Degradation Diagnostics of Ni-Rich NMC in Lithium-Ion Batteries Aged in Different Degrading Conditions

Abstract

Ni-rich Nickel Manganese Cobalt Oxides (NMC) such as LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) have been widely used as the cathode active materials in lithium-ion batteries (LIB) of electric vehicles due to their high capacity. Thus, we can foresee millions of tons of end-of-life LIB will be accumulated by the next decades. This means that the efficient recycling method of batteries material, especially NMC which has the highest value, should be developed to reduce the cost and increase the efficiency of recycling. Direct recycling which is a novel method to recover cathode active materials in lithium-ion batteries by directly regenerating the chemical and physical structure of cathode materials, skipping the usage of harsh chemical and high energy consumption such as those of other recycling methods i.e. pyrometallurgy and hydrometallurgy. Such an attempt to directly reprocess the spent NMC back into the as good as new cathode requires and understanding what has happened with the materials through the battery life. This implies that the understanding of degradation mechanisms of Ni-rich NMC in different usage conditions is required to up-scale the direct recycling of Ni-rich NMC from degraded lithium-ion batteries.In this work, we investigated failure mode in controlled aged samples of LIB cells to understand the degradation mechanism of NMC622 and NMC811 which are to be connected with the method for direct regeneration. Lithium-ion batteries composed of NMC622 or NMC811 cathode active materials were aged through 1C-rate charge/discharge cycling at 25°C, 45°C or 55°C until 20% or 40% capacity fading. The degraded cells were analyzed via incremental capacity analysis (ICA), volume change measurement, electrochemical impedance spectroscopy (EIS). Ex-situ X-ray powder diffraction (XRD), Scanning electron microscopy analysis (SEM), X-ray photoelectron spectroscopy (XPS) and Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) were used to characterize the chemical and physical structure of degraded NMC from each different degrading conditions.Inspection of degraded cells shows that NMC811 cells exhibit larger cell expansion compared to NMC622 cells going through the same degrading condition. EIS results show that the solution resistance (R S) does not vary much between the different cells, while solid-electrolyte interphase resistances (R SEI) were found to increase with temperature at approximately similar level for both NMC622 and NMC811. Fig. 1a and Fig. 1b show the contour plot on the variation of charge transfer resistance (R CT) ratio after and before degradation of NMC622 and NMC811 cells. It can be seen that R CT in NMC811 cells were found to increase for the more highly cycled NMC811 coupled with high temperature. However, temperature does not seem to have much role in the increment of R CT in NMC622. Investigating the change in structure of the cycled NMCs at different temperatures and to level of different degradation, it was found that the results show that at the same level of capacity remaining, NMC622 has a much larger change in c/a ratio after vs. before compared to NMC811.The above impedance analysis coupled with the XRD results imply that the main cause of capacity fade of NMC811 cells is likely due to electrolyte decomposition which generates gas, resulting in high degree of cell expansion, causing loss of electrical contact. The electrolyte decomposition in NMC811 cell is further facilitated by the increment of temperature due to the lower onset potential limit for gas evolution in NMC811, causing more capacity fading in NMC811 cell at high temperature [1]. On the other hand, the capacity fading in NMC622 cells is mainly from the chemical/structural change in the cycled NMC622 cathode.These insight information on the degradation mechanism of LIB cells and structural variation of Ni-rich NMC after ageing in each degrading condition is then further inspected to elucidate the method to effectively directly regenerate both NMC622 and NMC811 cathode in the spent Li-ion batteries.