Valence-Modulated Na₄Fe₃(PO₄)₂(P₂O₇) Cathode Tuned by Orbital-Delocalization for Extreme-Temperature Sodium Storage

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Iron-based polyanionic Na4Fe3(PO4)2(P2O7) (NFPP) is a cost-effective cathode material for sodium-ion batteries (SIBs).
• Commercialization of NFPP is limited by slow sodium-ion kinetics and poor capacity utilization.
• Existing NFPP frameworks exhibit sluggish Na+ diffusion and insufficient activation of specific sodium sites.

Purpose of the Study:

• To address the limitations of NFPP by enhancing its electrochemical performance.
• To improve sodium-ion kinetics and capacity utilization in polyanionic cathodes.
• To develop a strategy for creating high-performance, durable, and high-power SIBs.

Main Methods:

• A valence modulation strategy using high-valence Mo6+ doping to stabilize the lattice and reduce Na+ diffusion barriers.
• Utilizing the partially filled 3d orbitals of molybdenum for enhanced electron delocalization and conductivity.
• Employing a reversible Mo4+/Mo6+ redox couple for charge compensation, enabling complete Na+ extraction/insertion and suppressing structural distortion.

Main Results:

• The optimized Na4Fe2.91Mo0.09(PO4)2(P2O7) cathode achieved a high discharge capacity of 130.74 mAh g-1 at 0.1 C.
• Demonstrated exceptional cycling stability with 87.23% capacity retention after 10,000 cycles at 50 C.
• Exhibited stable operation across a broad temperature range (-40 to 60 °C).

Conclusions:

• The orbital-delocalization assisted valence modulated strategy effectively enhances NFPP cathode performance for SIBs.
• Molybdenum doping provides synergistic reinforcement, improving Na+ kinetics, electronic conductivity, and structural stability.
• This approach establishes a universal paradigm for developing high-performance polyanionic cathodes, advancing durable and high-power SIB technology.