No offense but it's a "I wasn't paying attention in high school physics" comment. It being beta decay with a half-life of 100 years should already indicate it's relatively safe. In fact someone else in this thread already already added the references showing how safe it is. If it's safe enough to power a pacemaker it's safe enough to sit in your phone that sits your pocket.

Personally I think that battery would have much bigger issues than safety, such as power requirements which are much harder to control with nuclear decay. Also obviously the device itself deprecating before the battery because tech will definitely advance a lot in 50 years, I imagine after a decade the phone will be useless. And finally the pricing considering Ni-63 doesn't occur in nature which means you need a specific process to create the materials necessary for the batter.