Modulating Cation Intermixing Behavior Enables Wide-Temperature-Stable Na_2+2<i>x</i>Fe_2-<i>x</i>(SO₄)₃ Cathode for Sodium-Ion Batteries.

Abstract

Sodium-ion batteries hold promise for grid-scale energy storage thanks to abundant resources and superior safety, but their wide-temperature operation is hindered by sluggish electronic-ionic transport and structural instability of cathode materials. Herein, a cation-intermixing strategy driven by stoichiometric regulation is proposed for Na_2+2<i>x</i>Fe_2-<i>x</i>(SO₄)₃ cathodes, which can simultaneously enhance structural stability, improve charge transfer, and facilitate Na⁺ transport kinetics. Specifically, derived Fe vacancies and concomitant Na⁺ insertion reconstruct the electronic environment, strengthening Fe-O bonds to stabilize the crystal framework while optimizing Fe 3d electron energy level distribution to facilitate charge transfer. This alteration concurrently widens Na⁺ migration channels and reduces diffusion barriers, enabling rapid ion transport. Consequently, the Na_2.48Fe_1.76(SO₄)₃ cathode (<i>x</i> = 0.24 in Na_2+2<i>x</i>Fe_2-<i>x</i>(SO₄)₃, with a Na/Fe molar ratio of 1.4) with optimal cation intermixing exhibits exceptional wide-temperature performance. It delivers 85.9% capacity retention following 3000 cycles at 30 C (25 °C) and 88.3% following 4000 cycles at 1 C (-20 °C). Even at an ultrahigh 100 C (60 °C), it still retains 83.2% relative to its capacity measured at 25 °C and 0.1 C. This work provides a stoichiometry-driven approach to designing superior-performance sulfate-based cathodes for wide-temperature sodium-ion batteries.