Toward unlocking the Mn3+/Mn2+ redox pair in alluaudite-type Na2+2zMn2−z(SO4)3−x(SeO4)x cathodes for sodium-ion batteries

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 (XO4m+). To manipulate the operating potential, we synthesized a series of mixed sulfate-selenate alluaudites, with structure formulas Na2+2zMn2-z(SO4)3−x(SeO4)x and Na2.81Ni1.60(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. Na2.81Ni1.60(SO4)1.43(SeO4)1.57 is isostructural to the Mn-based alluaudites. Although the Na2+2zMn2-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 Na2+2zMn2−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.