A robust malic acid-assisted displacement reaction to form carbon-coated submicron FeSn2 with superior lithium storage reversibility enabled by the solid solution effect

Abstract

Tin (Sn) has emerged as a promising anode for lithium-ion batteries (LIBs) with a high theoretical capacity of 994 mAh g⁻¹. However, conventional Sn-based anodes suffer from severe volume expansion and Sn coarsening issues, leading to rapid capacity degradation. Here, a robust malic acid (MA)-assisted displacement reaction route is developed to controllably fabricate carbon-coated submicron FeSn₂ and micron Sn particles (denoted as FeSn₂ SMPs@C and Sn MPs@C) by adjusting the molar ratio of Fe nanoparticles to Sn²⁺ ions (1 : 2 and 1 : 4, respectively). Notably, FeSn₂ SMPs@C delivers exceptional cycling stability and rate capability, delivering a reversible capacity of 1053 mAh g⁻¹ after 500 cycles at 1000 mA g⁻¹, far surpassing that of Sn MPs@C (56 mAh g⁻¹). Beyond the particle size effect, the underlying mechanisms of the solid solution effect induced superiority of FeSn₂ over Sn are elucidated in detail. In situ XRD measurements reveal that Sn MPs@C undergoes a complex multi-phase transformation involving various Li_xSn (0 < x ≤ 4.4) intermediates, resulting in sluggish kinetics and inhomogeneous stress distribution. In contrast, FeSn₂ SMPs@C exhibits reversible weakening and strengthening of FeSn₂ diffraction peaks during the discharging and charging processes, respectively, indicating the reversible decomposition–recovery of the FeSn₂ phase and the formation of an amorphous Li-FeSn₂ solid solution intermediate. Furthermore, the small particle size, homogeneous carbon coating, and abundant Fe/Li_xSn interfaces generated within the cycling electrode of FeSn₂ SMPs@C effectively facilitate enhanced capacitive charge storage. First-principles investigations unveil that Fe incorporation in FeSn₂ accelerates electron transfer and reduces Sn and Li adsorption energies, signifying improved electronic conductivity, diminished Sn coarsening and augmented delithiation kinetics. This study highlights the effectiveness of the MA-assisted displacement reaction in synthesizing high-performance carbon-coated Sn-based intermetallics and underscores the critical role of the solid solution effect in enhancing performance of alloy electrodes for alkali-ion batteries.