What is 1.8 to 5.5 VDC?
VDC is an abbreviation for volts DC. DC means Direct Current you can read more a detailed explanation about it here. Basically what powered by 1.8 to 5.5 VDC means is that the DC voltage on the input of your sensor can vary from 1.8 volts up to 5.5 volts. So you can use either 3 AAA batteries in series since the sum of their voltage will be: 1.5V * 3 = 4.5V what is in the range of recommended voltages for your sensor. Also, you can use a Li-ion battery in order to power your sensor since the voltage of this type of battery is 3.7V it is also in the range of recommended voltages for your sensor. However you would need an additional charge/discharge module in order to recharge and keep the battery in a good condition. You can read more about it on the web.