Synergy of Li₂CO₃ promoters and Al-Mn-Fe stabilizers in CaCO₃ pellets enables efficient direct solar-driven thermochemical energy storage

Calcium carbonate (CaCO₃) pellets are suitable for scalable solar thermochemical energy storage but suffer from low solar absorptance, poor stability, and slow reaction kinetics, which lead to low solar energy storage efficiency. Here, for the first time, we successfully achieve fast and efficient solar thermochemical energy storage via synergistically employing Al, Mn, and Fe stabilizers for improving both cyclic stability and solar absorptance, and Li₂CO₃ promoters for accelerating decomposition of CaCO₃ pellets. The average solar absorptance is 1500% higher than that of pure CaCO₃ pellets. After 60 cycles, the energy storage density is still as high as 1671 kJ/kg with a decay rate of only 4.14%, in stark contrast to energy density of 1121 kJ/kg and decay rate of 56.73% for pure CaCO₃ pellets. The average decomposition rate is about twice as fast as that of pure CaCO₃ pellets due to enhanced Ca²⁺ diffusion by doped Li₂CO₃. Under direct light irradiation, novel CaCO₃ pellets demonstrate high energy storage rate due to both high solar absorptance and fast decomposition rate, while pure CaCO₃ pellets cannot even reach the decomposition temperature. This work opens new opportunities for scalable deployment of direct solar-driven thermochemical energy storage techniques based on Li₂CO₃-doped dark CaCO₃ pellets.