The voltage of a battery cell is determined by its chemical makeup. The common alkaline batteries use Zinc and Manganese which react to produce a certain “concentration of free electrons” (aka voltage). The reaction is at equilibrium when no current is being drawn (moving those electrons out). When the battery is connected to a device, those electrons are drawn out of one end and pushed into the other, which changes the equilibrium of the reaction. The zinc and manganese react to replenish the electrons and reestablish equilibrium until all of the reactants are used up. At that point, the reaction can no longer produce electrons and the battery is dead. Using different reactants will change the equilibrium “concentration” (voltage) of electrons. Since each cell can only produce a certain electron pressure/concentration/voltage according to its chemistry, the way to get higher voltages is to stack them on top of one another.