sudden shutdown on my iphone6

iPhone 6 is an outdated iPhone (9 september 2014) -> 7 years old, which also affects the battery life. The hardware remains old, so does the battery.

It is no longer possible to have the battery replaced at Apple, see here: 

https://support.apple.com/iphone/repair/service/battery-power

You may want to consider switching to a newer type of iPhone.

OR if the problem persists, visit an Apple Store or an Apple Authorized Reseller in your region for assistance to diagnose the problem. And they could offer a possible solution.

You will first have to make an appointment due to the pandemic.