Why does my phone randomly go down to zero and shut down?

I started noticing this problem outside in the cold. I didn’t take much notice of this problem as I thought this was because of the cold. Now when my phone is indoors and at room temperature the battery might randomly go down to zero and it turns off. This is very annoying now especially when I’m in the middle of doing something important. In addition I don’t always have access to a charger so if I want to call/text someone I can’ because my phone has no battery. I’m already considering purchasing a new phone at this point. What might be the problem?