(40k) Degradation Modeling of Nickel-Rich Nickel-Cobalt-Manganese Oxide Cathode

As one of the promising cathode materials, nickel-rich nickel-cobalt-manganese oxide (Ni-rich NCM) has attracted major attention due to its large capacity and low manufacturing cost. However, its relatively poor cycle life hinders its realization. To achieve better cyclability, developing a comprehensive simulation model could be the most efficient way for preventing the degradation of Ni-rich NCM materials.

Several degradation behaviors have been revealed such as transition metal dissolution, electrolyte oxidation, salt decomposition and gas generation.^1,2 Especially for gas generation, the residual Li on the surface of the cathode such as LiOH and Li₂CO₃ is the main source of this phenomenon.³

In this study, we develop a electrochemical model coupled with the physics-based side-reaction for simulating the degradation of Ni-rich NCM cathode including gas generation. Most of known cathode degradation mechanisms are also considered in this study and implemented to the well-known Pseudo 2D electrochemical model.^2,4 To model the gas generation, oxygen activation and related role of residual Li are included by assuming CO₂ generation and oxygen activation reactions. Moreover, by constructing phase diagram, a critical level of delithiation of Ni-rich materials for oxygen generation is estimated. The corresponding equilibrium potential of oxygen activation reaction (U_O2) is estimated from the open circuit potentials of Ni-rich NCM materials, which is obtained from DFT calculation. The suggested model successfully reproduces the experimental charge/discharge voltage profiles within errors less than 5% up to 50 cycles. Especially, the large discrepancy of capacity between 1^st and 2^nd cycle observed from experiments is also captured from the simulation by only changing the critical U_O2. Based on the model, it will be discussed about the effect of residual Li and the positive correlation of metal dissolution and gas generation.

References

¹ K. Min, S.-W. Seo, Y.Y. Song, H.S. Lee, and E. Cho, Phys. Chem. Chem. Phys. 19, 1762 (2017).

²X. Lin, J. Park, L. Liu, Y. Lee, A.M. Sastry, and W. Lu, J. Electro. Chem. Soc. 160, A1701(2013).

³S. Hy, F. Felix, J.Rick, W.-N. Su, and B.J. Hwang, J. Am. Chem. Soc. 136, 999(2014).

⁴M. Doyle, J. Newman, J. Electrochem. Soc. 143, 1890(1996).