iphone 6 Battery question

Having frustrating phone battery problems and hoping for insight from the community.

Long story short, my battery is performing very inconsistently if at all. As of a month ago, it began taking the charge very slowly from the charger I use in my car, if at all. It got to the point where I would have to put the phone to sleep overnight for it to charge. Then brought a new Apple cable back from home. This worked fine until the phone decided to reject the cable anytime I tried to use it with that warning screen about the charger not being "certified" for the phone, or whatever it is. Now we're back to the other charger, which now needs to be positioned just so in order to charge at all, and was working fine until yesterday, when it started not charging again. I left it overnight with the phone shut down and it didn't charge up at all.

Why is this happening? A very similar thing happened to my last iphone, the 5, and it resulted in that phone just dying completely. The only things I can tihnk of that I could have done that would have cuased this: I used a pin to clean some lint out of the charging port, maybe this could have somehow scratched or damaged something? And before this all started, I was using an Apple cable that was frayed, and toward the end started getting really hot when I would try to use it to charge the phone (eventually it started smoking and I had to ditch it). Could that have fried the battery?