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Introducing oxygen into Na metal by thermal annealing is implemented for the first time to reduce its chemical activity and to yield a thin and homogeneous SEI. Such oxygen-regulated spontaneous SEI provides a more homogeneous plating/stripping process that inhibits the formation of interfacial voids. The ultra-stable cycle lifespan of symmetric cell (over 6600 h) is achieved and the full cell exhibits excellent cycle performance over 500 cycles at 0.5C without significant capacity degradation. [Display omitted] Solid-state sodium metal batteries utilizing inorganic solid electrolytes (SEs) hold immense potentials such as intrinsical safety, high energy density, and environmental sustainability. However, the interfacial inhomogeneity/instability at the anode-SE interface usually triggers the penetration of sodium dendrites into the electrolyte, leading to short circuit and battery failure. Herein, confronting with the original nonuniform and high-resistance solid electrolyte interphase (SEI) at the Na-Na 3 Zr 2 Si 2 PO 12 interface, an oxygen-regulated SEI innovative approach is proposed to enhance the cycling stability of anode-SEs interface, through a spontaneous reaction between the metallic sodium (containing trace amounts of oxygen) and the Na 3 Zr 2 Si 2 PO 12 SE. The oxygen-regulated spontaneous SEI is thin, uniform, and kinetically stable to facilitate homogenous interfacial Na+ transportation. Benefitting from the optimized SEI, the assembled symmetric cell exhibits an ultra-stable sodium plating/stripping cycle for over 6600 h under a practical capacity of 3 mAh cm−2. Quasi-solid-state batteries with Na 3 V 2 (PO 4) 3 cathode deliver excellent cyclability over 500 cycles at a rate of 0.5 C (1 C = 117 mA cm−2) with a high capacity retention of 95.4%. This oxygen-regulated SEI strategy may offer a potential avenue for the future development of high-energy-density solid-state metal batteries. [ABSTRACT FROM AUTHOR]

The pursuit for high‐energy and intrinsically safe energy storage is significantly driving the development of solid‐state alkali metal batteries. The interfacial contact between the metal anode and the solid electrolyte plays a key role in enabling stable cycling of solid batteries. However, the sluggish alkali atom replenishment rate during stripping unavoidably leads to the interface deterioration that destroys the initial physical contact by forming interfacial voids and triggering the dendrite growth. Herein, a hybrid bulk Na anode approach is proposed by incorporating an ion‐conducting phase into a metallic Na matrix, constructing an abundant interfacial electrochemical reaction area and enabling a balanced Na replenishment and consumption to minimize cycle‐induced interface deterioration. Specific attention is paid to the effects of the second phase on the wettability and creep ability of hybrid Na metal. A high critical current density (3.1 mA cm‐2) and long cycling life (6000 h in 0.5 mA cm‐2) are achieved for the symmetric cells. Full cells coupling the hybrid anode with the Na3V2(PO4)3/C cathode deliver excellent cyclability over 7300 cycles at a high rate of 5 C. The viewpoint of balancing the consumption and replenishment rate of metal atoms paves a new way for designing cycle‐stable anode/electrolyte interface in solid‐state batteries. [ABSTRACT FROM AUTHOR]