With respect to its high specific capacity, silicon would be an ideal replacement for carbonaceous anodes in lithium-ion batteries. But the usage of silicon-based active materials has been hindered by its tremendous volume change and related problems during lithiation cycles. To mitigate the problems, the silicon particles are often used as (expensive) nanoparticles, or some recent strategies include confined Si precipitates embedded in a matrix material. Among others, for example, Si/Ti₄Ni₄Si₇ (STN) has been recently proposed [1]. However, the lithiation mechanisms remain poorly understood for either pure silicon or silicon alloys, due to the silicon (and silicon alloy) amorphization induced by lithiation. We consider Atom Probe Tomography (APT) as a powerful tool [2] to understand the mechanisms of lithium insertion and interaction in silicon alloy electrodes. APT is enabling for a sub-micron sample prepared by Focused Ion Beam (FIB) to address the elemental distribution at the atomic level.  Our results propose a strong contribution of interfaces in such nano-scale Si alloys [3]. In the discharged state we revealed a core-shell structure with the formation of a Li-rich shell surrounding a pure silicon core.  Hereby, not only an accurate composition of this shell is determined by APT, but also the lithium atom position in the matrix is provided.  We believe that this study will be beneficial not only for silicon alloys, but also for understanding pure silicon lithiation mechanisms.

[1] Son  et al. “A Highly Reversible Nano-Si Anode Enabled by Mechanical Confinement in an Electrochemically Activated Li_XTi₄Ni₄Si₇ Matrix.” Advanced Energy Materials 2 (2012), 1226.

[2] Diercks et al., Microscopy of Chemical and Mechanical Heterogeneities in Lithium Cobalt Oxide, Microscopy and Microanalysis 21 (2015), 523.

[3] Y.Zheng, KG. Pradeep, F.U. Renner et al., to be published.

Figure 1. The atom probe tomography reconstruction of Silicon atoms in pristine Si/Si₇Ti₄Ni₄.