A novel spherical Mg/Sn co-doped alluaudite-type Na_2+2<i>x</i>Fe_2−<i>x</i>(SO₄)₃ cathode material for durable low-temperature sodium-ion batteries

Alluaudite-type Na_2+2<i>x</i>Fe_2-<i>x</i>(SO₄)₃ has been a promising cathode material for sodium-ion batteries (SIBs) due to its high operating voltage and stable structure. However, its actual electrochemical performance suffers from intrinsic sluggish kinetics and poor electronic conductivity. In this work, for the first time, we propose a Na_2.48(Fe_0.89Mg_0.03Sn_0.04)_1.76(SO₄)₃ cathode material prepared <i>via</i> a Mg/Sn co-doping strategy. Inactive Mg²⁺ stabilizes the structure, while Sn⁴⁺ inhibits the decomposition of electrolytes under high voltage. The Mg/Sn co-doping strategy enhances the kinetics of sodium ion diffusion reactions, leading to improved electrochemical properties, especially at low temperatures. The optimal NFMS/C-Sn0.03 cathode exhibits a long-term capacity retention of 91.6% after 1500 cycles at 5C and outstanding reversible capacities of 74.3 and 58.3 mAh g^-1 at 10C and even at 50C, respectively. Furthermore, the NFMS/C-Sn0.03 cathode demonstrates a high capacity retention of 95.5% at -5 °C and 88.4% at -15 °C, with a remarkable capacity retention of 93.9% after 1000 cycles at room temperature and 85.5% after 700 cycles at -15 °C, respectively. Electron paramagnetic resonance (EPR) and atomic force microscopy (AFM) techniques confirmed that the presence of unpaired electrons and enhanced electronic conductivity could be attributed to the Mg/Sn co-doping. This work provides a feasible approach for designing low-cost, durable, low-temperature, and high-performance cathode materials for SIBs.