A few months ago my beloved Jolla-Phone started to reboot at random times. I was not sure what the reason was, first thought it is a software problem. Then I ordered a new Polarcell Accu from wannsee-electronic in germany (btw in my case a very friendly dealer offering a first class service) via ebay.
Since I use the new battery in my phone it is stable again. No reboots so far.
Some words about the capacity: original black jolla battery is now at 7132mWh when fully charged. Polarcell accu started with 7259mWh. After 5 discharge/charge cycles it is now up to 7613mWh. This is really a really nice performance so far.
Hope this post helps other people in need of a replacement battery.