__-Ion Battery; Rechargeable Power Cell
Puzzle 4 of Energy Sources Hard Pack
Question is: __-Ion Battery; Rechargeable Power Cell
and the answer is: LITHIUM
Word Meaning:
LITHIUM — (uncountable) The simplest alkali metal, the lightest solid element, and the third lightest chemical element (symbol Li) with an atomic number of 3 and atomic weight of 6.94. It is a soft, silvery metal.
Definitions from Wiktionary.