Publication date: 15th December 2025
Sodium-ion batteries are attracting attention as cost-effective and sustainable energy-storage systems. Their performance—including energy density, power capability, cycle life, and safety—depends strongly on the choice and design of cathode active materials (CAMs). Among CAM families, polyanionic compounds are particularly versatile due to their structural diversity and intrinsic stability. These materials, generally represented as NaxMy(XzO₃z₊₁), combine redox-active transition metals with various polyanion groups. The strong covalent character and high electronegativity of these anions enhance the inductive effect, stabilizing the Na–O framework and enabling relatively high operating voltages. The flexibility to incorporate different transition metals and polyanion types—including phosphates, pyrophosphates, silicates, carbonates, and sulfates—provides multiple avenues for tuning electrochemical properties.
Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a representative polyanionic cathode, notable for its earth-abundant composition and stable framework. Its structure of PO₄³⁻ and P₂O₇⁴⁻ groups forms one-dimensional Na⁺ diffusion channels, allowing reversible sodium intercalation. This architecture supports a theoretical capacity of 129 mAh g⁻¹ at an average voltage of ~3.0 V vs. Na⁺/Na. Partial substitution of Fe by Mn has been investigated in recent studies. While Mn substitution appears to enable participation of the Mn²⁺/Mn³⁺ redox couple and may influence the operating voltage, its effects on Na⁺ mobility, capacity, rate performance, and long-term structural stability remain uncertain and require further systematic study.
Synthesis conditions remain a critical factor in determining phase purity and electrochemical performance. Solid-state preparation can lead to secondary phases, emphasizing the importance of precursor composition, reaction control, and carbon-coating strategies. These considerations highlight the broader need for careful compositional and structural design when developing NFPP-based polyanionic cathodes—whether pristine or Mn-substituted—for sodium-ion battery applications.
This work received financial support from the German Federal Ministry of Education and Research (BMBF) under the 4NiB project (03XP0572A).
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