A Novel Spherical Mg/Sn Co-doped Alluaudite-type Na2+2xFe2-x(SO4)3 Cathode Material for Durable Low-Temperature Sodium-ion Batteries

Abstract

Alluaudite-type Na2+2xFe2-x(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 Na2.48(Fe0.89Mg0.03Sn0.04)1.76(SO4)3 cathode material via 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 cyclability of remaining 91.6% after 1500 cycles at 5C and outstanding reversible capacity of 74.3 and 58.3 mAh g−1 at 10C and even at 50C, respectively. Furthermore, NFMS/C-Sn0.03 cathode demonstrates a high-capacity retention of 95.5% at −5 ℃ and 88.4% at −15 ℃, with a remarkable capacity retention of 93.9% after 1000 cycles at room temperature and 85.5% after 700 cycles at −15 ℃, respectively. The electron paramagnetic resonance (EPR) and atomic force microscopy (AFM) techniques confirmed that the unpaired electronic information 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.