Its been awhile, have you solved your problem. The power shield has a built in voltage regulator. This is from particle data sheets page.
The Power Shield, as the name implies, allows the Particle device to be powered from different types of power sources. The shield has an intelligent battery charger and power management unit along with a wide input voltage regulator and an I2C based fuel-gauge. You can power a Particle device with either a USB plug or a DC supply of anywhere from 7 to 20VDC and charge a 3.7V LiPo battery all at the same time.
Iām trying to make something similar.