The possibilities for batteries are endless. But you have to have a use-case or some type of desired runtime between charges/replacements to determine what batteries you can use. For super low power applications, consider the 15uA quiescent current for the mesh voltage regulators as discussed in this post. I haven’t seen specs on nominal current while awake yet but the promise is that the mesh devices will have a fairly low power consumption compared to the Electron and Photon.