Abstract
Replacement of graphite with alloying and conversion materials, having high specific capacity, has emerged as versatile route to increasing the energy density of Li-ion batteries. A key challenge is the large volume change in these materials, which leads to an unstable solid electrolyte interphase (SEI). The use sacrificial electrolyte additives, such as fluoroethylene-carbonate (FEC), has been established as an effective strategy for considerably improving cycling stability, but a mechanistic understanding of the underlying processes has been lacking so far. Here, we present an in-depth chemical and morphological study of the FEC-based interphase on graphite and SnO2–graphite model electrodes. We found that the FEC decomposition products aggregate first into spherical particles, whose growth depends on the cell medium and follows the laws of crystal-growth theory, before forming a continuous carbonate-rich film. The discrimination of the chemical composition of the FEC-derived particles from the rest of the electrode was obtained by X-ray photoemission electron microscopy (XPEEM) due to the high lateral resolution of this technique. The obtained understanding of SEI formation in fluorine-rich electrolytes should help to guide future designs of sacrificial fluorine-based additives.
Highlights
Lithium-ion batteries (LIBs) have been used as power sources for portable electronics for about 30 years
A detailed study of the morphological and chemical changes, occurring during FEC decomposition in the 1st cycle revealed that FEC, a commonly used electrolyte additive for anodes, at first decomposes into spherical particles and only afterwards the more familiar layer-like solid electrolyte interphase (SEI) is formed
We find that the properties of the active and passive cell components, such as electrolyte salt, solvent and electrode material chemistry, all have a major role on defining the final morphology of the FEC reduction products
Summary
Lithium-ion batteries (LIBs) have been used as power sources for portable electronics for about 30 years. New applications, such as electric mobility, and the need for more energy from smaller volumes for portable devices necessitate the development of LIBs with higher energy density and cycle life than offered by the current generation of commercial batteries. Intensive work has been devoted to alloying and conversion anode materials such as Si, SiOx, Sn, and SnO2 [1,2,3,4,5,6,7,8], whose high theoretical capacity, on the order of thousands of mAh g−1, promises to increase the overall energy density of LIBs [9]. This, in turn, results in an increase of the overall cell overpotential, low coulombic efficiency (CE), and rapid capacity fading
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