The lattice energy of a crystalline solid is usually defined as the energy of formation of the crystal from infinitely-separated ions, molecules, or atoms, and as such is invariably negative.
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When NaCl dissolves in water, the crystals disappear on the macroscopic scale. On the atomic scale, the Na^+ and Cl^- ions in the crystal are released into solution. The lattice energy of a salt therefore gives a rough indication of the solubility of the salt in water because it reflects the energy needed to separate the positive and negative ions in a salt.
Opposite of this picture is this example: The lattice energy of NaCl is the energy given off when Na^+ and Cl^- ions in the gas phase come together to form the lattice of alternating Na^+ and Cl^- ions in the NaCl crystal.
