One of the most auspicious materials for lithium-ion batteries is Nickel cobalt aluminum oxide (NCA). It provides a high capacity, excellent rate capability and low costs [1]. NCA-electrodes and NCA-based lithium-ion batteries have been observed to exhibit a distinct discharge/charge asymmetry in the low SOC regime, shown both in the published literature [2,3] and in our own full-cell (see Figure) and half-cell experiments.

The origin of this behavior has not yet been explained. Using elementary kinetics theory, we show that the asymmetry is caused by a two-step electrochemical reaction mechanism, where charge transfer is associated with electroadsorption of lithium ions from the electrolyte onto the NCA surface (s) according to

Li⁺(electrolyte) + e^–+ (s) ⇄ Li(s),

followed by incorporation into the bulk active material,

Li(s) + V[NCA] ⇄ Li[NCA] + (s).

The overall reaction is dominated by the surface reaction, because of the limited surface spots. Both thermodynamics (via Nernst equation) and kinetics (transition state theory) of the electroadsorption reaction are nonlinearly influenced by surface concentrations of the involved surface species. Surface coverages, in turn, nonlinearly depend on the thermodynamics of the incorporation reaction, where different assumptions for the surface thermodynamics and bulk intercalation range are shown to cause different levels and shape of asymmetry.

The asymmetry is shown to prevail in commercial NCA/LTO pouch cells, leading to a strong temperature and rate sensitivity of the cell capacity. Newman-type numerical models that take the two-step behavior into account are able to reproduce experimentally-observed cell discharge/charge measurements over a wide range of temperature and C rate.

The Figure shows a) the experimental charge-discharge curves of a commercial cell in the low SOC regime, b) simulations using a standard model with single-step Butler-Volmer kinetics, and c) simulations using an extended model with an elementary kinetic two-step mechanism.

Literature:

1.   W.-S. Yoon, K. Y. Chung, J. McBreen, and X.-Q. Yang, “A comparative study on structural changes of LiCo_1/3Ni_1/3Mn_1/3O₂ and LiNi_0.8Co_0.15Al_0.05O₂ during first charge using in situ XRD,” Electrochemistry Communications 8, 1257–1262 (2006).

2.   P. Albertus, J. Christensen, and J. Newman, “Experiments on and Modeling of Positive Electrodes with Multiple Active Materials for Lithium-Ion Batteries,” J. Electrochem. Soc. 156, A606 (2009).

3.   Y. Xu, X. Li, Z. Wang, H. Guo, and B. Huang, “Structure and electrochemical performance of TiO₂-coated LiNi_0.80Co_0.15Al_0.05O₂ cathode material,” Materials Letters 143, 151–154 (2015).