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

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I 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

Tonga

In Tonga, outlets of type I are in use.

Type I