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Challenges of Synthesizing Ni-rich ‘Single Crystal’ Layered Oxide Cathodes for Lithium Ion Batteries

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Secondary particle cracking has been identified as a major failure mechanism in Ni rich NCMs (Li[NixCoyMnz]O2 with x ≥ 0.8; x+y+z=1) layered oxides as it allows surface-related degradation phenomena such as ongoing electrolyte decomposition on newly formed, highly reactive surface cycle after cycle. As opposed to their polycrystalline (PC) counterparts, ‘Single-crystal’ (SC) NCMs are comprised of separated micron-sized primary particles and are therefore promising candidates to mitigate this issue. In this work, a systematic study on the high temperature synthesis of SC-NCM811 is carried out. Lithium-to-transition metal ratio (Li/TM) and calcination temperature (Tcalc.) are investigated in terms of their impact on the crystal structure, particle morphology and electrochemical performance.

Regular spherical Ni0.8Co0.1Mn0.1(OH)2 precursors were synthesized by co precipitation using a Couette-Taylor Flow Reactor (CTFR). The rotational speed of the reactor was varied to obtain precursors of standard (D50 ≈ 8 μm) for PC-NCM811 and reduced size (D50 ≈ 4 μm) for SC-NCM811 synthesis. Tcalc. and Li/TM were systematically varied from 850 °C to 1050 °C and 1.05 to 1.25, respectively.

The particle morphology was analyzed by means of SEM. Higher calcination temperatures strongly increased the diameter of primary particles from the sub-micron range to several microns, while the Li/TM ratio only had a minor effect on size. While larger size and higher Li/TM ratio favored the separation of primary particles from each other, no satisfactory deagglomeration could be achieved. Separation of primary particles could only be achieved by an altered calcination protocol and a subsequent ball-milling step. The latter, however, damaged the morphology and led to amorphization of the material. Therefore, the as-prepared samples were used for further investigations. An analysis of powder X-ray diffraction (PXRD) data revealed that higher Tcalc. led to a dramatic aggravation of the Li/Ni disorder. The Li/Ni disorder had a direct negative impact on the electrochemical results. While the sample synthesized at 850 °C (Li/TM = 1.05) could match the specific discharge capacity of the reference PC NCM811 sample, samples synthesized at higher temperatures and equal Li/TM ratios showed a strongly reduced specific discharge capacity as well as worse C-rate performance.

In conclusion, this work highlights the challenges and potential issues for the synthesis of Ni-rich NCMs. Results reported herein show that the synthesis of SC-NCM811 is challenging as the Ni-rich material hardly tolerates the high calcination temperatures necessary for sufficient crystal growth (> 1 μm) without encountering an increase in Li/Ni disorder. Furthermore, particle agglomeration complicates the synthesis. Agglomerated particles, strictly speaking, cannot be characterized as ‘single-crystal’. As opposed to NCMs with lower Ni-content, Ni-rich SC-NCMs seem to require more sophisticated synthesis approaches.

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