Nonstoichiometric Na3.1Fe3(PO4)2P2O7 polyanion for sodium-ion batteries: Electrochemical performance in an organic electrolyte

The pure sodium-deficient Na 3.1 Fe 3 (PO 4 ) 2 P 2 O 7 (Na 3.1 FPP) composition, encapsulated in amorphous carbon, is synthesized via the gel-combustion route. Sodium deficiency results from charge compensation by Fe 3+ ions (approximately 33%) arising from ammonium-ferric complexes formed during synthesis. The presence of Fe 3+ does not limit the specific capacity of the Na 3.1 FPP cathode in a half-cell, as these ions are electrochemically reduced during discharge and participate in further cycling. This is followed by an additional insertion of Na ions into vacant sites, which improves structural ordering and plateau-related performance. As a result, a capacity of approximately 120–135 mAh g −1 is achieved at current densities of 1C-C/3. Still, Fe 3+ introduces local structural disorder, especially at high states of charge, leading to lattice instability and gradual capacity fade during prolonged cycling. This results in a capacity drop of 0.05% per cycle at 5C across 1000 cycles. However, the reduction of voltage limits prevents pronounced distortion, thus enabling lower capacity fade (0.018% per cycle at 1C over 1000 cycles). Detailed electrochemical characterization reveals two distinct capacity-loss regimes: deactivation of the surface-distorted regions associated with Na4 sites and subsequent gradual deactivation of the bulk crystalline phase along the Na1-Na4-Na3 sequence, with both processes proceeding in parallel beyond a certain level of cycling depth. Furthermore, we propose a revised XPS fitting methodology for this family of mixed polyanions. The results provide a design perspective for the third generation of Na-ion battery cathodes.