I wear a % oxygen meter when visiting our manufacturing sites. I've disassembled them when they expire. The sensor looks similar, and has a 24 month life. I'll pull it apart to see whether it is a similar electro-chemical. I've used a multimeter to try measuring voltage across the sensor pins, but nothing shows up.
It has a nice 3 volt haptic motor inside to alert you in case you can't hear the piezo speaker alarm.
The battery is a non-rechargeable lithium. I guess they assume since the oxygen sensor wears out, you don't need a rechargeable battery.
Another great teardown.