You can find official Park and Ride on the Rheinbahn website. Open the "Düsseldorf" tab and watch out to select only Park & Ride facilities, they also list plenty of Bike & Ride facilities.
Alternatively, gratisparken.de ("parking for free") has a list + map of free parking spots around Düsseldorf. In your case I would select only those without temporary restrictions, i.e. where it says "zeitlich beschränkt: nein", otherwise you may find yourself in violation e.g. on market days on a spot that was perfectly fine and empty when you arrived. There are descriptions of the spots in German which you might want to have auto-translated if in doubt, however the maps should give a good indication (click "Detailansicht anzeigen"). What is not shown is the number of spots and how likely they are occupied (but likely by commuters, so if you arrive e.g. in the evening you should have better chances). Also many of the descriptions state if there is easy access to public transport + GPS coordinates.
Example screenshot from this parking.
You should of course check if the parking is really free before leaving your car. There can always be events or construction works or outdated info.
