“Unfortunately car batteries contain mostly lead oxides”
Nope, not true. No lead oxides... Lead sulfates. The plates in a lead acid battery are a lead grid filled with sponge lead. The sulfur goes between lead sulphate and sulfuric acid, depending on the charge level.
Take one apart and melt it down. You get most off the weight as lead...
On a standard car battery at the time of manufacture the negative plates are filled with lead oxide which is known as “red lead” and the negative plates are filled with lead dioxide also known as “spongy” lead. Basically the active materials absorbs sulfate from the acid during discharge and give up the sulfate while the battery is being recharged.
Although the lead can be recovered from car batteries... it is a more complicated process than simply taking them apart and melting them down. This is too bad because I have dozens of old car batteries and I cast my own bullets. So don't think that I haven't thought of it. If you know of plans for a simple way to recover the lead I would love to give it a try. ; )