I was thinking something more like 6x AA-sized cells wrapped into two bundles to make a trianglular core for the grip. That would get around 18-20V of juice.
All that fancy electronics and OLED. Programmable temperature curves? LOL, no. Replace that with a buck converter and charging IC for the integral battery, so it can charge with a USB cord that I CAN lose as many as I want. And for controller, simple temp regulation +- 10 degrees or so is fine. A thermocouple in the tip, a couple opamps, a comparator, a pot, and a mosfet. Controls: on/off switch and a knob; don't even need degree markings, just a dial indicator and a scale from 1-10. I don't want to hold a button for X seconds to start up and shutdown or to cycle thru menus, or to use a touch capacitive control to slide the temp up/down. And I don't want a soldering iron to give me warning messages thru an OLED. If I can't tell my iron is malfunctioning, then it's working fine. If the iron knows the iron is too hot, then it can switch off the heater like it's supposed to and keep that info to itself.
Better yet, instead of using a 24V heater element designed for a bench station, how about a design made to run off 6-7V at high current. Run from 2x high output 18650 cells. It doesn't matter if it lasts 10 minutes, because you're probably using this kind of iron to fix something, not to assemble a board. Manufacturing a heater and tip is the hard part, though.