Rational Quantification of Deep Sodium Reversibility with a Utilization-Corrected N/P Descriptor toward Low-N/P-Ratio Solid-State Na–S Batteries

Abstract Solid-state Na–S batteries offer high energy density, yet their practicality deteriorates at low N/P ratios because limited Na inventory cannot compensate for irreversible loss. Under lean-Na conditions, the key issue is whether the restricted Na reservoir remains reversibly accessible during deep cycling. Here, we introduce a utilization-corrected N/P descriptor, φ = n/α, where n is the N/P ratio and α is the initial deep-stripping utilization. This descriptor corrects the apparent N/P advantage by accounting for incomplete Na utilization; as α decreases, φ exceeds n, while φ approaching 1 represents the ideal lean-anode limit. A self-generated 3D Na–Na3P/Cu foam anode is constructed via molten-Na conversion. The in-situ-formed Na3P enhances sodiophilicity, Na+ transport, and interfacial stability during deep stripping. The anode achieves 94% stripping utilization with φ = 1.28, sustains over 50 cycles at 90% depth of discharge, and delivers 976 mAh g–1 at 60 °C and N/P = 1.2.