ESD protection for device with isolated PSU
The ESD9B3.3ST5G is a dual back-to-back zener. 

For positive ESD voltages this will clamp to about 11 volts so it's possible that this may be too high to adequately protect your circuitry. Also, given that it does the same in reverse (i.e. -11 volts) then this could indeed by too high in magnitude to offer any protection when the ESD is negative.

I suggest you read the data sheet for the device and for the connected chips inside what you call "Li-ion battery charger" and do a full analysis.