Na-Fe-P Stoichiometry-Driven Heterostructure Design With a P₂O₇-Rich Buffering Phase for Stable NASICON-Type...

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion batteries (SIBs) are promising for grid-scale energy storage due to the abundance of sodium.
• NASICON-type cathodes, like Na₄Fe₃(PO₄)₂P₂O₇ (NFPP), offer low cost and good structural properties.
• A key challenge for NFPP is the formation of inactive maricite NaFePO₄ (m-NFP) during synthesis, hindering electrochemical performance.

Purpose of the Study:

• To develop a synthesis strategy for NFPP that suppresses the formation of electrochemically inactive phases.
• To enhance the electrochemical performance and cycling stability of NFPP-based cathodes for SIBs.
• To establish a structure-property relationship for NASICON-type materials through controlled stoichiometry.

Main Methods:

• Stoichiometry engineering to induce in situ formation of Na_3.12Fe_2.44(P₂O₇)₂ (N_3.12F_2.44PO) within NFPP.
• Fabrication of a heterogeneous NFPP/N_3.12F_2.44PO composite cathode.
• Electrochemical testing, including long-term cycling at high rates (40C), to evaluate performance.
• Phase evolution mapping across a controlled Na-Fe-P compositional window.

Main Results:

• The heterogeneous NFPP/N_3.12F_2.44PO composite effectively suppressed m-NFP formation.
• The optimized heterostructure demonstrated improved reversible capacity and remarkable structural robustness.
• An ultralong cycling life of 10,000 cycles at 40C with 83.6% capacity retention was achieved.
• Na and Fe content were identified as key factors governing the formation of the beneficial N_3.12F_2.44PO phase.

Conclusions:

• Stoichiometry engineering provides a viable route to create heterogeneous NASICON-type cathodes for advanced SIBs.
• The N_3.12F_2.44PO phase acts as a structural buffer, enhancing cycling durability.
• This approach offers a cost-effective method to improve NFPP performance without additional elements or synthetic steps.