are insufficient
to elucidate the atomistic origin of the anisotropic swelling behavior.
In this study, we perform in situ sodiation experiments with single-crystalline
Sb anodes followed by atomic simulations to determine the diffusion
kinetics governing the sodiation of Sb and its associated swelling
behavior. In situ sodiation experiments demonstrate that the rate
of diffusion of Na into single-crystalline Sb anodes differs by more
than a factor of 2 depending on the orientation of the Sb crystal,
causing the crystal to swell anisotropically. This observed anisotropic
diffusion is explained here by determining the orientation-dependent
diffusion kinetics, while the associated structural origins are clarified
by studying the interfacial Na diffusion in the atomically thin layer
preceding the advancing interface.