Not all, but Apple has a program in place for iPhone 6s owners who are experiencing an "unexpected shutdown issue". It affects iPhone 6s that were manufactured between September and October 2015. If you think it applies to you, run your serial number at apple.com to find out if you're eligible for a free battery replacement.
I'm so sorry to learn about the issue you're having with your device. We can take a closer look into this. When did this begin? Does the phone power off by itself? When turning back on, is there any charge percentage left, or is it at zero? Please clarify some details for me.
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