Optimized Small Polaron Hopping Enhances the Conductivity Barrier of Polyanionic Frameworks

ABSTRACT The low intrinsic electronic conductivity of polyanionic cathodes, caused by electronic localization of isolated anionic groups, restricts the fast‐charging performance of sodium‐ion batteries. To address this challenge, we strategically introduce low‐valence Cs + into Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) to enhance small polaron hopping and electronic conductivity. The Cs + triggers a charge‐compensation effect that pre‐stabilizes Fe 3+ species and establishes an Fe 3+ reservoir in NFPP. This configuration creates a built‐in electric field that introduces electron‐relaying states above the valence‐band maximum, significantly lowering the small‐polaron hopping barrier between Fe 2+ /Fe 3+ couples. Crucially, Hall‐effect measurements directly validate this mechanism, revealing enhanced carrier mobility and concentration that overcompensate for conventional doping strategies with reduced carrier mobility trade‐off due to carrier scattering from doped foreign ions. Consequently, the optimized NFPP‐Cs cathode overcomes carrier transport bottlenecks and hence enhances charge storage kinetics, achieving an 80% state‐of‐charge in just 7.9 min at 5 C and achieving full charge in 58 s at 50 C with a high specific capacity of 81.0 mAh g −1 . Distribution of relaxation times analysis further substantiates that this charge‐compensation‐driven small polaron hopping constitutes the core mechanism for the enhanced electrochemical reaction kinetics. This work provides a compelling paradigm for designing polyanionic frameworks with enhanced reaction kinetics.