Reason for high electrode potential of Lithium?

Answer 1

I would add the fact that, although lithium does not have the lowest ionisation potential, the Li+ iron is very small and therefore has a very favourable hydration energy. This also enters into the energy bookkeeping.

Answer 2

The electrode potential of any metal depends (in part) on how easy it is for the metal to lose electrons. On net, the smaller the metal atom's nucleus, and the fewer the electrons in its outermost orbit, the easier it will be for the electron to be lost.

Lithium is literally the first metal in the periodic table - http://www.corrosionsource.com/handbook/periodic/periodic_table.gif - and its atoms find it very easy to lose an electron and slip into a solution into which an electrode made of Lithium is dipped. The consequent potential difference between the solution and the (now negatively charged) electrode is higher with Lithium than any other metal - http://hyperphysics.phy-astr.gsu.edu/HBASE/Chemical/electrode.html