Electrochemical Modeling and Performance of a Lithium- and Manganese-Rich Layered Transition-Metal Oxide Positive Electrode

The impedance of a lithium- and manganese-rich layered transition-metal oxide (LMR-NMC) positive electrode,   specifically Li_1.2Ni_0.15Mn_0.55Co_0.1O₂, is compared to two other transition-metal layered oxide materials, specifically LiNi_0.8Co_0.15Al_0.05O₂ (NCA) and Li_1.05(Ni_1/3Co_1/3Mn_1/3)_0.95O₂ (NMC). A more detailed electrochemical impedance spectroscopy (EIS) study is conducted on the LMR-NMC electrode, which includes a range of states-of-charge (SOCs) for both current directions (i.e. charge and discharge) and two relaxation times (i.e. hours and one hundred hours) before the EIS sweep. The LMR-NMC electrode EIS studies are supported by half-cell constant current and galvanostatic intermittent titration technique (GITT) studies. Two types of electrochemical models are utilized to examine the results. The first type is a lithium ion cell electrochemical model for intercalation active material electrodes that includes a complex active material/electrolyte interfacial structure. The other is a lithium ion half-cell electrochemical model that focuses on the unique composite structure of the bulk LMR-NMC materials.

The LMR-NMC electrode impedance is much higher than the impedance of the NCA and NMC electrodes. The higher impedance of the LMR-NMC electrode suggests that it would be better utilized in a high energy battery rather than a high power battery. The intercalation active material electrochemical model and supporting studies indicate the source of the higher impedance can be associated with a much higher electronic contact resistance between the oxide active material and the conducting carbon additive, as well as the material’s interfacial and bulk transport and kinetic characteristics being significantly worse than the other oxides, as typified by the active material lithium diffusion coefficients for the surface layer and bulk material (D_Si and D_Sb) and the kinetic exchange current density (i_o).

As typical of layered oxide active materials the LMR-NMC electrode impedance increases at low and high SOCs, corresponding to D_Si, D_Sb, and i_o all decreasing. Further, the rate of decrease in the parameters with SOC (i.e increase in impedance) is higher than observed for the other layered oxides. When the LMR-NMC electrode is allowed to relax for a very long time (i.e. approximately 100 hours) the impedance is observed to be significantly higher. Also, the resulting D_Si, D_Sb, and i_o parameters determined by the intercalation electrochemical model from the EIS studies during charge and discharge where found to correlate with electrode voltage. This unique behavior for the LMR-NMC electrode is explained by the LMR-NMC bulk reaction and transport electrochemical model, which accounts for the individual domains in the active material. Specifically, the Li₂MnO₃ domains when allowed to relax are either nearly full or empty depending on the SOC. Because of the large characteristic time constant for the Li₂MnO₃ domains’ transition, when the LMR-NMC material is used in transportation applications the Li₂MnO₃ domains typically never relax and the electrode exhibits a lower impedance.

The LMR-NMC bulk reaction and transport electrochemical model describes much of the observed behavior that cannot be accounted for using a standard intercalation active material electrochemical model. This includes the voltage hysteresis as shown in Figure 1, as well as the slow relaxation phenomena. However, the complexity of this model makes determining the parameters for the individual domains quite a challenge.

 

ACKNOWLEDGMENT

Support from the Vehicle Technologies Program, Hybrid and Electric Systems, David Howell (Team Lead) and Peter Faguy, at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Figure 1.  Electrochemical model simulation of LMR-NMC standard electrode half-cell charge and discharge curves at a C/300 rate.