Accessing Four-Sodium Storage in Na_xV₂(PO₄)₃ (1≤x≤5) Cathodes via Precise Chemical Pre-Sodiation Toward High-Energy...

Anode-free sodium batteries (AFSBs) promise high energy density and low cost but are fundamentally constrained by severe active sodium loss, leading to rapid cell failure. Here, we propose a precise chemical presodiation strategy that unlocks the full 4e-/4Na⁺ redox chemistry of vanadium phosphate cathodes by converting Na-stoichiometric Na₃V₂(PO₄)₃ (Na₃VP) into Na-saturated Na₅V₂(PO₄)₃ (Na₅VP). Through voltage-controlled sodium compensation, chemically sodiated Na₅VP serves as a versatile cathode platform for the flexible design of AFSBs tailored for either ultra-high energy density or exceptional longevity. With two sodium compensations, the (Na₅VP→Na₃VP)||carbon-coated Al (C@Al) delivers an exceptional 1000-cycle lifespan with a decent energy density of 334 Wh kg^-1, markedly outperforming the Na₃VP counterpart (305 Wh kg^-1 and a 10-cycle lifespan). Particularly, the (Na₅VP→Na₄VP)||C@Al cell with one sodium compensation achieves an ultra-high energy density of 430 Wh kg^-1 and a competitive lifespan of 630 cycles, representing a state-of-the-art benchmark. This work establishes precise sodium compensation as a powerful lever for designing high-performance and application-specific AFSBs.