The 7805 is a rather poor choice for this task since it is a so called linear voltage regulator. This means that it has no choice but to convert the voltage drop into heat. In this case 11.4 V - 5 V = 6.4 V. In other words, the regulator is going to produce more heat than the Pi itself and sucking your LiPO battery empty at the same time.
According to the datasheet and assuming the TO-220 package without heat sink has a thermal resistance junction-ambient (R_thJA) of 50 K/W (and even lower values for other packages such as D2PAK, DPAK, and TO-220FP). With P_D = (T_J(max) – T_A) / R_thJA and at an ambient temperature (T_A) of 25°C and the maximum junction temperature (T_J(max)) of 125°C - where the internal thermal-overload protection should kick in - a maximum power dissipation (PD) of 1.67 W is possible, which would be reached at about 260 mA. Besides wasting energy - not a preferable thing for a mobile application - operating the regulator at a higher temperature can affect reliability.
Now Power consumption of Pi Zero W? suggests the the Pi Zero W with WiFi enabled should consume less than that 260 mA which indicates it should be possible to run the Pi with this setup. Luckily the effect is rather simple to test - just check the temperature of the regulator. Warning, might be hot...
Possible (short term) solutions:
- add a heat sink to reduce R_thJA and get some more headroom
- reducing power dissipation with dropping resistor according to this circuit (note a required high power rating of the resistor, which will now share the burden with the regulator), (source):
Better (long term) solution:
- go for a switching regulator, i.e. a step-down UBEC