Work

Polyanion-Mixed Off-Stoichiometric Alluaudites Na 3−δ Fe 2±β (PO 4 ) y (SO 4 ) 3– y as Sustainable Positive Electrode Materials for Na-Ion Batteries

Year: 2025

Type: article

Abstract: The transition to renewable energy sources requires cost-effective and scalable energy storage solutions based on abundant elements, such as Na-ion batteries with sustainable positive electrode materials, based on Na, Fe, and S. The sulfate-based alluaudite Na 2+2δ Fe 2−δ (SO 4 ) 3 that exhibits excellent cycling performance inspired the investigation of mixed PO 4 3– /SO 4 2– polyanion-based compounds with a view to increase the phase stability of sulfates. Herein, we report on various synthesis methods, such as solid-state, mechanochemical, and ionothermal treatments, to obtain nonreported until now compositions in the mixed phosphate-sulfate iron sodium alluaudite system, using the cost-effective precursors, Na 3 PO 4 and FeSO 4 . Quite surprisingly, solid-state synthesis followed in situ using the synchrotron X-ray powder diffraction technique revealed the presence of an intermediate phase closely resembling the NaSICON phase Na 2.65 Fe 2 PO 4 (SO 4 ) 2 along with Na 6 Fe(SO 4 ) 4, prior to alluaudite formation. Physicochemical investigations of the alluaudite Na 2.65 Fe 1.9 (PO 4 ) y (SO 4 ) 3– y phase, obtained via solid-state synthesis at 450 °C, confirm that the phosphate incorporation enhanced the thermal stability while preserving promising electrochemical properties, i.e., rate capability and long-term stability with no capacity loss after 50 cycles: a reversible capacity of ≈90 mAh/g is obtained at an average discharge voltage of 3.32 V vs Na + /Na and for an electrode mass loading of 16 mg/cm 2 . This study proposes easy and effective synthesis approaches to obtain series of compounds and opens the perspective to explore conditions of transitions between NaSICON and alluaudite structural types.

Source: ACS Applied Energy Materials

DOI: 10.1021/acsaem.5c02412