Elucidating the Effect of the Morphology of Ni-Rich Cathode Active Materials on Their Long-Term Cycling Performance: Poly- Vs. Single Crystalline NCM851005

Abstract

The market share of nickel-rich layered lithium nickel cobalt manganese oxides (NCMs, Li1+δNixCoyMnzO2, x+y+z+δ = 1, and 0 < δ < 0.05) as attractive cathode active materials (CAMs) for lithium-ion batteries is constantly growing owed to their high specific capacity. With higher nickel content, however, the increased capacity is often accompanied by a somewhat shorter cycle life due to undesired side reactions. These processes result from the decomposition of residual lithium salts and surface contaminants that are present in greater amounts on Ni-rich materials after synthesis and storage,[1] as well as from the reaction of evolved lattice oxygen with the electrolyte,[2] which occurs at lower potentials for higher nickel content.[3] These processes degrade the NCM active material upon charge-discharge cycling and dramatically increase the impedance of the entire cell, eventually leading to cell failure. One way to minimize side reactions is to reduce the specific surface area of the CAM by increasing the NCM crystallite size, being known in the literature as so-called single crystals,[4] which maintain their pristine surface area upon cycling.[5] In this work, we investigate two NCM851005 materials and their long-term cycling behavior in full-cells: a polycrystalline material (PC, 0.27 m²/g) with secondary agglomerates of ~5-10 µm in diameter that are comprised of primary crystallites with particle diameters on the order of ~0.1 µm, and a single crystalline material (SC, 0.51 m²/g) with primary crystallites of ~1 µm without distinct secondary structure, both shown in the SEM images in Figure 1. Due to the different particle morphology of the two materials, one would expect significant differences in the rate capability, in the long-term cycling behavior, in the extent of gassing at high state of charge (SOC), and in the change of CAM surface area over the course of extended charge/discharge cycling.To examine these aspects, calendered PC or SC NCM851005 electrodes with mass loadings of ~10 mgAM/cm² were assembled in a first set of experiments in half-cells with an additional lithium reference electrode and then subjected to discharge rate tests with potential cutoffs controlled by the lithium reference electrode. In a second set of experiments, the materials were cycled in coin full-cells using graphite as counter electrode for >200 cycles, applying different upper cutoff potentials chosen to be either below or above the limit of oxygen release of NCM851005. In addition, these full-cells were cycled at 25 °C or at 45 °C to illuminate the effect of elevated temperatures on the capacity loss. After cycling, both anode and cathode were harvested from the cycled cells and investigated by post mortem experiments: The NCM cathodes were analyzed in half-cells to differentiate between the contributions of lithium inventory losses and cathode active material losses; the graphite anodes were investigated regarding their elemental composition after cycling to understand the effect of particle morphology (or specific surface area, respectively) on the dissolution of transition metals from the cathode and their deposition at the anode. Finally, the picture of the mechanism behind the capacity losses was completed by in situ capacitance measurements tracking the surface area of the CAMs upon cycling[6] as well as by on-line electrochemical mass spectrometry (OEMS) quantifying the released gas amounts during the first four cycles to >80 %SOC. It is shown that the ratio of the surface area of both materials at high SOC correlates well with the ratio of the amounts of gas released during electrochemical cycling as well as with the detected amounts of deposited transition metals on the graphite anode.In summary, this study gives comprehensive insights into the effects of the morphology on the long-term cycling performance of poly- and single crystalline CAMs and highlights the advantages of SC NCMs for the use in commercial lithium-ion batteries. Reference s : [1] J. Paulsen and J. H. Kim, Patent US2014/0054495A1 (2014).[2] A. T. S. Freiberg, M. K. Roos, J. Wandt, R. de Vivie-Riedle, and H. A. Gasteiger, J. Phys. Chem. A 122 8828 (2018).[3] R. Jung, M. Metzger, F. Maglia, C. Stinner, and H. A. Gasteiger, J. Electrochem. Soc., 164 (7) A1361 (2017).[4] Y. Kim, ACS Appl. Mater. Interfaces 4 2329−2333 (2012).[5] S. Oswald, M. Bock, and H. A. Gasteiger, Meet. Abstr. MA2020-02 144 (2020).[6] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J. Electrochem. Soc., 167 100511 (2020). Figure 1