$Li_2CO_3$ decomposes on heating because the Li+ ion exerts a strong polarising action and distorts the electron cloud of the nearby oxygen atom of the large $CO_3^{2–}$ ion. This results in the weakening of the C—O bond and strengthening of the Li—O bond. This ultimately facilitates the decomposition of $Li_2CO_3$ into $Li_2O$ and $CO_2$. The lattice energy of $Li_2O$ is higher than the lattice energy of $Li_2CO_3$. This also favours decomposition of $Li_2CO_3$. Na+ ion being bigger in size, the lattice energy of $Na_2O$ is less stable than that of $Na_2CO_3$. Therefore, $Na_2CO_3$ does not decompose on heating.