When Lithium has just a strong nuclear charge then how can it have such a low reduction potential? How is it able to donate electrons inspite of the large force exerted over them by the nucleus?

Dear Student,
Li  has such a low reduction potential despite it small size, the reduction potential depends upon the  atomization energy , hydration energy and ionization energy. 
$∆H = H_{tomization }+ Ionization energy + ∆H_{hydartion}$
This value of $∆H$ decides the reduction potential. Lithium has a very high  heat of hydration  because of its small size large amount of energy is released when we the gaseous ions change to aqueous form  and therefore the reduction potential value becomes very low (that means large negative). This explains why lithium despite having such a strong nuclear charge .
It is able to donate electrons despite the force exerted because of achieving stable octet.
Regards!