Cross‐System Recycling of Spent Layered Oxides Boosts Synergistic Optimization of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Cathodes via Site‐Selective Ni/Mn Co‐Doping

ABSTRACT To address the economic inviability of recycling spent sodium‐ion batteries (SIBs) and the intrinsic drawbacks of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) cathodes, a “cross‐system recycling” strategy is proposed to transform the waste layered oxides cathodes (NaNi 1/3 Fe 1/3 Mn 1/3 O 2 , NFM) into high‐performance polyanionic cathodes (Na 4 Fe 2.9 Ni 0.05 Mn 0.05 (PO 4 ) 2 P 2 O 7 , NFNMPP). Nickel and manganese in the waste NFM that degrades due to “sodium deficiency” and irreversible phase transitions are leached out and doped into the NFNMPP lattice. Benefiting from the synergistic optimization of NFNMPP via site‐selective Ni/Mn co‐doping, the impurities and [P 2 O 7 ] distortion are suppressed, while the conductivity and Na + diffusion kinetics are enhanced. Consequently, the high‐purity NFNMPP (91.2%) exhibits an ideal initial coulombic efficiency of 92.84%, remarkable rate capability (75.21 mAh g −1 at 50 C), and excellent cycling stability (86.74% capacity retention after 12 000 cycles at 20 C). Furthermore, paired with hard carbon (HC) anode, NFNMPP//HC pouch cells exhibit high safety, a record‐long cycling life of 5557 cycles, 82.37% capacity retention at 30 C, and a wide operating temperature range (−40°C to 55°C), demonstrating great potential for energy storage applications and powering low‐speed electric vehicles. This study presents a viable pathway for optimizing cathode materials by recycling spent SIBs, and facilitates its practical applications validated by Ah‐level pouch cells.