theoretical 25-years-between-chargings batteries are being proposed/developed for applications such as satellite power, the primary fabrication method is to place a micro-layer of tritium suspension polymer on a diode wafer...
Tritium is so safe that it is used in novelty glowing keychains, watch faces, and exit signs due to its faint luminosity (things for which the more dangerous radium is no longer used).
Tritium is relatively rare, and is only manufactured in nuclear reactors. Its high cost is not likely to come down any time soon and in fact would likely only skyrocket should tritium become a widely-used fuel. Tritium also has applications in nuclear weaponry, and any significant quantities of it would be highly regulated.