Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered →
rock-salt phase transformation in the layered
dioxide cathodes for Li-ion batteries is believed to result in a “core–shell”
structure of the primary particles, in which the core region remains
as the layered phase while the surface region undergoes a phase transformation
to the rock-salt phase. Using transmission electron microscopy, here
we demonstrate the formation of an “anti-core–shell”
structure in cycled primary particles with a formula of LiNi0.80Co0.15Al0.05O2, in which the surface
and subsurface regions remain as the layered structure while the rock-salt
phase forms as domains in the bulk with a thin layer of the spinel
phase between the rock-salt core and the skin of the layered phase.
Formation of this anti-core–shell structure is attributed to
oxygen loss at the surface that drives the migration of oxygen from
the bulk to the surface, thereby resulting in localized areas of significantly
reduced oxygen levels in the bulk of the particle, which subsequently
undergoes phase transformation to the rock-salt domains. The formation
of the anti-core–shell rock-salt domains is responsible for
the reduced capacity, discharge voltage, and ionic conductivity in
cycled cathodes.