I know salt water batteries are less efficient than lithium-ion, but water is safer, can be found anywhere, is cheap and 100% ecological. Even if it doesn't last long, you can refill anywhere. Why can't salt water batteries be used to power electric cars?
Water has one major setback: It is chemically stable only up to a voltage of 1.22 volts. This means that a water cell supplies three times less voltage than a customary lithium ion cell with 3.7 volts, which makes it poorly suited for applications in electric cars. However, water-based batteries, could be interesting for stationary electricity storage applications.
Secondly it is not true that water is lighter; Lithium is the least dense of all the metals, it has a density nearly half that of water $0.534 g/cm^3$.