This seems like a rather touchy solution looking for a problem.
Unless you really enjoy buying replacement hardware; the need to have battery power in order to trigger the kill switch is a problem. If you don't configure the device to self-destruct when its battery is on the verge of no longer having enough energy to perform a self destruct; all the attacker has to do is run the battery down. If you do configure it that way, forgetting to put it on the charger could get expensive and tedious rather fast(in addition to the various other issues that can interrupt battery power: overtemp protection kicking in in a hot car; current delivery capability falling under freezing conditions, etc.)
Plus, the battery, and its connection to the logic boards, tend to be among the larger and more obvious parts of a modern electronic widget. That makes them good candidates for controlled disconnection/destruction, even if you can't open the case without tripping some sort of anti-tamper mechanism.
Finding a good self-destruct temperature is also a bit tricky. The lower you go, the closer you get to the high end of normal operating conditions or the 'device won't operate; but should not be permanently damaged' range. 80 degrees is high for flash memory; but most CPUs will be happy enough to run that hot. The higher you go; the more power you need to be able to deliver to kick off the destruction; and the more vulnerable you are to an attacker who is able to apply coolant to slow you down; limit current or voltage delivered to the resistive heater, or both.