Pulse Measurements on LiNi0.6Co0.2Mn0.2O2 Cathodes with Various Electrode Parameters Affecting the Charge-Discharge Rate Performance
Authors: Hwang Soo Min , Kim Dong-Won , Kim Young-Jun 
Published: December 2020

Li-ion batteries (LIBs) are used globally as power sources for various applications, such as portable electronics, electric vehicles, and large-format stationary energy storage systems, driving a wireless, fossil fuel-free sustainable society [1,2]. Together with the increasing demands for higher energy and longer lifespan, faster charging capability and higher (discharging) power characteristic are of significant interests. In this study, we investigated the rate capability of LiNi0.6Co0.2Mn0.2O2 (NCM) cathodes, as one of Ni-rich, layered oxide cathodes, for LIBs. We examined the rate performance and the limiting factors for both charging and discharging at current rates of 0.2-10C using electrochemical impedance spectroscopy (EIS) and pulse polarization measurements. The electrodes were prepared by varying the electrode density from 1.9 g cm-3 to 3.6 g cm-3 at a fixed electrode thickness of ~63 μm and the mass loading from 3 mg cm-2 to 23 mg cm-2 at a fixed electrode density of 2.7g cm-3.

With increasing C-rate, both the discharge and charge capacities are decreased for all the electrode densities (see Fig. a and b). A low C-rates, no prominent difference in the charge and discharge capacities is found with respect to the electrode density. The moderate electrode density of 2.7 g cm-3 shows the best rate capability owing to a balance between electron conduction and ion transport in the electrode. The 1.9 g cm-3 electrode displays lower discharge capacities at C-rates of 0.2C-2C, as compared to the 3.6 g cm-3 electrode. However, capacity reversal is found at high C-rates (>2C). On the other hand, the 3.6 g cm-3 electrode shows better rate properties than the 1.9 g cm-3 electrode for charging. In order to probe the electrochemical kinetics behind the different rate properties of the NCM electrode with respect to the electrode density, we carried out EIS measurements; however, the resulting Nyquist plots could not differentiate the limiting factors affecting the kinetics for charge and discharge, in terms of the relationship between the electrode density and charge (electron and Li-ion) transports (not shown).

We performed pulse experiments to generate pulse polarization profiles for charge and discharge of the electrodes with different electrode densities. Pulse polarization measurements are techniques used to measure voltage differences during time-limited charging or discharging of a cell at a CC and given state-of-charge [3,4]. Fig. c depicts a schematic of the pulse measurements for discharge and charge. When a certain amount of current is applied to the cell upon charging or discharging, a voltage difference can occur due to various polarization factors according to DC-IR = Eocv + EIR-drop + ∆Et, where EIR-drop is an initial voltage change upon charging/discharging (IR drop) and ∆Et is a voltage change for the pulse region at a CC and time. The DC-IR, EIR-drop, and ∆Et at a pulse time (∆tp) of 100 s with respect to the current density for pulse discharge and charge are plotted in Fig. 2b and 2c, respectively. For pulse discharging (Fig. d), the 3.6 g cm-3 electrode exhibits a relatively large ∆Et with increasing current density, compared to those of the 1.9 g cm-3 electrode, which is contrast to the variation of DC-IR. These pulse polarization data suggest the limiting factors affecting the charge and discharge rate capabilities of the electrodes with different electrode parameters. The pulse measurements with respect to the mass loading (electrode thickness) will be also discussed.

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (2020R1C1C1013253).

[1] Nat. Energy 3, 290-300 (2018).

[2] ACS Energy Lett. 5, 807-816 (2020).

[3] J. Electrochem. Soc. 164, E3539-E3546 (2017).

[4] J. Power Sources 196, 412-427 (2011).

Figure. Rate capability of the NCM622 cathodes with electrode densities of 1.9-3.6 g cm-3 upon (a) discharging and (b) charging; (c) Schematic of the pulse measurements of the NCM622 cathodes with electrode densities of 1.9-3.6 g cm-
3 for charge and discharge and the resulting parameters including (d) discharge DC-IR, EIR-drop, and ∆Et (∆tp=100 s) and (e) charge DC-IR, EIR-drop, and ∆Et (∆tp=100s) versus the current density.

Figure 1