2.2. Cell Assembly and Electrochemical Analysis

NMC 622 cathodes were assembled versus 0.25 mm lithium foil (Merck KGaA, Darmstadt, Germany) in coin cells CR2032 in an argon-filled glove box (LAB master pro sp, M. Braun Intergas-Systeme GmbH, Garching, Germany) with H₂O < 0.1 ppm and O₂ < 0.1 ppm. A mixture of ethylene carbonate and ethyl methyl carbonate (EC/EMC 3:7) with 1.3 M lithium hexafluorophosphate (LiPF₆) as conducting salt and 5 wt.% fluoroethylene carbonate (FEC) as additive was used as electrolyte. The separator was polypropylene (PP, Celgard, Charlotte, NC, USA) foil with a thickness of 25 µm and a diameter of 15 mm. A total amount of 120 µL electrolyte was added to the cell. After stacking, all cell components were pressed using a digital pressure controlled electric crimper (MSK-160D, MTI Corporation, Richmond, CA, USA). The assembled batteries were stored at room temperature for 20 h in order to allow the cathode and separator to be fully wetted with liquid electrolyte.

The “constant current–constant voltage” (CCCV) method was carried out for rate capability tests using a battery cycler (BT 2000, Arbin Instruments, College Station, TX, USA). The first 3 cycles were carried out at C/20 as the formation step, followed by increasing C-rates from C/10 to 5C. C/10 and C/5 were carried out for 5 cycles and the others were carried out for 10 cycles. The C-rate was calculated based on the discharge time and applied current during the formation step. In this work, 1C = 0.99 h⁻¹, as well as a specific capacity of 155 mAh/g, were applied for the calculation of currents under different C-rates. The lower and upper cut-off voltages were 3.0 and 4.2 V, respectively. The CV measurements with a scan rate at 0.02 mV/s were carried out to find out the redox reaction that occurred during the charge and discharge processes. The operating window was set to 3.0 to 4.3 V. The electrochemical analyses were performed at room temperature in an ambient atmosphere.