LiNi_0.8Co_0.15Al_0.05O₂ (NCA) has been widely used as cathode in commercialized lithium-ion batteries (LIBs), but its fundamental electrokinetic properties such as interfacial and bulk transport, are poorly understood. In our previous study, diffusivity and conductivity of bulk NCA at different states of charge (SOC) have been investigated using dense single-phase pellets¹. Based on that, we further attempt to understand the electrochemical charge-transfer kinetics at NCA/liquid electrolyte interface, and find out the rate-limiting transport factor of NCA cathode in LIBs.

From Butler-Volmer equation, exchange current density j_o can be utilized to evaluate the kinetics at solid/electrolyte interface. Thus, we applied two methods - electrochemical impedance spectroscopy (EIS) and potentiostatic intermittent titration technique (PITT) to study the exchange current density of NCA at different SOC using the same dense bulk pellets². Primary results of these measurements are shown in Fig. 1. Exchange current density from PITT is found to decrease with increasing SOC, whose value is nearly 10²smaller at high SOC (70 %) than that of pristine NCA. However, EIS results show discrepancy above 50 % SOC.

Based on the PITT results from bulk pellets, relative contributions of bulk and interfacial transport of the particles can be modeled using Biot numbers Λ and time constant analysis. Fixing the acceptable overpotential as 0.1 V, the rate of bulk diffusion and interfacial reaction of the 5.6 um commercial spherical NCA particles are plotted as Fig. 2. With increasing SOC, the maximum C-rate decreases by almost 10² and Λ is of order 10⁰ indicating the mixed-control kinetics. In other words, the interfacial reaction clearly becomes the rate-limiting kinetic step and thus determine the rate-performance of NCA.

This work was supported as part of the North East Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583.

References:

[1]. R. Amin, D.B. Ravnsbaek, Y.-M. Chiang, J. Electrochem. Soc. 2015, 162, A1163-A1169.

[2]. J. Li, X. Xiao, F. Yang, M. W. Verbrugge, Y.-T. Cheng, J. Phys. Chem. C. 2012, 116, 1472–1478.