Assuming it's a weather balloon, it's gonna just ride the wind until it eventually deflates. It's possible that it was supposed to deflate but didn't, and it drifted out over the ocean and they were like "guess it's gone forever." If they don't burst they can stay aloft for weeks, which would be plenty of time to drift from China to the US.

Edit: Appears the Pentagon has said it's not a weather device. They said it can maneuver, which I don't know much about balloons, but appears they do that by changing altitude, which doesn't seem like something that you'd bother have a weather balloon be able to do. Looking at the pics of it, it appears to have solar panels on it too.

Sodium sulfur batteries aren't anything new. The main issue with them is they need to be kept at like 300 degrees Celsius to work. This article is claiming a working sodium battery at room temperature.