You Do Need power plug adapter while traveling from Sierra Leone to Saint Lucia.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in Sierra Leone are D and G. This is how they look like.

Sierra Leone

In Sierra Leone, outlets of type D and G are in use.

Type D

Type G

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G