Relay Driver with 24VDC Relay.... Zener diode?
You need to add a switch (at the battery) to your diagram to see the issue. When the switch opens, the inductance of the relay coil will cause the current to want to keep flowing. With nowhere to go, a huge negative voltage will form at the positive side of the relay. This voltage will stress whatever switch (mechanical or electrical) that you use. The diode provides a safe path.

Now that you understand the issue, you should be able to determine what type of diode will help.