Why not use Zinc–bromine batteries for cars?

The cycle life at 100% depth of discharge may be better for zinc-bromine batteries than for lithium ion batteries. That's an advantage. Zinc and bromine are available on a larger scale than lithium and cobalt. That's a potential future advantage.

The energy density of zinc-bromine batteries is lower than lithium ion batteries. The cost per unit of storage is higher than lithium ion batteries. Liquid bromine is much more acutely toxic and corrosive than the hydrocarbon fuels we let consumers pump themselves. In the battery's charged state, the zinc is a solid, so you can't recharge a car with such a battery simply by pumping fluids.

Zinc bromine batteries have some advantages that might make them viable for large scale grid tied storage in stationary applications. The disadvantages mean that they are unlikely to be adopted in passenger transportation applications.

I remember this concept being the cover story in an issue of Popular Science back in the 1970s -- a "gas station" where you exchange electrolytes.

philipkglass' comment brings up one risk: what happens if the batteries are breached in a severe collision?