Phase change materials (PCMs) are promising alternative candidates for a variety of thermal energy storage and related applications. Among the PCMs, alkali carbonates have received particular attention in recent years due to their favourable thermophysical properties at high temperatures. However, the use of such carbonates as PCM requires the understanding of the structural information of the material particularly at both the solid and molten states and the composition dependence of the latent heat, which are largely lacking. Here we study, by both experiments and molecular dynamics simulations, the latent heat of the binary carbonates Na2CO3/Li2CO3. The results show that the binary carbonates can convert to LiNaCO3 within a wide concentration range. A eutectic composition (EC) region of about 51 mol. % Li2CO3 has shown the largest conversion to the LiNaCO3. Any excess loading of Na2CO3 or Li2CO3 beyond the EC region of the carbonates could lower the activation energy of diffusion. The solidification of the carbonates leads to a higher phase change resistance in the EC region with a higher activation energy E-a((Pc)) and a larger latent heat during the phase change.