Block current flow in one direction with FET transistors
You ARE talking about reverse polarity current protection, you said:  


  ".. if Vbat>VS the current flow from battery to power supply."


This is a reversed polarity current. 

Every MOSFET has a parasitic body diode structure. So even when it is turned OFF, it doesn't block the current if it goes in opposite direction. To completely block any current in OFF state, two transistors are needed, which is illustrated in the Figure 4.