Comment by conductr

Those exist and are still available but are fairly outdated in the US. The sealed lithium 10-year disposable is the newer standard. And, actually, building codes for last several year requires them to be hardwired so no batteries at all.

The landlord special on older construction (maybe >10 years old, can't remember when the hardwire code went into effect) will usually be the 9v. Because they don't care about you having to get on a ladder to change the battery every year. They get to save $5-10 per smoke detector. Practically any homeowner is going to choose the 10 year option as the batteries don't have to be swapped.