Temperature Dependent Discharge Rate Tests and Cycling Stability of High-Nickel Ncm Cathodes Employing a µ-Reference Electrode in All-Solid-State Pouch Cells
Authors: Schuster Robin , Sedlmeier Christian , Conforto Gioele , Bohn Moritz , Kutsch Tobias , Gasteiger Hubert Andreas 
Published: February 2024

All-solid-state batteries (ASSBs) are widely investigated in the field of lithium-ion battery (LIB) research, since they might offer several advantages over the conventional LIB technology based on liquid electrolytes, for example a possible lower flammability of solid electrolytes compared to organic solvents in LIBs.[1] However, Li6PS5Cl (LPSCl), a commonly used sulfidic solid electrolyte undergoes exothermic reactions with de-lithiated nickel-rich NCM already above 150°C.[2] Establishing a temperature window for save long-term operation is therefore required for large-scale applications in, e.g., battery electric vehicles.
Additionally, a fundamental challenge for the development and electrochemical characterization of individual electrodes for ASSBs is to achieve accurate potential control of the working electrode (WE). In general, this can be done by either the use of a reference electrode (RE) or, alternatively, by the use of a counter electrode (CE) with a well-defined constant potential and little overpotential, whereby the later condition is difficult to meet at high C-rates for electrodes with industrially relevant areal capacities. Recently, we reported on a single-layer pouch cell configuration with an integrated µ-RE, which consist of a thin (~50 µm Ø) in-situ lithiated gold wire that provides a long-term stable reference potential.[3] This configuration was adapted from a µ-RE based three-electrode setup for liquid electrolyte LIBs.[4]

In this study, we utilize the described setup to investigate the temperature dependent rate capability and cycling stability of ASSB cathodes, composed of a LiNi0.85Co0.10Mn0.05O2, solid electrolyte (LPSCl), conductive carbon additive, and a binder in sulfidic ASSB single-layer pouch cells at a comparatively low measurement pressure of 30 MPa. Besides the NCM WE with an application-oriented areal capacity of 3 mAh/cm2, a sulfidic solid electrolyte sheet-type separator,[5] and an InLi alloy counter electrode are used. In the cathode discharge rate tests, the upper and lower cathode cutoff potentials are controlled via the lithiated gold wire µ-RE. Stepwise increased current densities from 0.1C up to 10C (30 mA/cm²) are investigated, while the charge rate is kept constant at 0.2C (CCCV).
Figure 1 exemplarily shows the discharge rate data obtained at 25°C for upper/lower cathode cutoff potentials of 2.6/4.2 V vs. Li+/Li, demonstrating a specific capacity of ~180 mAh/gCAM at 0.1C, which decreases by only ~15% when increasing the discharge rate to 1C. To the best of our knowledge, the here demonstrated rate capability is quite remarkable for cathodes with such high areal capacities. The deconvolution of the half-cell potentials shows that the InLi CE has a constant overpotential of ~400 mV at the highest here examined current density of 30 mA/cm2 (at 10C) and is therefore not interfering with the cathode discharge rate test. In our presentation we will discuss the temperature dependence of the discharge rate capability as well the temperature dependent cycling stability at a discharge rate of 1.0C, including intermittent rate tests.

Acknowledgements:

BASF (Ludwigshafen, Germany) is gratefully acknowledged for supplying the NCM material.

Literature:

[1]S. Hess, M. Wohlfahrt-Mehrens, M. Wachtler,J. Electrochem.
Soc., 2015, 162 (2) A3084-A3097
[2] T. Kim, K. Kim, S. Lee, G. Song, M. S. Jung, K. T. Lee, Chem.
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2022, 34, 9159−9171
[3] C. Sedlmeier, R. Schuster, C. Schramm, H.A.Gasteiger, J. Electrochem. Soc., 2023, 170, 030536
[4]S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, H. A. Gasteiger,J. Electrochem. Soc., 2016, 163(10) A2265-A2272
[5] C. Sedlmeier, T. Kutsch, R. Schuster, L. Hartmann, R. Bublitz, M. Tominac, M. Bohn, H. A. Gasteiger. J. Electrochem. Soc.,
2022, 169, 070508

Figure 1