Anode-Free Sodium Metal Batteries: From Materials Design to System-Level Integration.

Anode-free sodium metal batteries (AFSMBs) offer a lithium-independent route to high-energy, low-cost storage, but their deployment is constrained by limited sodium inventory and interfacial instability. Without excess sodium, small inefficiencies in nucleation, interphase formation, and cross-electrode reactions accumulate into irreversible capacity loss, and advances in materials design frequently fail to survive practical conditions. Here we establish a mechanism-informed framework that resolves the sodium inventory into reversible, dead, and chemically bound fractions, each tied to a measurable quantity, and apply it across current collectors, electrolytes, cathodes, separators, and solid-state architectures. Because Coulombic efficiency reports only the sum of two loss channels that scale differently, we derive the efficiency required for practical operation rather than assuming it: 500 cycles at N/P = 0 demands an average above 99.95%. Screened against lean-electrolyte and high-areal-capacity criteria, few reported systems approach this threshold, and most omit the electrolyte loading at which their efficiency is obtained. We identify operando diagnostics and quantitative morphological descriptors as the missing tools for predictive design. AFSMBs are therefore integration-limited rather than materials-limited, and progress depends on translating interfacial control into manufacturable cell architectures.