Morphology and facet effects on the charge and discharge mechanisms in FeSe2-based lithium-ion storage

Abstract

Small and large FeSe₂ spheres with {111}-bound triangular protrusions, plus stars and {011}-exposed bars, have been synthesized and used as an anode material for lithium-ion batteries. Crystal facets regulate electrical conductivity and influence ion diffusion. A long-term cycling test at 1 A g⁻¹ revealed four stages with large electrochemical capacity changes. Small spheres with a combination of high electrical conductivity and a relatively large surface area exhibited the best capacity stability at current densities of 1 and 10 A g⁻¹. Through synchrotron X-ray diffraction (XRD) analysis, a novel reaction mechanism for lithium ion storage with FeSe₂ quickly converted to FeSe and then to Fe and Li₂Se during discharging was identified. At different stages, the Li₂Se lattice constant varies slightly, suggesting a correlation between crystal lattice and lithium-ion insertion and extraction. Cyclic voltammetry analysis showed variations in the reduction peak contributions at different stages. This study illustrates that control of crystal face, electrical conductivity, and surface area are critical to the optimization of battery performance.