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Surface-layer formation by reductive decomposition of LiPF6 at relatively high potentials on negative electrodes in lithium ion batteries and its suppression

机译：锂离子电池负极上较高电位的LiPF6还原分解形成的表面层及其抑制

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In using a LiPF6/ethylene carbonate–dimethyl carbonate electrolyte for lithium ion batteries (LIBs), a certain reductive reaction is known to occur at a relatively high potential (ca. 2.6 V vs. Li[+]/Li) on Sn electrode, but its details are still unknown. By means of in-situ X-ray reflectometry, X-ray photoelectron spectroscopy, scanning electron microscopy observations and electrochemical measurements (by using mainly Sn electrode, and additionally Pt, graphite electrodes), we have found out that this reduction eventually forms an inactive passivation-layer consisting mainly of insulative LiF ascribed to the reductive decomposition of LiPF6, which significantly affects the battery cyclability. In contrast, a solid-electrolyte interphase (SEI) is formed by the reductive reaction of the solvent at ca. 1.5 V vs. Li[+]/Li, which is lower than the reduction potential of LiPF6. However, we have found that the formation of SEI preempts that of the passivation layer when holding the electrode at a potential lower than 1.5 V vs. Li[+]/Li. Consequently, the cyclability is improved by suppressing the formation of the inactive passivation layer. Such a pretreatment would be quite effective on improvement of the battery cyclability, especially for a relatively noble electrode whose oxidation potential is between 1.5 V and 2.6 V vs. Li[+]/Li.
机译：在将LiPF6 /碳酸亚乙酯-碳酸二甲酯用于锂离子电池（LIB）时，已知在Sn电极上以相对较高的电位（约2.6 V vs. Li [+] / Li）发生某种还原反应，但其细节仍然未知。通过原位X射线反射法，X射线光电子能谱，扫描电子显微镜观察和电化学测量（主要使用Sn电极，另外还使用Pt石墨电极），我们发现这种还原最终形成了惰性钝化层主要由绝缘LiF组成，归因于LiPF6的还原分解，这大大影响了电池的可循环性。相比之下，固体电解质间相（SEI）是通过溶剂在约200℃下的还原反应形成的。 1.5 V vs. Li [+] / Li，低于LiPF6的还原电位。然而，我们发现当将电极保持在相对于Li [+] / Li的电势低于1.5 V的电势时，SEI的形成优先于钝化层的形成。因此，通过抑制非活性钝化层的形成，提高了可循环性。这种预处理对于改善电池的可循环性将是非常有效的，特别是对于相对贵的电极，其相对于Li [+] / Li的氧化电势在1.5V和2.6V之间。

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