Why is $Li_{2}CO_{3}$ decomposed at a lower temperature whereas $Na_{2}CO_{3}$ at a higher temperature?

(N/A) As we move down the alkali metal group,the electropositive character increases,which leads to an increase in the thermal stability of alkali metal carbonates.
Lithium ion $(Li^{+})$ has a very small size and a high charge density,which allows it to polarize the large carbonate ion $(CO_{3}^{2-})$ effectively.
This polarization makes $Li_{2}CO_{3}$ covalent in nature and thermally unstable,causing it to decompose at a lower temperature:
$Li_{2}CO_{3} \xrightarrow{\Delta} Li_{2}O + CO_{2}$
In contrast,$Na_{2}CO_{3}$ is more ionic and stable,requiring a much higher temperature for decomposition.