Is the ‘single-crystal’ approach really feasible for Ni-rich layered oxides? – A fair and realistic performance comparison in NCM||Graphite coin cells

Is the ‘single-crystal’ approach really 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_xCo_yMn_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 (i.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_0.8Co_0.1Mn_0.1]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.