While it's true that a significant portion of our power grids energy comes from natural gas, it's also true that power plants are far more efficient at turning fossil fuels into usable energy than a car, which spits most of its produced power out the tail pipe (heat).

In other words, even if 100% of our power grid was fossil fuel based, by producing the power in plants instead of in your cars engines, it would still be a net positive for the environment as less fuel would be used to power said cars.

We do need to find better battery types than lithium while this transition happens, however.