Solid-state electrolytes overcome many challenges of present-day lithium ion batteries, such as safety hazards and dendrite formation^1,2. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificity. Here we investigate a prototypical solid-state electrolyte using linear and nonlinear extreme-ultraviolet spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet-second-harmonic-generation spectroscopy, we obtained a direct spectral signature of surface lithium ions, showing a distinct blueshift relative to bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1 s state to hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies to develop these electrochemical devices via interfacial engineering of lithium ions.

固态电解质克服了当今锂离子电池的许多挑战，例如安全隐患和枝晶形成^1,2 。然而，由于缺乏化学和界面特异性的操作测量，因此缺乏对所涉及的锂动力学的详细了解。在这里，我们使用线性和非线性极紫外光谱研究了一种原型固态电解质。利用极紫外二次谐波产生光谱的表面灵敏度，我们获得了表面锂离子的直接光谱特征，显示出相对于体吸收光谱的明显蓝移。第一性原理模拟将这种转变归因于从锂 1 s态到表面杂化 Li- s /Ti- d轨道的转变。我们的计算进一步表明，由于抑制低频震颤模式，锂界面迁移率降低，这是该材料中大界面电阻的根本原因。我们的研究结果为通过锂离子界面工程开发这些电化学装置的新优化策略铺平了道路。