Monitoring the Electrochemical Capacitance By in Situ Impedance Spectroscopy As Indicator for Particle Cracking of (Nickel-Rich) Cathode Active Materials: Development of a Simplified Measurement Setup
Authors: Oswald Stefan , Riewald Felix , Gasteiger Hubert Andreas 
Published: July 2022

Due to their high specific capacity, the market share of nickel-rich layered lithium nickel cobalt manganese oxides (NCMs, LiNixCoyMnzO2, x+y+z = 1) as cathode active materials (CAMs) for lithium-ion batteries is constantly growing. With higher nickel content, however, the increased capacity is often accompanied by shorter cycle life due to undesired side reactions on the CAM/electrolyte interface. As the optimization of the electrochemical performance of lithium-ion batteries by the adjustment of the composition of the cathode active materials has come to a limit, the focus has shifted to the modification of the morphological aspects. One way to minimize side reactions is to reduce the specific surface area of the CAM through a greater NCM crystallite size or through a customized particle morphology, both exhibiting less particle cracking upon charge/discharge cycling [1]. However, new methodologies for the quantification of aspects such as particle size, particle cracking, and surface area change are needed.

Recently, we have developed a novel in situ analytical method which is able to quantify the increase of the CAM surface area upon extended charge/discharge cycling using electrochemical impedance spectroscopy (EIS) [2]. There, we make use of the direct correlation between the capacitance and the surface area of the electrode, whereby an increase of the capacitance indicates cracking of CAM particles, caused by the repeated volume change of the NCM material upon (de)lithiation and/or oxygen release at high state of charge [3]. In these works, the direct relationship between capacitance and NCM particle surface area was validated by ex situ krypton physisorption measurements.

Unfortunately, this impedance-based method relies on a sophisticated experimental setup, using a micro-reference electrode (i.e., a gold-wire µ-RE) and a partially pre-lithiated lithium titanate (LTO) counter electrode. Therefore, in this study, we deduce a stepwise simplification of the capacitance measurements from the setup with a µ-RE and a pre-lithiated LTO counter electrode to a conventional coin half-cell setup, which is commonly used in industry for quick and routine material benchmarking. Additionally, it will be shown that the capacitance does not have to be extracted from a full impedance spectrum provided by an impedance analyzer, but that it can be obtained solely from a low-frequency single-point impedance measurement performed at a battery cycler. The working principle of this approach is validated using four different cell and potentiostat / battery cycler configurations over several charge/discharge cycles [4].

The here demonstrated setup provides a simple method suitable for conventional coin half-cells to identify surface area changes of cathode active materials, being easily implemented in standard cycling procedures.

Reference
s

[1] W. Li, E. M. Erickson, and A. Manthiram, Nature Energy
5 26 (2020).

[2] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J. Electrochem. Soc.
167 100511 (2020).

[3] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J
.
Electrochem
.
Soc
.
168 120501 (2021).

[4] S. Oswald, F. Riewald, H. A. Gasteiger, manuscript
in
preparation.

Acknowledgements

This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research.