Editors' Choice—Washing of Nickel-Rich Cathode Materials for Lithium-Ion Batteries: Towards a Mechanistic Understanding

By Daniel Pritzl, T. Teufl, A. Freiberg, Benjamin Strehle, J. Sicklinger, H. Sommer, P. Hartmann, H. Gasteiger. Journal of the Electrochemical Society, 2019

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Washing is a commonly used method to remove surface impurities of cathode materials for lithium-ion batteries. However, a clear mechanistic understanding of the washing process is missing in the literature. In this study, we will investigate the effect of washing and subsequent drying of nickel-rich NCM cathodes (85% nickel) with respect to gassing and impedance of the washed cathodes. By on-line electrochemical mass spectrometry (OEMS), we will show a drastic reduction of the O 2 release above 80% SOC for the NCM washed with deionized water, suggesting the formation of an oxygen-depleted surface layer on the NCM particle surface. The modification of the surface can be confirmed by a strong impedance buildup of cathodes composed of washed NCM (using a micro-reference electrode in a full-cell), revealing that the impedance increases strongly with increasing drying temperature after washing. Last, we will propose a comprehensive mechanism on the processes occurring during the washing/drying process of nickel-rich NCM materials and identify the drying temperature after washing as the dominant factor influencing the surface properties. troscopy (EIS) and OEMS that the drying temperature most strongly affects the properties of the newly formed NCM-851005 surface. Its effect on the cycling stability of graphite/NCM-851005 full-cells will be discussed. charge/discharge cycling.— The impedance of the cathode was measured with a micro-reference electrode based on our previously introduced gold wire reference electrode (GWRE) which is integrated into a standard Swagelok T-cell. 40 After assembly of the cell with a graphite counter electrode, an NCM-851005 working electrode, the GWRE, two glass fiber separator, and 60 μ l of LP57 electrolyte (from BASF SE, Germany), the GWRE was lithiated using the NCM-851005 working electrode (1 h at 150 nA, consuming only ≈ 0.005% of its capacity). Before measuring the cathode impedance, two formation cycles of the graphite/NCM-851005 full-cells were carried out at 25°C between 4.2–3.0 V cell voltage at a rate of C/10 (referenced to 215 mA/g for the first charge). The formation protocol consisted of a constant-current constant-voltage (CCCV) charge with a cutoff for the CV phase at C/20 and a constant-current (CC) discharge. After formation, the cells were charged to 50% SOC by a 5h charge with C/10 based on the second-cycle discharge capacity. After 1h of an OCV rest phase, the cathode impedance was recorded in potentiostatic mode from 100 kHz to 100 mHz with a voltage perturbation of 10 mV. Afterwards, charge/discharge cycling at 25°C for another 198 cycles was carried out with a CCCV charge to 4.2 V with C/2 (cutoff for the CV phase at C/20) and a CC discharge to 3.0 V with 1C.

Subfields: Automotive Engineering, Electrical and Electronic Engineering, Mechanical Engineering

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