You Do Need power plug adapter while traveling from Sri Lanka to Benin.

Here is Why You Need An Adapter

The outlet and plug type used in Benin are C and E and in Sri Lanka is G. This is how they look like.

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G

Benin

In Benin, outlets of type C and E are in use.

Type C

Type E