Access control with NFC smartphones

I'm studying NFC technology and how it can be used in access control applications, mostly with smartphones.

If I understood correctly, smartphones with NFC support provide a NFC "active element" that can communicate with passive card/tags (Reader/Writer mode) or with another active transceiver (Peer mode). Initially I thought they were simple NFC *passive* tags that worked even without power supply, such as RFID tags.

I think, in access control applications, the smartphone works in Peer mode, because the control system is another active element that "opens the door" only if it sees an authorized smartphone. Maybe the smartphone is the initiator and the control system reader is the target. I imagine the user *must* launch an app on his smartphone (configuring/activating NFC functionality in Peer mode), put it near the reader (maybe on the door handle) and the door magically opens.

If what I explained is right, I have two questions.

Should a custom app be developed for the smartphone? I know NFC can be used in many applications, but I hoped for authorizing access it was sufficiently standardized and already implemented in hw and sw in modern smartphone, without installing app. As with RFID passive tags, I thought the NFC smartphone introduces itself with a unique ID, without the intervention of a custom app.

The other question is what happens when the battery is too low in such a way the OS is in standby mode... or worse all the electronics isn't supplied at all. Is it possible to use the "dead" smartphone for "opening the door"? If not, it's difficult to think to NFC as a good technology for access control (imagine what happens if you come back home at the end of the day and the smartphone is smartphone battery is completely discharged).

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