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

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 ]O 2 ( x + y + z = 1, NCM xyz 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[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 (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