Peace of mind is serenity, quietude, or an absence of mental stress.
To give a piece of your mind is to tell someone what you think, usually angrily or self-righteously.
I then gave the poor student volunteer on the other end of the phone a piece of my mind. [Parkersberg News]
If it’s peace of mind that you are after, then opting for a fix now could be the best move. [Telegraph]
But one guest, getting fed up with these ostentations, gave the host a piece of his mind, something that apparently had never happened before. [The Standard]
It forms part of a well-maintained granite tenement building which is protected by a security entry system – giving peace of mind to those who live alone. [Aberdeen Press and Journal]