Resolving the Origins of Dynamic Match Mechanism in Nonstoichiometric Phosphates for Mg Batteries

Magnesium-based hybrid battery systems have been regarded as promising systems for electrochemical energy storage because they leverage the rapid ion transport capabilities of flexible cathodes and the intrinsic safety of dendrite-free Mg anodes. However, their further development is restricted by the strong Coulombic interaction of Mg 2+ and dynamic mismatch issues in cathode materials. Herein, a novel nonstoichiometric Na 4− x Fe 2+ x /2 (P 2 O 7 ) 2 @C cathode is proposed for a Mg-based hybrid system to construct a Mg 2+ /Na + dynamic equilibrium battery. The cathode modulated at the atomic level exhibits a working voltage plateau of 2.1 V and a discharge capacity of 115 mAh/g, with favorable cost performance. Furthermore, multidimensional electrochemical and structural analysis were employed to verify the functional utility of nonstoichiometric composition in enhancing the kinetics and to clarify the origin of the kinetic compatibility of hybrid battery systems. This work offers new insights and structural modulation protocols for developing and applying polyanionic cathodes for Mg-based batteries.