We report the first direct experimental evidence of the dynamic behavior of the solid-electrolyte-interphase (SEI) on copper electrodes upon electrochemical cycling. Synchrotron-based soft X-ray absorption spectroscopy (sXAS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) consistently show that both the chemical composition and the thickness of the SEI change with electrochemical potential throughout the slow formation process. In particular, sXAS results show that the nascent carbonate species in SEI show redox reversibility and decompose during the delithiation (oxidation) process, which leads to a significant shrinking of the SEI thickness as confirmed by TOF-SIMS. Meanwhile, the carbonates also matures and become more and more inactive at every lithiation (reduction) process. These experimental observations reveal unambiguously that SEI formation is much more complicated than a simple and monotonous deposition of electrolyte decomposition product; instead, it could be an oscillating process with a breathing growth.

中文翻译：

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电化学循环过程中固体电解质间相的呼吸和振荡生长

我们报告了电化学循环在铜电极上的固体电解质中间相（SEI）的动态行为的第一个直接实验证据。基于同步加速器的软X射线吸收光谱（sXAS）和飞行时间二次离子质谱（TOF-SIMS）始终表明，在整个缓慢形成过程中，SEI的化学成分和厚度均随电化学势而变化。sXAS结果特别表明，SEI中新生的碳酸盐物质在脱锂（氧化）过程中显示出氧化还原可逆性并分解，这导致SEI厚度显着缩小，这已由TOF-SIMS确认。同时，碳酸盐也逐渐成熟并在每个锂化（还原）过程中变得越来越不活泼。这些实验观察清楚地表明，SEI的形成比电解质分解产物的简单单调沉积要复杂得多。相反，这可能是一个伴随着呼吸增长的振荡过程。