The energy density of this betavoltaic battery is only about half that of an LiMnO2 primary-cell battery, about 370 mWh/cc versus 650 mWh/cc. Examples of LiMnO2 batteries are the extremely common CR2032 or CR2450 coin cells.

Self-discharge isn't much of an issue with LiMn02 batteries, either, at only 1% per year.

And, of course, LiMnO2 cells have the benefits other commentators have mentioned, such as the ability to supply more than just 75 uA short-circuit.