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Subject of the Master's internship

Operando study of the Solid-Electrolyte Interphase in Li-ion batteries

Master's internship which can be continued in thesis.
Published on 21 October 2019
This internship will focus on the interaction between the electrolyte and the electrode in Li-ion batteries (LiBs). At the interface between the former and the latter, a complex transition layer – so-called “Solid-Electrolyte Interphase” (SEI) – is formed during the cycling of the battery. A deeper understanding of the SEI is of paramount importance as it mediates the Li-ion exchange between the electrolyte and the electrode.
The study will be conducted on model electrode of silicon. Silicon is considered a very promising candidate for LiB anodes as it has about 10 times the capacity of graphite, the usual material for LiB anode. Operando X-ray reflectivity data have already been acquired during synchrotron measurements and will provide a sound basis for the study. Further characterizations can be envisioned within the institute.

Full description of the subject
Li-ion batteries (LiBs) are now ubiquitous and they are expected to drive the next mobility revolution with the advent of the electrical vehicles. They are therefore at the center of intense research, from materials development to device designing, at all fundamental, applied and technological levels. Many questions are being addressed to improve in fine the performances of the LiBs in terms of energy and power density, lifetime, cost-effectiveness, sustainability or safety.
One of the major interests in the field of battery research is the formation of the “Solid-Electrolyte Interphase” (SEI) on the electrodes. The SEI usually results from the degradation of the electrolyte at the electrode surface and is usually made of both organic and inorganic compounds. It starts forming from the first cycle of the battery and will typically continue to grow slowly during the lifetime of the battery. The SEI is a fundamental aspect of LiB functioning, as it may consume electrolyte and trap Li ion, thus negatively affecting the performance of the battery. Since it is an interface layer usually few nanometers thick, it is also difficult to investigate experimentally. Most experimental surface characterization techniques require to dismount the electrode after battery cycling (post-mortem measurements) and to wash the electrolyte prior to the experiment. It is thus difficult – though not impossible – to obtain representative results.
In this internship, operando X-ray reflectivity (XRR) will be used in a model system, namely silicon wafers. In short, X-ray reflectivity can probe the electron density gradient along a profile normal to a flat surface. Using high intensity hard X-rays (such as found in a synchrotron), the measurements can be performed through an electrochemical cell, i.e. during the battery cycling. Thus, this approach alleviates all the difficulties previously discussed. Si is considered a very promising material for Li, as it can store 10 times more Li ion than graphite for the same weight. Si wafers, typically used in the microelectronics, are also extremely flat, thus allowing very precise XRR measurements. Such experiments have already been performed, considering different types of surface preparation and different formulations of the electrolyte (Fig. 1). A large amount of data is therefore readily available for further processing. The European Synchrotron Radiation Facility (ESRF) in Grenoble is currently under a major upgrade program. Synchrotron beam is expected to resume between March and September, which might allow additional measurements to be performed. Laboratory-based measurements can also be considered. As a large research effort in the field of battery research is currently undertaken at the Institute, numerous interactions with other students and researchers on closely related topics will be possible and encouraged, and there will be a possibility to continue on a PhD thesis.

Left: Raw data of the operando X-ray reflectivity measurement on a Si wafer anode over the first 7 partial cycles, (right) experimental setup on beamline BM32 at the ESRF.

Requested skills
Background in physics and/or electrochemistry, large dataset analysis, Python programming language

Li-ion batteries, X-ray reflectivity, operando

Samuel Tardif  - Phone: 04 76 88 28 19

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