Is the ‘Single-Crystal’ Approach Truly Feasible for Ni-Rich Layered Oxides? – a Fair and Realistic Performance Comparison in Ncm||Graphite Coin Cells

Abstract

Secondary particle cracking induced by anisotropic volume change during cycling has been identified as a major failure mechanism of state-of-the-art Ni-rich layered oxide cathodes like Li[Ni x Co y Mn z ]O2 (x + y + z = 1, NCMxyz where x ≥ 0.8) as it allows surface-related degradation phenomena such as inactive phase formation, transition metal dissolution and ongoing electrolyte decomposition to continue on newly formed, highly reactive surface cycle after cycle.1 The approach of designing ‘single-crystal’ (SC) particles is believed to mitigate this issue by replacing polycrystalline secondary agglomerates (PC) by well-separated micron-sized primary particles that can prolong the cycle life of the material by improving its morphological integrity.2 Particle cracking in layered oxide cathode materials is exacerbated with increasing Ni-content which qualifies the ‘single-crystal’ approach especially for Ni-rich NCMs (x ≥ 0.8). Despite this, meaningful long-term cycling studies are still scarce as two main challenges obstruct a fair evaluation of the ‘single-crystal’ approach: The synthesis of Ni-rich SC-NCMs is not straightforward as the conditions required for enhanced crystal growth (e. higher calcination temperature and/or a molten-salt environment requiring a washing step) are inherently damaging to Ni-rich materials.3 This often leads to comparisons between materials where factors other than morphology cannot be excluded (e.g. synthesis from different precursors, different post-processing steps, undisclosed dopants/coatings in commercial materials, etc.). The larger crystal size of SC-NCMs compared to PC-NCMs is believed to cause kinetic limitations impacting achievable capacities and rate performance.4 As a consequence, either the ‘single-crystal’ sample is cycled in a lower state-of-charge window despite the same same upper cut-off voltage or the polycrystalline reference sample is cycled at an unnecessarily high voltage. Both conditions favor the ‘single-crystal’ material. The objective of this work was to establish a fair comparison between the two morphologies. For this purpose, a series of ‘single-crystal’ Li[Ni0.8Co0.1Mn0.1]O2 (SC-NCM811) materials with varying particle sizes where synthesized. To deconvolute the effect of particle size and morphology from other influences, a molten salt-assisted synthesis was followed, so that bulk properties of SC-NCMs remained constant and PC-NCM reference samples could be synthesized from the same home-made precursors and with the same post-processing steps. The samples were thoroughly characterized in terms of physicochemical properties and their electrochemical performance was evaluated in NCM||Li and NCM||Graphite coin cells. The performance of PC- and SC-NCM811 was compared at the same upper cutoff voltage and again at the same state-of-charge window to compare the materials as fairly as possible. References: (1) de Biasi, L.; Schwarz, B.; Brezesinski, T.; Hartmann, P.; Janek, J.; Ehrenberg, H. Chemical, Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni-Rich NCM and Li-Rich HE-NCM Cathode Materials in Li-Ion Batteries. Advanced Materials 2019, 31 (26). https://doi.org/10.1002/adma.201900985. (2) Zhao, W.; Zou, L.; Zhang, L.; Fan, X.; Zhang, H.; Pagani, F.; Brack, E.; Seidl, L.; Ou, X.; Egorov, K.; Guo, X.; Hu, G.; Trabesinger, S.; Wang, C.; Battaglia, C. Assessing Long‐Term Cycling Stability of Single‐Crystal Versus Polycrystalline Nickel‐Rich NCM in Pouch Cells with 6 MAh Cm −2 Electrodes. Small 2022, 2107357, 2107357. https://doi.org/10.1002/smll.202107357. (3) Langdon, J.; Manthiram, A. A Perspective on Single-Crystal Layered Oxide Cathodes for Lithium-Ion Batteries. Energy Storage Mater 2021, 37, 143–160. https://doi.org/10.1016/j.ensm.2021.02.003. (4) Ryu, H. H.; Namkoong, B.; Kim, J. H.; Belharouak, I.; Yoon, C. S.; Sun, Y. K. Capacity Fading Mechanisms in Ni-Rich Single-Crystal NCM Cathodes. ACS Energy Lett 2021, 6 (8), 2726–2734. https://doi.org/10.1021/acsenergylett.1c01089. Figure 1