Using QSense EQCM-D analysis to investigate SEI formation
Erik Nilebäck Jul 12, ’22 ~ 9 min

Using QSense EQCM-D analysis to investigate SEI formation

A stable solid electrolyte interphase (SEI) is key to reversible and stable cycling of a battery. Ideally, this interfacial layer, which forms on the battery anode through electrolyte decomposition during battery charge and discharge cycles, should be mechanically robust, electrically insulating and ionically conductive. In this post, we describe a user case where QSense EQCM-D, Electrochemical Quartz Crystal microbalance with Dissipation monitoring, was used to investigate SEI-formation. The analysis was used to explore how the electrolyte affects the build-up, evolution, and the mechanical properties of the formed interfacial layer.

QSense EQCM-D - combining QCM-D and electrochemistry

QSense EQCM-D, Electrochemical Quartz Crystal microbalance with Dissipation monitoring, is a time-resolved surface sensitive technology which is used to characterize molecular interactions and reactions at surfaces and interfaces while monitoring electrical properties through electrochemical measurements. These interfacial processes can be monitored with nano-level sensitivity via two parameters, frequency (∆f) and energy dissipation (∆D) at multiple harmonics, so called multi-harmonic EQCM-D, where the frequency relates to mass changes at the surface and the dissipation relates to the softness or viscoelasticity of the surface-adhering layer. Monitoring changes in mass and viscoelasticity, the method can provide quantitative information on interfacial processes such as the formation, evolution, and mechanical properties of SEIs in different solvents and in the presence of different additives.

Case example: Towards the development of stable SEI:s1

Over the years, significant efforts have been made to reveal the conditions at which optimal SEI properties are achieved. In this case study,1 the researchers wanted to understand how the SEI structure, composition and mechanical properties evolve on an active electrode surface in different electrolytes.

Specifically, the goal was to answer the following questions:

  • What does the SEI build-up process look like?
  • What are the SEI mechanical properties?
  • How are the build-up and mechanical properties affected by the electrolyte composition?

Experimental details

To answer the questions above, the researchers used QSense EQCM-D to monitor and analyze the SEI build-up and mechanical properties in three different electrolytes.

The electrolytes used all contained 1.0 mol/L LiClO4, and were:

  1. propylene carbonate (PC)-based electrolyte
  2. ethylene carbonate/diethyl carbonate (EC/DEC)-based electrolyte
  3. tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte

A two-electrode cell was used, and the electrode configuration was as follows:

  • Working electrode (WE): copper coated QCM sensor
  • Counter electrode (CE): Li coated stainless steel

Measurements

Multi-harmonic EQCM-D measurements were performed from open circuit voltage (OCV) to 0V to form SEI:s on the copper coated sensor surfaces, i.e. the WE, in the three different electrolytes. The discharge from OCV to 0V was at a current density of 0.1 mA/cm2.

Experimental results

The EQCM-D analysis revealed several insights on the SEI build-up process and the evolution of the mechanical properties of the formed layers in the three different electrolytes.

The SEI build-up process

Looking at the Δf and ΔD vs discharge time for the PC-, EC/DEC-, and TEGDME-based electrolytes, Fig. 1, the researchers concluded the following

  • The decrease in Δf reveals that there is mass uptake at the sensor surface as a function of discharge time in all three cases, i.e., layers form at the surfaces.
  • After an initial period, there is a considerable increase in ΔD for the carbonate based electrolytes, PC and EC/DEC, which indicates that the SEI:s formed are initially rigid where after a soft, i.e. viscoelastic, layer forms on top
  • The low dissipation in the TEGDME based electrolyte indicates that the SEI formed is rigid.
  • The electrolyte decomposition starts at different potentials

SEI build up process

Figure 1. EQCM-D measurements of the SEI build-up process showing Δf and ΔD for three harmonics, n = 5, 7, and 9, and the working electrode potential vs discharge time for the A) PC-, B) EC/DEC-, and C) TEGDME-based electrolytes.

Analyzing the mass and viscoelastic properties of the SEIs

Using the QCM-D data, the researchers quantified the mass and the viscoelastic properties of the layers formed at the WE in the three different electrolytes, PC-, EC/DEC- and TEGDME. The mass of the SEI:s were 20.9 μg/cm2, 4.7 μg/cm2, and 2.4 μg/cm2 in the PC-, EC/DEC- and TEGDME – based electrolytes respectively. The researchers also calculated the mass change per mole of electron transfer (mpe) to extract information on what species that were generated at different voltages in the three cases.

Viscoelastic modelling of the SEI:s formed in the ether- based electrolytes revealed that the shear modulus decreases, and the viscosity increases during the build-up in both PC and EC/DEC- electrolytes. This indicates that soft layers are formed at the electrode/electrolyte interface. It is also noted that the shear modulus is higher for the SEI in the EC/DEC based electrolyte that in the PC based one, which indicates that the formed SEI is more rigid.

Concluding remarks

Stable SEI:s have been declared critical for reliable operation of Li-ion batteries and there are ongoing efforts to increase the understanding on how to achieve SEI:s to support optimal battery performance. Even though efforts have been made to elucidate how solvents and other components affect the formation, evolution and properties of the SEI:s, however, there still is a lack of understanding in this area. In the study here presented, the researchers wanted to fill some of this knowledge gap. Using EQCM-D analysis, they investigated the SEI build-up process and mechanical properties, and how these depend on the electrolyte composition. Three different electrolytes were explored, a propylene carbonate (PC)-based electrolyte, an ethylene carbonate/diethyl carbonate (EC/DEC)-based electrolyte, and a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte. The time-resolved EQCM-D measurements revealed several key findings. The analysis showed that the SEI build-up process vary greatly between the different electrolytes, and so do the mechanical properties and mass of the SEI:s formed. Based on the study, the authors conclude that in situ EQCM-D helps the understanding of SEI formation and its stability, and that the technology can inspire to new ideas for SEI and electrolyte design in the future.1

QSense EQCM-D analysis to explore SEI formation

Download the case study to learn more about the user case and how QSense EQCM-D was used to investigate SEI formation.

SEI formation and mechanical properties
Case Study

Learn more about EQCM-D in in the analysis of SEI formation

Case study  Analyzing SEI formation with QSense EQCM-D  Download

Reference

  1. Chai, et. al., Chinese Chemical Letters, 2021, 32, 1139 – 1143

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   QSense Explorer Electrochemistry Enables the combination of QCM-D and electrochemistry on the same surface.

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