Phone dies at random percentages?

So for the past few weeks / month-ish my IPhone 6 (which I’ve only had for almost a year) has been dying at random percentages, it even died at ~88% a few days ago after charging it for around an hour. Not only does it die randomly, but the battery drains insanely quick, like it went from 80 to 40 in ten minutes. As well as charging insanely slow, I plugged my phone in last night at 20 and 8 hours later only got it up to 99, not even a full charge !! Help??