magnetic car mounts problem for Qi charging?

I have the LG circle case with Qi charging. Love it.

Was thinking of getting a magnetic car mount (requires putting a small metal plate behind the back cover.

I did a little test, and place the small metal plate between my phone and my Qi charger, and sure enough the metal plate got pretty hot.

My question is, will this interfere with the charging? Would it be bad for the phone to do this?