What is my iPhone dying so quickly

When using my iPhone I usually charge my phone to 100%. However my phone tends to die around 70% to 90%. Then after a few minuets I turn it on to find it at around 70% to 90%. When it's not connected to a charger it would turn off right after the device has been powered on. i drained the battery but it would again power off if no charger is connected to the phone. I was wondering if I should buy another battery for the iPhone 6?