New Understanding of Defect Formation in Silicon Electrodes

Jülich, 7 July 2020 – In theory, it is possible to significantly increase the storage capacity of commercially available lithium ion batteries – using an anode based on silicon instead of graphite. However, in practice, these batteries with pure silicon anodes start to deteriorate after just a few cycles of charging and discharging. An international team of researchers at Jülich’s Institute of Energy and Climate Research (IEK-9) has now observed with a unique level of detail how the defects form on the anode. As a result, they discovered previously unknown structural inhomogeneities in the boundary layer between the anode and the liquid electrolyte to be the origins of the destructive processes that cause premature battery decay. Their findings have been published in the current issue of the journal Nature Communications.

In principle, silicon-based anodes can allow lithium ion batteries to store over nine times as much charge as the commercially used graphite anodes of the same weight and size. If equipped with these batteries, electric cars would be able to drive much further without stopping to charge than previously, and smartphones would be operational for longer without being plugged in to recharge so often. But there is one serious problem: after just a few hours of battery operation, cracks form in the silicon anode and parts of the material pulverize. A team headed by the Jülich researchers Dr. Chunguang Chen and Prof. Peter Notten has now unveiled how the stability of the silicon anodes could potentially be improved – a result of a variety of studies that combine four innovative methods.

The studies produce a detailed picture of what happens when charging takes place, a process which the researchers also explain in a video: During the very first instance of lithiation, lithium ions from the liquid electrolytes move to the atomically smooth surface of the silicon crystal. Two layers of a solid electrolyte interphase (SEI) form there one after another.

The first “inner” SEI layer is harder and is mainly composed of lithium fluoride and other inorganic lithium compounds. The second “outer” layer is softer and mainly consists of organic lithium compounds, i.e. materials containing carbon species. While the outer SEI layer is forming, lithium ions enter into the silicon crystal below the SEI. There, an amorphous (i.e. non-crystalline) lithium–silicon alloy is formed.

“It is interesting that the SEI does not form everywhere uniformly – or homogeneously – but instead there are thicker and thinner areas as well as areas with major differences in lithium ion mobility in the outer SEI layer in particular,” explains Dr. Chunguang Chen. The SEI inhomogeneity has significant consequences. Because SEI acts as the lithium-ion conductor for silicon, the underlying lithium–silicon alloy also does not form in a homogeneous manner, regardless of the use of originally atomic-flat silicon crystal. Starting from the second cycle, defects (lattice tilts) in their early stage are visualized on the boundary between the amorphous alloy and the crystal.

“During further recharging processes, these early-stage defects turn out to be the hot-spots for large-scale structural deformations of the entire anode,” says Prof. Peter Notten. As the lithium ions are incorporated and released by Si, the amorphous alloy layer expands and shrinks by up to 300 %. This change in volume applies stress to the silicon crystal beneath it. The crystal then tends to succumb to this stress and form these defects.

“If you want to increase the structural stability of the silicon anode during cycles, the early-stage defect formation should be suppressed from the very beginning,” concludes Dr. Chen. He argues that a promising approach is to ensure that a homogeneous inner SEI forms because of its direct contact or to build a homogeneous artificial SEI for Si.

In order to observe the formation of early-stage defects at the boundary between the silicon crystal and the lithium–silicon alloy, the researchers used full-field diffraction X-ray microscopy (FFDXM), a novel synchrotron technique. They performed the FFDXM studies at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Meanwhile, the inner and outer SEIs were studied at their home lab at Forschungszentrum Jülich (IEK-9) using the in operando atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), and electrochemical strain microscopy (ESM).

Original publication:

Chunguang Chen, Tao Zhou, Dmitri L. Danilov, Lu Gao, Svenja Benning, Nino Schön, Samuel Tardif, Hugh Simons, Florian Hausen, Tobias U. Schülli, R.-A. Eichel, Peter H. L. Notten
Impact of dual-layer solid-electrolyte interphase inhomogeneities on early-stage defect formation in Si electrodes
Nature Communications (published 1 July 2020), DOI: https://doi.org/10.1038/s41467-020-17104-9 (Open Access)

Further Information:

Institute of Energy and Climate Research – Fundamental Electrochemistry (IEK-9)

Contact:

Dr. Chunguang Chen
Institute of Energy and Climate Research (IEK-9)
Forschungszentrum Jülich
Tel.: 02461 61-2011
E-Mail: c.chen @fz-juelich.de

Press contact:

Dr. Regine Panknin
Corporate Communications
Tel.: 02461 61-9054
E-Mail: r.panknin@fz-juelich.de