Effects of HF on the Lithiation Behavior of the Silicon Anode in LiPF₆ Organic Electrolyte Solution

As one of the major impurities in the organic electrolyte, HF can react with the alkali components in the solid electrolyte interphase (SEI), such as lithium alkoxide and lithium carbonate, to form more LiF-rich SEI. Here, the effects of HF on the lithiation behavior of the single crystal Si(111) anode were studied using scanning electron microscopy, soft X-ray emission spectroscopy, and windowless energy-dispersive X-ray spectroscopy. When the Li-Si alloy is formed in 1.0 M LiPF₆ in the propylene carbonate solvent, it has a layered structure that contained the first layer of crystalline Li₁₅Si₄ (c-Li₁₅Si₄) alloy pyramids, the second layer of amorphous Li₁₃Si₄ (a-Li₁₃Si₄) alloy, and a third layer of Li-diffused Li Si alloy. When the more concentrated HF is in the electrolyte solution, less amount of the c-Li₁₅Si₄ alloy is formed in the first layer. It suggests that the Si lithiation can form only amorphous Li Si alloy relative to the components in the electrolytes. The study also explains why only amorphous Li Si alloy formation was observed in some previous studies.