https://www.theregister.co.uk/2018/06/04/russias_nuclear_battery_ambition_see_a_10x_increase_in_power/

Russian boffins at the Moscow Institute of Physics (MIPT) have emitted a prototype nuclear battery packing 3,300 milliwatt hours of energy per gram.

The paper, published in Diamond and Related Materials, describes a betavoltaic battery powered by the beta decay of the nickel-63 isotope giving 10 times the power of conventional commercial cells.