Sounds very similar to the old Soviet pacemakers with radioisotope batteries. After the collapse of the Soviet Union, records about them got lost and so a bunch of people have been buried with pretty radioactive stuff in their chest. I don't think we (as developed societies) are going to take that risk for some phone batteries...
I noticed, by reading the article, that Nickel 63 decays to Copper 63 which is stable.
"Betavolt further states the battery is environmentally friendly. “After the decay period, the nickel 63 isotopes become a stable isotope of copper, which is non-radioactive and does not pose any threat or pollution to the environment,” the company explains. “Therefore, unlike existing chemical batteries, nuclear batteries do not require expensive recycling processes.” "
Here's the summary for the wikipedia article you mentioned in your comment:
Naturally occurring nickel (28Ni) is composed of five stable isotopes; 58Ni, 60Ni, 61Ni, 62Ni and 64Ni, with 58Ni being the most abundant (68.077% natural abundance). 26 radioisotopes have been characterised with the most stable being 59Ni with a half-life of 76,000 years, 63Ni with a half-life of 100.1 years, and 56Ni with a half-life of 6.077 days. All of the remaining radioactive isotopes have half-lives that are less than 60 hours and the majority of these have half-lives that are less than 30 seconds. This element also has 8 meta states.