Low-Electromotive Force-Driven Sodium Compensation for Optimizing Na Deposition in Rechargeable Sodium Batteries

Sufficient active Na⁺ and stable electrode-electrolyte interfaces enable anode-free sodium batteries to achieve high energy densities and long operational lifespan. Here, we establish the critical role of low electromotive force (EMF) in promoting uniform Na deposition and stable interfacial chemistry by minimizing overpotential and mitigating steep concentration gradients. We accordingly designed a versatile cathode, Na-replenished P2-type oxides (NRP2), with reversible overstoichiometric Na-ion insertion that incorporates sodium compensation and promotes Na leveling deposition. By releasing the prestored Na-ions at a low voltage, the NRP2 cathode supplements the loss of active Na⁺ during the initial formation of the solid electrolyte interphase (SEI). Simultaneously, it creates a low EMF to facilitate uniform and dense metal deposition at the anode surface. The deposited Na metal shows a regular two-dimensional polygonal morphology, in contrast to the dendritic morphology with the high EMF condition. As a result, the rechargeable Na-metal batteries that employ the NRP2-50 demonstrate a capacity retention of 90.1% after 500 cycles and capacity retention of 97.4% over 50 cycles in an anode-free geometry.