What separates the inner core from the outer core?
The inner core (from 5100 km depth to center) is solid with density up to 13 gm/cc, nearly The outer core (2800 - 5100 km) has extremely low-viscous liquid that is distinct, in form, from liquid..
Enveloped by mantle-core boundary, the outer core might not be spherical. Propagation of seismic waves, partly with reflection, marks the separation between mantle and outer core. Only primary waves enter. Very strong primary waves enter and exit the inner core. This research has to continue for ever, for better-than-before precision.