Cation-inspired polyhedral distortion boosting moisture/electrolyte stability of iron sulfate cathode for durable high-temperature sodium-ion storage

Abstract

Alluaudite-type iron-based sulfates are prospective positive-electrode active materials for sodium-ion batteries given their low-cost and high operation voltage, yet suffer from poor intrinsic ionic conductivity and (electro) chemical instability at high temperatures. Herein, a cation-modified Na_2.466Fe_1.724Mg_0.043(SO₄)₃ with micron-sized spherical structure was reported. The substitutive MgO₆ octahedron featured stronger covalent bonding interactions and enriched the ion transfer pathways within the crystals, facilitating the ionic kinetics in bulk. Using in situ mass spectrometry and quartz crystal microbalance techniques, Mg cations were demonstrated to lower the electron density around O atoms and surficial nucleophilicity of materials, which effectively suppressed their side reactions with H₂O in air and active ester molecule in electrolyte. This interaction enables an inorganic-rich and uniform interphase to stabilize the cathode/electrolyte interface at high voltage (4.5 ​V vs. Na⁺/Na). The as-prepared cathode exhibits a high discharge capacity of 102.2 mAh g⁻¹ (voltage platform at 3.74 ​V), remarkable reaction reversibility (average Coulomb efficiency of 99.3 ​% over 300 cycles) at high loading (9.0−9.6 ​mg ​cm⁻²) and temperature (60 ​°C), as well as long-lasting cyclability (70.8 ​%, 5000 cycles). Its application was verified in assembled sodium-ion full cells with a hard carbon negative electrode, showing a long cycling lifetime over 190 cycles.