So, in my mind lets say you have two metal plates all by themselves. Their not connected to anything. They must both already have negative electrons and positive protons stored in them. Then when you hold them close together and fill the space with dielectric and connect them to a battery; the negative electrons that are on the positive side are then drawn to the positive side of the battery and go stack up on the negative side. This reduces the number of positive protons on that side and at the same time, increases the number of negative electrons on the other side. This action will continue until both sides are balanced out, meaning one side is negative and one side is positive.