MnSn2 negative electrodes for Na-ion batteries: a conversion-based reaction dissected

Abstract

To date, the most common negative electrodes used in Na-ion batteries are based on hard carbons, offering around ca. 250 mAh g⁻¹ gravimetric capacity but only 400 mAh cm⁻³ volumetric capacity due to their low density. Negative electrode materials based on intermetallics could outperform this with both a higher gravimetric capacity (>400 mAh g⁻¹) and a higher volumetric capacity (>1000 mAh cm⁻³) but often struggle with cycling stability. Here MnSn₂ is investigated as an electrode material for Na-ion batteries for the first time and delivers 400 mAh g⁻¹ for over 50 cycles, by far outperforming its parent (Sn) in terms of cycling stability. The 1^st cycle and the 10^th cycle of the Na/MnSn₂ reaction are probed using electrochemical methods and operando XRD to reveal the formation and ageing reaction mechanisms. It is shown that MnSn₂ benefits from a robust reaction mechanism where all features seen in the 1^st cycle (insertion into MnSn₂, formation of Na₁₅Sn₄, Na_15−xSn₄, Na₇Sn₃ and MnSn₂ reformation) are still visible in the 10^th cycle, explaining the cycling stability.