Synergistic Defect Engineering in Ca‐Based Pellets for Durable and High‐Absorptance Solar Thermochemical Energy Storage

Abstract Solar thermochemical energy storage (TCES) holds immense promise for a carbon‐neutral future. However, its large‐scale, low‐cost implementation is severely hampered by the poor solar absorptance, rapid performance degradation, and low volumetric energy density of conventional calcium carbonate/oxide materials. Herein, a multi‐scale synergistic defect engineering strategy is proposed to resolve this conflict fundamentally. Guided by predictive ray‐tracing models and density functional theory calculations that pinpoint dopant d‐electron effects as the key modulator of optical properties, Mn/Mg co‐doped pellets are rationally designed. These engineered pellets demonstrate a suite of outstanding properties: a dramatic leap in solar absorptance from a mere 3.4% to 78.5%, a 1.54‐fold enhanced thermal conductivity, and exceptional cycling durability. Over 123 cycles, the pellets achieve a remarkable average volumetric energy density of 877.9 (2.1 times that of the pristine powder), maintaining a high average of 848.3 over 217 cycles. This unprecedented performance comes from a unique multi‐scale defect synergy, where 0D point defects induce beneficial lattice strain, while in situ formed 3D nano‐precipitates strategically pin 2D grain boundaries to suppress high‐temperature sintering effectively. This work not only delivers a record‐performing TCES material but also establishes a promising, theory‐guided strategy for the intelligent design of next‐generation energy storage materials.