Accessing Four‐Sodium Storage in Na <i> _x </i> V ₂ (PO ₄ ) ₃ (1≤ <i>x</i> ≤5) Cathodes via Precise Chemical Pre‐Sodiation Toward High‐Energy and Long‐Life Anode‐Free Sodium Batteries

Abstract

ABSTRACT 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 Na 3 V 2 (PO 4 ) 3 (Na 3 VP) into Na‐saturated Na 5 V 2 (PO 4 ) 3 (Na 5 VP). Through voltage‐controlled sodium compensation, chemically sodiated Na 5 VP 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 (Na 5 VP→Na 3 VP)||carbon‐coated Al (C@Al) delivers an exceptional 1000‐cycle lifespan with a decent energy density of 334 Wh kg −1 , markedly outperforming the Na 3 VP counterpart (305 Wh kg −1 and a 10‐cycle lifespan). Particularly, the (Na 5 VP→Na 4 VP)||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.