Battery
what should I do when my phone unexpected shutdown
iPhone 6
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91 is a good % (start worrying when that value goes down close to 80%).
So the unexpected shutdown shouldn't be related to you battery, unless it has been a temporary power glitch, but we, just users like you, can't dignose that issue remotely, sorry.
Apple direct support can do it: try contacting them using this link:--> Get Support
Regards
Giulio