Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at higher temperature?
As we move down the alkali metal group, the electropositive character increases. This causes an increase in the stability of alkali carbonates. However, lithium carbonate is not so stable to heat. This is because lithium carbonate is covalent. Lithium ion, being very small in size, polarizes a large carbonate ion, leading to the formation of more stable lithium oxide.
On Heating:
Li2CO3 → Li2O + CO2