I think IMEI is only for 3G tablets. Try using
ANDROID_ID
In particular, Settings.Secure.ANDROID_ID. This is a 64-bit amount that is generated and saved the first time the device boots. When erasing the device, reset.
ANDROID_ID seems like a good choice for a unique device identifier. There are drawbacks: firstly, it is not 100% reliable in Android releases prior to 2.2 ("Froyo"). In addition, at least one well-known mistake in a popular phone from a major manufacturer, where each instance has the same ANDROID_ID.
Look here
user370305
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