This question already has an answer here:
I want to encrypt my phone to protect sensitive information. But I do not want to enter a password every time I pick up my phone. That seriously hampers the usability. Particularly because the password must be of sufficient complexity for the encryption to be secure.
What I want is a boot time only password.
Most thieves switch off the phone after stealing it. Or remove the battery to change the SIM (so it won't get tracked). When he starts it again it'll be locked. There is a risk that he may mail my sensitive information to himself before switching off the phone but that is a risk I'm willing to live with.
How do I encrypt my phone with only a boot time password?