Cheap Step-Down converter as Solar Charge controller
In a buck converter, the schottky diode (that you may be referring to) is absolutely needed. Don't even think about leaving it out - you'll be just wasting your time. When the transistor "opens" after pushing current into the inductor, that current needs to keep flowing to carry on passing energy into the output capacitor. Without the diode (schottky or otherwise) this current can't flow into the output cap so instead it creates a big voltage spike and destroys the switching transistor in the chip.

If you are referring to a diode placed in series with the power feed to the battery then it's likely this is also needed for two reasons: -


With no power fed to the buck convertor, the battery terminal voltage would reverse bias everything and possibly cause circuit failure.
Even if the switcher circuit self protected without the diode there is astill a good chance you'd drain the battery.