The reaction mechanism of FeSb2 as anode for sodium-ion batteries

Abstract

The electrochemical reaction of FeSb₂ with Na is reported for the first time. The first discharge (sodiation) potential profile of FeSb₂ is characterized by a gentle slope centered at 0.25 V. During charge (Na removal) and the subsequent discharge, the main reaction takes place near 0.7 V and 0.4 V, respectively. The reversible storage capacity amounts to 360 mA h g⁻¹, which is smaller than the theoretical value of 537 mA h g⁻¹. The reaction, studied by ex situ and in situ X-ray diffraction, is found to proceed by the consumption of crystalline FeSb₂ to form an amorphous phase. Upon further sodiation, the formation of nanocrystalline Na₃Sb domains is evidenced. During desodiation, Na₃Sb domains convert into an amorphous phase. The chemical environment of Fe, probed by ⁵⁷Fe Mössbauer spectroscopy, undergoes significant changes during the reaction. During sodiation, the well-resolved doublet of FeSb₂ with an isomer shift around 0.45 mm s⁻¹ and a quadrupole splitting of 1.26 mm s⁻¹ is gradually converted into a doublet line centered at about 0.15 mm s⁻¹ along with a singlet line around 0 mm s⁻¹. The former signal results from the formation of a Fe-rich Fe_xSb alloy with an estimated composition of ‘Fe₄Sb’ while the latter signal corresponds to superparamagnetic Fe due to the formation of nanosized pure Fe domains. Interestingly the signal of ‘Fe₄Sb’ remains unaltered during desodiation. This mechanism is substantially different than that observed during the reaction with Li. The irreversible formation of a Fe-rich ‘Fe₄Sb’ alloy and the absence of full desodiation of Sb domains explain the lower than theoretical practical storage capacity.