What effect could millions of cobalt batteries powering cars have on the environment?

Cobalt is an important component of vitamin B12.

...of medical devices and other materials.

Like most gamma radiation sources, it can be used to destroy powerful carcinogens like organohalides.

It is also used in cancer treatment.

It's half-life is about 5.2 years

It can be made by neutron bombardment of cobalt-59, but will be contaminated with the naturally occurring monoisotopic Co-59, an element which is of extreme economic and technical importance.

Pure Co-60 can be obtained as a decay product from the radioactive isotope Fe-60, which is generally formed in small quantities from the neutron bombardment of iron, as in steel in the cores of nuclear reactors.

It is not possible to obtain large amounts of iron-60, however, because the heaviest isotope of iron is Fe-58. Fe-59 has a relatively short half-life of around 44 days, and owing to Bateman Equilibrium can only reach a low concentration - depending on neutron flux - before it is decaying as fast as it is formed. In addition, at these low concentrations, it is necessary for a neutron to collide with the Fe-59 to form iron-60, which has a half-life of around 2.6 million years.

I have been interested in these iron isotopes and cobalt isotopes because of the eutectics formed by plutonium and iron, which was utilized in the LAMPRE reactor in the 1960's. The reactor, which is pictured in the link, might easily fit into a high ceiling living room in a McMansion, produced about 1MW power during its experimental operation.

This was an outstanding reactor concept, and in my view, given advances in materials science, is worth another look.

The cobalt version, which was investigated but never utilized, relied on a ternary eutectic of plutonium, cobalt and cerium.

Co-60 decays to stable nickel.

The long term irradiation of iron is in theory a way to transmute iron into the rarer cobalt, albeit after a cooling period of several decades.

However since the energy density of plutonium is so high - making it the most attractive fuel for humanity from an environmental standpoint - only trivial amounts of cobalt would accumulate in such a scenario.