Electrically charging robe

[Crake]

So I wanted to put together an item for one of my players that slowly built up an electric charge that could discharge when an enemy attacked them. I was thinking something along the lines of for every 3 points of electricity damage that the player deals or takes, the robe would store 1 point of retaliatory damage that would discharge at an enemy who successfully attacked the wearer (no save, kinda like fire shield). The charges would remain for something like 1 minute or so before dispersing, so no pre-emptively charging the robe with a bunch of damage and discharging it at an enemy.

I'm looking for balance advice and pricing help if anyone has any suggestions?

Or, if something like this already exists, can someone point it out to me?