The lattice energy is a measure of the strength of the ionic bonds in a solid ionic compound. It depends on the charges of the ions and the distance between them.
In the case of LiCl and NaCl, both compounds have the same crystal structure (face-centered cubic) and similar ionic radii. However, the charges of the ions differ: Li+ has a charge of +1, and Cl- has a charge of -1, while Na+ has a charge of +1, and Cl- has a charge of -1.
Lattice energy generally increases with increasing charges of the ions and decreasing distance between them. In this case, the charge of the ions is the same for both compounds, but the ionic radius of Li+ is smaller than that of Na+.
Since Li+ has a smaller ionic radius, the distance between the ions in LiCl will be smaller than in NaCl, resulting in a greater lattice energy for LiCl compared to NaCl. Therefore, LiCl has a higher lattice energy than NaCl.