High-Power and Long-Lifespan Rechargeable Ion Batteries based on Na+-Confined Na+/Mg2+ Coinsertion Chemistry.

Magnesium-sodium hybrid ion batteries (MSHIBs) are expected to achieve excellent rate capability. However, existing MSHIB cathodes exhibit low ionic conductivity and poor structural stability, resulting in low power density and cycle lifespan. Herein, sodium-rich Na_3.7V₆O₁₆·2.9H₂O (Na-rich NVO) nanobelts are synthesized as MSHIB cathodes. Excess Na⁺ induced NaO₅ and NaO₃ interlayer pins, which ensures NVO structural stability to accommodate Mg²⁺ and Na⁺. They also confine the migration pathway of cations to the diffusion direction, lowering the migration barriers of Mg²⁺ and enhancing the ionic conductivity. Excess interlayer Na⁺ increases the electronic conductivity of the involved Na-rich NVO cathode. The cathode exhibits a high Mg²⁺ diffusion coefficient, and the resulting MSHIBs exhibit a power density of 3.4 kW kg^-1 and a lifespan of 20 000 cycles at 5.0 A g^-1, with a capacity retention rate of 85%. Overall, this study paves the way for designing and developing fast-charging secondary batteries.