iPhone boot only when plugged

Hi everyone, i have a question,

when i try to turn on my iphone 6s, the iphone display a dead battery logo and then it boots only if i plugged it to a charger and then it show 30 % of battery. after 5 minutes i shut it down turn it on again without charger and it can boot but several days later, same problem, i i have to plug it to the charger and it boot instantly. Battery is in good shape more than 80 %.

Thanks