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Soft X‐ray absorption spectroscopy with different probe depth was employed to characterize the solid electrolyte interphases (SEIs) formed on β‐Sn single crystals with two different surface orientations. Based on comparative studies of C‐K, O‐K, and F‐K absorption spectra between the SEIs and reference samples, SEI on Sn (100) mainly consists of porous Li2CO3 species with electrolyte uptake, while SEI on Sn (001) essentially consists of LiF and organic molecules, with a small amount of –CO3 and electrolyte buried inside. Theoretical calculation suggests that Sn (001) surface is more reactive than (100), especially after air exposure. The reactive (001) surface facilitates the decomposition of LiPF6 to form a LiF layer. In contrast, SEI on (100) surface is predominately from the typical decomposition of carbonate‐based electrolyte. While the LiF passivates Sn (001) electrode after one cycle, the porous carbonate layer on (100) surface does not prevent further decomposition of electrolyte after many cycles. This leads to drastically different electrochemical behavior and morphology of the two SEIs. The result is a direct proof that surface properties of active materials could strongly impact the SEI formation on electrodes even with the same electrolyte. Such effect could lead to distinct SEI formation and electrochemical performance.

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This study demonstrates the unique capability of infrared near-field nanoscopy combined with Fourier transform infrared spectroscopy to map phase distributions in microcrystals of Li(x)FePO4, a positive electrode material for Li-ion batteries. Ex situ nanoscale IR imaging provides direct evidence for the coexistence of LiFePO4 and FePO4 phases in partially delithiated single-crystal microparticles. A quantitative three-dimensional tomographic reconstruction of the phase distribution within a single microcrystal provides new insights into the phase transformation and/or relaxation mechanism, revealing a FePO4 shell surrounding a diamond-shaped LiFePO4 inner core, gradually shrinking in size and vanishing upon delithiation of the crystal. The observed phase propagation pattern supports recent functional models of LiFePO4 operation relating electrochemical performance to material design. This work demonstrates the remarkable potential of near-field optical techniques for the characterization of electrochemical materials and interfaces.

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Direct chemical analysis of electrode/electrolyte interfaces can provide critical information on surface phenomena that define and control the performance of Li-based battery systems. In this work, we introduce the use of ex situ femtosecond laser induced breakdown spectroscopy to probe compositional variations within the solid electrolyte interphase (SEI) layer. Nanometer-scale depth resolution was achieved for elemental and molecular depth profiling of SEI layers formed on highly oriented pyrolytic graphite electrodes in an organic carbonate-based electrolyte. This work demonstrates the unique ability of ultrafast laser spectroscopy as a highly versatile, light element-sensitive technique for direct chemical analysis of interfacial layers in electrochemical energy storage systems.

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