From the little I read
on this topic...
It's a sandwich of C14 and diamond... doesn't really matter... the power output is so low (100's of microwatts) that the applications are limited... even if it could be scaled up, it would need to be 10000 times larger just to get a watt.... better to use the RTGs that put out 100s of watts... they have a half life of 88 years, instead of 1000, but that's fine for any applications we are likely to have...
Again, beta decay batteries are not new, they just have very low power output and so are only useful in very limited applications.