Crystal Void Fraction-Engineered Fe-S Catalysts for Self-Sustaining Li-CO₂ Mars Batteries

Efficient energy storage is vital for self-sustaining Martian exploration. Li-CO₂ batteries are promising by utilizing the Martian atmosphere (∼95% CO₂) as active materials. Fe-S minerals, abundant on Mars, offer a viable candidate for cathode catalysts, yet their structural diversity necessitates a rational selection criterion. Here, we propose crystal void fraction as a governing descriptor correlating with affinity toward critical oxygen-containing species, Li₂CO₃ and singlet oxygen (¹O₂). Higher void fraction with decreased Fe-S₆ octahedra packing density upshifts the d-band center and brings the z-containing orbitals closer to the Fermi level. Given the pronounced O-2p_z character of Li₂CO₃ band-edge states and the π* orbital of ¹O₂ frontier orbital, symmetry matching along surface orbitals with z-directional components strengthens orbital coupling, correlating higher crystal void fractions with increased affinity for oxygen-containing species. Crucially, this affinity exhibits a dual role. High void fraction promotes Li₂CO₃ decomposition but ¹O₂-induced catalyst degradation, while low void fraction exhibits the opposite tendency. Marcasite with moderate void fraction achieves an optimal balance, achieving 88% energy efficiency and 1000 h cycle life. This work establishes crystal void fraction as a predictive metric for screening suitable catalysts for achieving activity-stability trade-off, and provides a promising landscape for in-situ resource utilization on Mars.