The lattice energy of potassium bromide is more exothermic than the lattice energy of rubidium iodide because lattice energy is inversely proportional to atomic radius.

Ions release some amount of energy when they get together to form a lattice. If it is needed to be seperated into ions again, the same energy should be absorbed. The energy to break 1 mole of a lattice at the standard temperature and pressure is defined as the standard lattice energy. Using formulae, the lattice energy at any conditions can be found as the standard values are tabulated.

it releases lattice energy

Ionic Charge and Ionic Radius - the smaller the radius the greater the lattice energy

Ionic Charge and Ionic Radius - the smaller the radius the greater the lattice energy