Phosphorus represents a promising
anode material for sodium ion
batteries owing to its extremely high theoretical capacity. Recent
in situ transmission electron microscopy studies evidenced anisotropic
swelling in sodiated black phosphorus, which may find an origin from
the two intrinsic anisotropic properties inherent to the layered structure
of black phosphorus: sodium diffusional directionality and insertion
strain anisotropy. To understand the morphological evolution and stress
generation in sodiated black phosphorus, we develop a chemo-mechanical
model by incorporating the intrinsic anisotropic properties into the
large elasto-plastic deformation. Our modeling results reveal that
the apparent morphological evolution in sodiated black phosphorus
is critically controlled by the coupled effect of the two intrinsic
anisotropic properties. In particular, sodium diffusional directionality
generates sharp interphases along the [010] and [001] directions,
which constrain anisotropic development of the insertion strain. The
coupled effect renders distinctive stress-generation and fracture
mechanisms when sodiation starts from different crystal facets. In
addition to providing a powerful modeling framework for sodiation
and lithiation of layered structures, our findings shed significant
light on the sodiation-induced chemo-mechanical degradation of black
phosphorus as a promising anode for the next-generation sodium ion
batteries
