Toward unlocking the Mn3+/Mn2+ redox pair in alluaudite-type Na2+2z Mn2−z (SO4)3−x (SeO4) x cathodes for sodium-ion batteries

Highlights: • Na_2+2zM_2−z(SO₄)_3−x(SeO₄)_x (M = Mn, Ni) compounds were synthesized by soft chemistry method. • The crystal structure was solved from X-ray powder diffraction data. • All prepared compounds demonstrate an alluaudite related crystal structure. • Mn³⁺/Mn²⁺ redox pair can be activated in Na-ion cell. -- Abstract: In polyanion cathodes, the inductive effect alters the potential of a M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺ redox couple (M − transition metal) according to the electronegativity of the X cation in the polyanion groups (XO₄^m+). To manipulate the operating potential, we synthesized a series of mixed sulfate-selenate alluaudites, with structure formulas Na_2+2zMn_2-z(SO₄)_3−x(SeO₄)_x and Na_2.81Ni_1.60(SO₄)_1.43(SeO₄)_1.57. Their crystal structure was determined from powder X-ray diffraction data, revealing that the Mn-based alluaudites form solid solutions with the same crystal structure for x = 0.75; 1.125 and 1.5. Na_2.81Ni_1.60(SO₄)_1.43(SeO₄)_1.57 is isostructural to the Mn-based alluaudites. Although the Na_2+2zMn_2-z(SO₄)_3−x(SeO₄)_x compound with the highest selenium content demonstrates a reversible discharge capacity of 60 mAh g⁻¹, only a small part of this electrochemical activity can be ascribed to the Mn³⁺/Mn²⁺ redox couple. The redox potential of the Mn³⁺/Mn²⁺ pair in Na_2+2zMn_2−z(SO₄)_3−x(SeO₄)_x decreases with increasing values of x, in agreement with the lower electronegativity of Se compared to that of S.