UNRAVELLING THE ORIGIN OF EARLY-STAGE DEFECTS IN SILICON ELECTRODES
Bulk Si electrodes undergo severe mechanical deformation during (de)lithiation. Using operando full-field diffraction X-ray microscopy (FFDXM) on beamline ID01, these deformations were studied in their early stage. Their origin is understood as due to a heterogeneous level of lithiation, correlated to inhomogeneities in the dual- layer solid-electrolyte interphase (SEI).
Silicon is one of the most promising anode candidates for the next-generation of lithium- ion batteries. In terms of specific capacity, Si (3579 mAh g-1) can store eight times more lithium-ions than conventional graphite electrodes (372 mAh g-1). What currently limits its application is the extreme volume change (up to 300%) during cycling, which results in mechanical failure of the Si electrodes. Tremendous efforts have been made to understand the mechanical deformation in Si electrodes. Most of these studies were performed without any spatial resolution, yielding only average information.
In this study, operando FFDXM at beamline ID01 was applied to study the formation of early- stage defects in single-crystal Si electrodes. As a novel strain imaging technique, FFDXM is
able to detect tiny mechanical deformations (lattice tilt of ~10-2 mrad and lattice strain of 10-4) at a spatial resolution of 100 nm. The Si electrode potential was scanned by cyclic voltammetry (CV), during which only the Si near the surface participates in the (de)lithiation process and is subsequently amorphised. By recording the dark-field snapshots at −0.03° off the Si (004) Bragg θ angle, the dark-field contrast in FFDXM reflects structural changes in single-crystal pristine Si just below the Si/LixSi interface. Specifically, the scattered intensity is sensitive to any deviation from the perfect single-crystal Si state and thus serves as an excellent measure of structural deformations at the Si/LixSi interface.
No defect was observed in the first CV cycle. The only feature was the SEI formation peak in the electrochemistry measurement. Two defects (Figures 65b to 65e) appeared at the Si/LixSi interface during the second CV cycle, as indicated in the Int. increase (Figure 65a). The nature of these defects was characterised by 3D reciprocal space mapping (Figure 65f) at the end of the third cycle, which revealed that these early-stage defects were dominated by lattice- tilts of as large as 0.03°. The lattices tilted inwards, indicating a smaller lattice parameter at the defect centre, which is characteristic of a heterogeneous degree of lithiation. These early- stage defects, despite their weak deformation and low density, are the seeds of large- scale defects formed after extensive cycling. Understanding the origin of these defects is thus crucial to improving the structural stability of Si electrodes.
The heterogeneous lithiation discovered by FFDXM is an intriguing phenomenon considering that the electrodes were initially atomic-flat single-crystal Si wafers. Since the early-stage defects were only observed after the initial SEI formation, this may point to a possible correlation. To complement the FFDXM measurements, operando atomic force microscopy (AFM), electrochemical strain microscopy (ESM) and sputter-etched X-ray photoelectron spectroscopy (XPS) were performed to study the SEI (Figure 66) and its possible correlation with early-stage
Fig. 65: a) Potential (E), current (I) and total scattered intensity (Int.) of the entire area (100×430 μm2) for the second CV cycle. b-e) show the evolution
of the observed defects. The labels of (b-e) match the marked annotations in (a), at which the FFDXM images were taken. f) 3D reciprocal space mapping
result of the defects at the end of the third cycle.