The Electrolyte - Electrode Interaction - Studying the Influence of the Electrolyte Decomposition Layer on the Kinetics of the Battery
Authors: Niehoff Philip , Schroeder Matthias , Schappacher Falko , Winter Martin 
Published: February 2020

Lithium-Ion Batteries (LIBs) are strongly relying on the electrolyte decomposition layers which are forming at the anode and cathode. These decomposition layers are protecting the battery from further degradation due to uncontrolled reaction of the lithium with the electrolyte [1-5]. Therefore, information about the characteristics of these decomposition layers is very valuable for the development of advanced LIBs. In this study we were able to quantitatively study the structure, composition and thickness of the electrolyte decomposition layer. Furthermore, the influences of the structure, composition and thickness on the kinetics and cycle performance are investigated. These characteristics are of special interest for the design of a proper electrolyte.
The electrolyte decomposition layers were studied using a sputter depth profiling X-ray photoelectron spectroscopy technique. This technique allows the determination of the composition and thickness of the electrolyte decomposition layers quantitatively [6, 7]. From the results it can be seen, that a significant electrolyte decomposition layer forms at the cathode (0.8 nm after formation 2.5 nm at state of initial capacity of 80% [7]). Although this layer is smaller compared to the anode (2.8 nm after formation [6]), from direct current resistance and impedance measurement it was found that the cathode resistance is significantly higher.
To further investigate the influence of the electrolyte decomposition layer on the electrode kinetics additives like fluro acetone and vinylene carbonate are used to influence the composition and thickness of electrolyte decomposition layers. The use of additives to change the electrolyte decomposition layer characteristics, instead of ageing, is carried out since with ageing also different causes for a change in the kinetics may occur. As electrode materials i.e. 1:1:1 lithium nickel manganese cobalt oxide, spinel lithium manganese oxide and iron (III) oxide are used to investigate a difference in interaction of the electrode material with the electrolyte decomposition layer. Measurement of the direct current resistance using a 3-electrode cell gives the influence of the decomposition layer on the kinetics of the battery. The results (Figure 1) show that even thicker layers can show lower resistances and that the effect on the kinetics of the different additive may be different for a different electrode material.
The reason for this finding is further evaluated by X-ray diffraction to estimate an influence of the electrolyte decomposition layer on the phase and structure changes of the active material. Additionally, thermo-gravimetric measurements and temperature dependent X-ray photoelectron spectroscopy are applied to investigate the strength of interaction.

References

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[6] P. Niehoff, S. Passerini, M. Winter, Langmuir 29 (2013).
[7] P. Niehoff, M. Winter, Langmuir 29 (2013).

Figure 1