I got a $11 battery replacement kit off of ebay and installed it a month ago. It solved all of the battery issues I had. Installation was very easy other than getting the old battery out, as it did not work anything like the video I watched showed about pulling the 2 sticky white strips up. I ended up spending like 40 minutes or so with a hair dryer and credit card to get that sticky crap off.
But other than that part, everything was a breeze and it's very easy.
This is the one I got:
https://www.ebay.com/itm/Brand-NEW-...6-Battery-1810-mAh-With-Free-Kit/262947800364
And this is the instruction set/video I followed:
https://www.ifixit.com/Guide/iPhone+6+Battery+Replacement/29363
I only wanted to make it to the iPhone X launch so it's lasted me just fine and the phone is like a new phone now with the new battery.