The energy stored on a capacitor can be expressed in terms of the work done by the battery. Voltage represents energy per unit charge, so the work to move a charge element dq from the negative plate to the positive plate is equal to V dq, where V is the voltage on the capacitor.
U =0.5CV^2
0.02 =0.5(500x10^-6) V^2
Solve for V
V = 8.94 volts