Investigation of the Solid Electrolyte Interface Structure and Kinetics

Importance of the Topic

The solid electrolyte interface (SEI) plays a critical role in determining the performance, safety and lifetime of rechargeable lithium batteries. A well-formed SEI layer regulates ion transport, minimizes side reactions and ensures stable cycling.

Study Objectives and Overview

This study aimed to elucidate the structure and transport kinetics of a model SEI formed on glassy carbon in a conventional LiPF6-based electrolyte with varying levels of LiBOB additive. Combining electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), redox probe analysis and complementary imaging provided a detailed assessment of SEI properties.

Instrumentation Used

• Multi Autolab M204 multichannel potentiostat/galvanostat with FRA32M module
• Autolab Microcell HC with TSC surface measuring cell and Peltier temperature control
• NOVA 2 software for data acquisition and temperature regulation

Methodology

Glassy carbon electrodes were polished to a mirror finish and assembled with lithium metal as both counter and reference electrodes in an inert atmosphere. Electrolyte solutions (1 M LiPF6 in EC/EMC/DMC) with LiBOB concentrations from 0.1 to 5.0 mmol/L were prepared under argon. SEI formation was carried out by three CV cycles between 3.0 and 0.01 V vs. Li/Li+ at 0.5 mV/s. EIS was performed at 3.0 V before and after SEI formation over 100 kHz to 0.3 Hz (5 mV rms). Redox probe experiments used ferrocene-containing electrolyte and CV scans between 2.8 and 3.6 V at 10 mV/s.

Main Results and Discussion

EIS Nyquist plots revealed a distinct high-frequency semicircle attributed to the SEI resistance (RSEI), which increased with LiBOB concentration. Fitting with a R–CPE equivalent circuit yielded SEI capacitances corresponding to thicknesses of 20–100 nm, depending on assumed permittivity. CV of the ferrocene/ferrocenium redox couple showed diffusion-limited currents that decreased at higher LiBOB levels. Calculated diffusion coefficients of ferrocene closely matched those of Li+ and both decreased as the SEI matured, indicating transport through electrolyte-filled pores within a solid-like SEI backbone.

Benefits and Practical Applications

The combined EIS and CV approach provides quantitative insights into SEI resistivity, thickness and ion transport pathways. Understanding the impact of LiBOB on SEI properties enables the rational design of electrolyte additives to optimize interface stability and extend battery life.

Future Trends and Opportunities

Future work may explore other additive chemistries, operando spectroscopic and imaging methods, microelectrode platforms and machine learning models to predict SEI evolution. Integrating multi-modal characterization will further unveil dynamic interphase processes under real-world cycling conditions.

Conclusion

This investigation demonstrates that LiBOB additives increase SEI resistance and thickness, while EIS and redox-probe CV collectively reveal transport kinetics. The porous SEI architecture supports distinct ion pathways, guiding future electrolyte formulation strategies.

References

• S. Kranz, T. Kranz, A. G. Jaegermann, B. Roling, Journal of Power Sources 418 (2019) 138–146
• K. Xu, Chemical Reviews 114 (2014) 11503–11618
• T. Kranz, S. Kranz, V. Miß, J. Schepp, B. Roling, Journal of The Electrochemical Society 164 (2017) 3777–3784