Before the emergency hits, get a $20 10,000 mAh or larger rechargeable USB power bank. It’s possible to get 2-3 phone recharges from this bank for each recharge of the bank.
9V battery cell phone charge hoax is from 2014 and has been disproved in my lab and independently by the Daily Mail.
A 9V battery has less than 100 mAh available at the 500-1000 mA charge rate demanded by modern smartphones. This means the 9V battery current output quickly drops to near zero, while the phone still sees the input voltage, kicking the phone CPU into full power mode.
When a smartphone sees a 5V input on its charge connector, it thinks it’s on a source of adequate power, and starts checking email, syncing photos, putting the CPU into full power mode, etc. Every phone I’ve had does NOT check if the charging source is actually adequate to have a net positive charge! This means the phone battery will discharge faster by having a 9V battery connected via a cigarette lighter adapter.