A lithium battery is nearly 100% efficient.
They both run through an inverter and electric motor so those efficiencies are the same.
A fuel cell is efficient compared to pretty much every other form of generating electricity from energy stored in a fuel that we know of.
Why would you do that? You could argue energy density, but looking at current fuel cell vehicles like the Mirai shows the have only a slim advantage in range dues to the weight of the fuel cells, high pressure tanks and tractions batteries and extra structure to protect the tanks.
The electrons in the battery are very light and energy dense but the battery to hold them and return them as useful current is not, same for hydrogen.
What hydrogen fuel cell car is not a proof of concept?
Efficiency is how much power is put in vs how much you get out.