Abstract: The low cost and high theoretical capacity of Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) make it an ideal candidate material for sodium-ion battery cathodes; however, the presence of NaFePO₄, which is electrochemically inactive and exhibits poor electronic conductivity, is a common issue during synthesis, limiting the application of NFPP. Herein, we report a synergistic La-Ce dual-doping strategy that simultaneously addresses both critical challenges by engineering the electronic structure and stabilizing the crystal lattice of NFPP. First-principles density functional theory (DFT) simulations indicate the La³⁺ and Ce³⁺ ions preferentially substitute Fe sites in the NFPP lattice. This substitution suppresses the nucleation and growth of NaFePO₄ impurities effectively via modification of the local thermodynamic environment, while simultaneously bringing about substantial changes in the electronic structure. Specifically, the hybridization between La/Ce 4f orbitals and Fe 3d-O 2p orbitals dramatically narrows the band gap, while simultaneously strengthening the Fe-O covalent bonds to enhance structural stability. Na₄Fe_2.91La_0.03Ce_0.03(PO₄)₂P₂O₇ (La-Ce-NFPP) exhibited a high-rate capacity of 92.47 mAh g^-1 at 20C, and reached 93.76% capacity retention after 3000 cycles at 10C. La-Ce dual-doping reduces the unit cell volume variation from 4.63% (NFPP) to 3.05% (La-Ce-NFPP), as evidenced by in situ XRD testing, this confirms that the doping optimizes Na⁺ diffusion pathways and mitigates structural strain during phase transitions. Furthermore, the La-Ce-NFPP//hard carbon full cell shows superior cycling stability, achieving 94.9% capacity preservation after 200 cycles, demonstrating great potential for practical applications.

Key words: Sodium-ion batteries, Cathode material, Na₄Fe₃(PO₄)₂P₂O₇, La-Ce dual-doping, Electrochemical performance