iPhone Xs Max battery acting up.

my iphone has suffered some issues lately, the battery charges so slowly past 80%, it takes a solid half hour to get from 80% to 90% with a 20W charger and the battery gets very hot while at it. Also my phone crashes and shuts down suddenly when it reaches 10%, the battery health was at 85% but it dropped to 83% in one day... what should I do? i'm using an iPhone Xs Max