Why is the lattice energy of Nacl more than that of CsCl? If size of Na is smaller than that of Cs, then according to Fajan's rule, Na should polarise Chlorine more and therefore, NaCl must have more covalent character than CsCl (and higher covalent character means low lattice energy).
Lattice Energy is directly proportional to the Charge on ion and inversely proportional to radius of atom . Now in between NaCl and CsCl the radius of Cs+ is way more than Na+ , even though CsCl is more ionic but still radius is more and hence the lattice energy is less .
It can also be explained using