Behaviour of Nanostructured Si-Based Anode Materials Studied By Atom Probe Tomography and Hard X-Ray Photoelectron Spectrocopy

Abstract

Lithium-ion batteries (LIB) are considered for electric vehicles (EV) as well as for home or off-grid energy storage for example in combination with photovoltaics. These specific applications drive the research on high energy density, cost-effective, safe and environmental friendly battery materials. To obtain deeper insight into the reaction mechanisms advanced characterization such as in-situ TEM or in-situ XRD1 are often used in combination with simplified model systems. For LIB anodes Si has the highest specific capacity among all anode materials. However, the mechanical fatigue (lack of microstructural integrity) associated with large volume expansion of pure Si during the lithiation process has hindered their wider use for commercial applications so far. Also, there are a series of side reactions which happen at the surface forming the solid electrolyte interface (SEI). In this work we investigate the role of mechanically confined nanocrystalline Si particles being embedded in a SiTiNi matrix2. Samples are produced by melt-spinning from a rapidly solidified SiTiNi alloy. The ribbon exhibits a nanocomposite microstructure with nano-Si particles (~50-200 nm) as potential sites for Li-storage embedded in an amorphous matrix of SiTiNi (STN). Such alloys have shown potential for anode materials in high capacity Li-ion batteries. Systematic lithiation and de-lithiation cycles were performed and selected samples after different cycles were investigated using Atom Probe Tomography (APT) to understand the Li diffusion mechanism in and out of the Si particles. We consider Atom Probe Tomography as a powerful tool to understand the phase changes at an atomic level3. Aided by Focus Ion Beam (FIB), we can address the nm-scale element distribution in any local structure. On the other hand, Hard X-Ray Photoelectron Spectroscopy (HAXPES) is adopted for a detailed SEI study. The employed high energy X-ray beams can probe deeper information of core level photoemission [1] Bach, P.; Valencia-Jaime, I.; Ruett, U.; Gutowski, O.; Romero, A.H.; Renner, F.U. Chemistry of Materials (2016), doi: 10.1021/acs.chemmater.5b04719; Renner, F.U.; Kageyama, H.; Siroma, Z.; Shikano, M.; Schröder, S.; Gründer, Y.; Sakata, O. Gold Model Anodes for Li-Ion Batteries: Single Crystalline Systems Studied by In Situ X-ray Diffraction, Electrochim. Acta 2008, 53, 6064-6069. [2] Son et al. “A Highly Reversible Nano Si Anode Enabled by Mechanical Confinement in an Electrochemically Activated LiXTi4Ni4Si7 Matrix.” Advanced Energy Materials 2 (2012), 1226. [3] Diercks et al., Microscopy of Chemical and Mechanical Heterogeneities in Lithium Cobalt Oxide, Microscopy and Microanalysis 21 (2015), 523.