Combination of Li Isotopic Tracing and Ex-Situ Characterizations for Imaging Li Dynamics in the SEI: Benefits and Issues
Authors: Berthault Manon , Porcher Willy , Gutel Thibaut , Bardet Michel , De Vito Eric 
Published: December 2020

The Solid Electrolyte Interphase (SEI) has been considered as “the Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries” by Winter in 2009 [1]. Being aware that its stability has a significant impact on their performance, lifespan and safety, the scientific community tries to improve knowledge on this interface. Experimental studies have been achieved to observe the global composition of the SEI on common anodes (lithium, graphite and silicon) [2], [3]. LiF, Li2CO3, polyolefins and semicarbonates detected in the SEI derives from electrolyte compounds degradation. Several SEI models have been discussed (mosaic, super capacitor, multi-layers, etc), but they are still being debated. Finding the “perfect” model is challenging because this model should be able to explain conductive (ions) and resistive (electrons) properties of the SEI, as well as its formation and ageing. As of today, most of the knowledge concerning Li transport properties and SEI formation comes from theoretical studies and are based on simulations.
Li isotopic tracing has already been successfully employed to study the Li dynamics in components of Li-ions accumulators. Since 2011 [4], Li isotope tracing has been used about 10 times in this field. In all literature, isotopes are introduced in specific parts of a cell (anode, cathode or electrolyte). In our study, we have added them at different states of charge (100% or 0%) in order to label selectively the SEI.
This study aims at investigating the lithium dynamics within the SEI during the first cycles of charge and discharge. Trapped lithium (in the SEI) can be distinguished from active lithium by using its natural isotopes 6Li and 7Li. Two specific systems have been studied: SEI on graphite and SEI on silicon. During a first step, the SEI have been generated in 7Li. Then, the electrode has been recovered and re-used in a new cell in front of a 6Li-enriched counter electrode and electrolyte. By using time-of-flight secondary ion mass spectrometry (ToF-SIMS), we have been able to track 7Li and 6Li distribution in the electrode. First results on graphite electrode have shown that “trapped” Li is eventually highly mobile, even when the system is down (without polarization of the electrode). Indeed, when electrolyte and electrode are in contact without any electric field, lithium initially present in the electrolyte have been detected in the SEI.
The same kind of experiments have been carried out on silicon-based electrodes. In addition to ToF-SIMS characterization, 7Li and 6Li Solid State Nuclear Magnetic Resonance (NMR) analyses have been performed. ToF-SIMS allows probing the surface chemistry on a small sample of the electrode (~1000 µm²), while NMR spectroscopy investigates the whole sample and allows knowing the chemical environment of the probed nucleus. The same observation has been made in these experiments: lithium “trapped” in the SEI is highly mobile. As no self-discharge has been observed, it is probably mainly related to isotope exchanges.
These results paves the way for calculations of diffusion coefficients, as well as for the estimation of a characteristic length and thickness of the probed SEI. Other isotope experiments have also been carried out. However, natural isotope exchanges, even without polarization of the electrode, tend to hide information by homogenizing isotope concentrations. Optimization of experiments and operando measurements may overcome such issues.
[1] M. Winter, Zeitschrift für Physikalische Chemie, 223, 1395–1406 (2009).
[2] E. Peled and S. Menkin, Journal of The Electrochemical Society, 164, A1703–A1719 (2017).
[3] S. J. An et al., Carbon, 105, 52–76 (2016).
[4] P. Lu and S. J. Harris, Electrochemistry Communications, 13, 1035–1037 (2011).

Figure 1