Hi:
After reading some articles this weekend about exploding lithium primary cells (after purchasing a second flashlight that relies upon CR123 Li cells, which might reside in my pocket :-( I looked this morning at an electrochemical series for alternatives to Li.
Calcium is -2.87V and magnesium -2.37V. Seems Ca or Mg might be useable in cells while perhaps reducing the hazard of lithium. Ca is a bit feisty itself, but significantly less so than Li. Mg at least doesn't readily react with water except at very elevated temperature. These metals are also quite light and abundant.
What does the future hold for high energy density primary and secondary cells?