Polyanion-Mixed Off-Stoichiometric Alluaudites Na _3−δ Fe _2±β (PO ₄ ) _<i>y</i> (SO ₄ ) _{3– <i>y</i>} as Sustainable Positive Electrode Materials for Na-Ion Batteries

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 Na2+2δFe2−δ(SO4)3 that exhibits excellent cycling performance inspired the investigation of mixed PO43–/SO42– 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, Na3PO4 and FeSO4. 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 Na2.65Fe2PO4(SO4)2 along with Na6Fe(SO4)4, prior to alluaudite formation. Physicochemical investigations of the alluaudite Na2.65Fe1.9(PO4)y(SO4)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/cm2. 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.