Is a strontium-fluorine battery the highest voltage battery using pure elements?
The half-potential you've given for strontium is only for the first ionisation. The half-potential you'd actually get is -2.899V for the stable dication to give a cell potential of 5.769V. From the CRC Handbook[1], lithium has the lowest element->stable ion potential of -3.0401V, which is why it's common in batteries.

[1]: Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.