On schematics, the "J" designator is normally used for connectors; in this case J2400 is the battery connector, not the resistor you're looking for.
The image @imicrosoldering posted is a board layout drawing that's used to generate the circuit board and shows the location of the electrical connections on the top surface of the board.
So J2400 is the battery connector, and the red square he marked shows the location relative to it of the two solder pads for the part Chris is pointing out to you.
In order to find a replacement for it, you'll need a board layout file, also known as a Gerber file, then locate the part you're looking for on it and determine the reference designator. For example, if this is a resistor (which I'm guessing it is), it will have a number like "R103". You then have to find a schematic diagram and locate that same number on it, which will have a label that will tell you what the part is and any information needed to identify the correct part such as resistance value, tolerance and power dissipation.
Yeah, it's a lot, but there are plenty of really smart people on this site who can help identify it for you, assuming of course that's what the problem is. If you can take a high resolution image of the battery connector and the parts next to it, we can try and help with that.
Adding images to an existing question - iFixit Repair Guide
Your next step is to take a magnifying glass and a bright light and figure out if that part really is missing on your motherboard. If not, then obviously something else is the problem and we'll have to try to diagnose it.
In response to your comment on @codyr08's answer, there is more to the battery than just the ability to power the phone. It also has sensors on it that the logic board reads to determine the current power level of the battery and its temperature. If it can't read those sensors once it boots, that can and does cause a boot loop; meaning basically the operating system crashes and then restarts.
Chris is indicating that he's seen cases where the component gets knocked off while trying to connect the battery, and that part carries one of the sensor signals that the phone is trying to read. When it can't - boot loop.
It is possible to find out what's causing your boot loop; there are files on your phone called panic logs that will point out exactly what sensor the phone can't read that's causing the boot loop. Read about it here.
iPhone Kernel Panics - iFixit
If you can get in and read those logs and post what you find here, we can help figure out exactly why you're stuck in a boot loop, although Chris has a lot of experience and most likely has already given you the answer.