I saw a video about a recent advancement in nuclear diamond batteries. Basically look like normal AA batteries but used depleted uranium and lab diamonds to make them save and long lasting
Sadly like most similar nuclear powered energy sources this is very low density, and provides very little power, despite lasting a long time. Think microwatts.
Only really viable in deployments that need very little power, where no other energy harvesting method is available, and periodically changing out batteries is not an option.