Off‐Stoichiometry in Alluaudite‐Type Sodium Iron Sulfate Na_2+2xFe_2−x(SO₄)₃ as an Advanced Sodium Battery Cathode Material

Abstract

An alluaudite‐type sodium iron sulfate has recently been discovered as a 3.8 V cathode material for low‐cost, high‐power, and efficient sodium‐ion batteries. To optimize the composition of the alluaudite phase and to explore further compounds, we have carefully surveyed the Na2SO4‐FeSO4 binary system. Solid‐state reactions at a moderate temperature of 623 K produce two stable phases: 1) vanthoffite‐structured Na6Fe(SO4)4 and 2) alluaudite‐type Na2+2xFe2−x(SO4)3 with a certain non‐stoichiometry. The possible compositional and structural flexibilities demonstrated in this work inspire further improvement of the alluaudite‐type sodium metal sulfates for advanced sodium‐ion batteries.

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

“…Further investigations on the electrochemical mechanisms and Na diffusion kinetics of off-stoichiometric phases of Na 2+2 x Fe 2-x (SO 4 ) 3 (0 ≤ x ≤ 0.4) were performed by the Yamada and Adam groups. [ 185,186 ] The Mn-based alluaudite Na 2+2 x Mn 2-x (SO 4 ) 3 and solid-solution phase of Na 2.5 (Fe 1-y Mn y ) 1.75 (SO 4 ) 3 were also investigated as cathodes for NIBs. [187][188][189] However, these electrodes exhibited marginal electrochemical activities of Mn 2+ /Mn 3+ in Na-ion cells.…”

Recent Progress in Iron‐Based Electrode Materials for Grid‐Scale Sodium‐Ion Batteries

“…Of particular interest is the alluaudite‐type Na 2 Fe 2 (SO 4 ) 3 material, which possesses edge‐sharing Fe 2 O 10 dimers bridged together by SO 4 units by the corner‐sharing model ( Figure a) . The Na 2 Fe 2 (SO 4 ) 3 material, which turned out to be a more precise phase with the composition of Na 2.4 Fe 1.8 (SO 4 ) 3 , has a C2/c symmetry, with Na + ions occupying three different sites . The electrode showed the highest Fe 2+ /Fe 3+ redox couple voltage (3.8 V) ever reported and was associated with good reversible capacity (102 mAh g −1 at 0.05 C) (Figure b) .…”

Systematic Studies on “Abundant” Battery Materials: Identification and Reaction Mechanisms

“…13). [35][36][37] Deviating sharply from most of the A x M 2 (XO 4 ) 3 type compounds adopting the NASICON-related structures, Na 2 Fe 2 (SO 4 ) 3 does not contain the lantern units [M 2 (XO 4 ) 3 ]. It would be convenient to denote AABBM 2 (XO 4 ) 3 as general alluauditetype compounds, where A = partially occupied Na(2), AB = partially occupied Na(3), B = Na(1), M = Fe 2+ , and X = S in the present case.…”