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

Here is Why You Need An Adapter

The outlet and plug type used in Sierra Leone are D and G and in Guinea are C, F and K. This is how they look like.

Guinea

In Guinea, outlets of type C, F and K are in use.

Type C

Type F

Type K

Sierra Leone

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

Type D

Type G