Microstructural Refinement and Surface Modification of Ni-Rich Cathodes for High-Performance Li-Ion Battery

Li[Ni1–x–y–zCoxMnyAlz]O2 (NCMA) cathodes have attracted public attention owing to their improved durability by leveraging the advantages of NCM and NCA cathodes. The combination of Mn and Al in a NCMA cathode effectively stabilizes the host layered structure to increase its resistance to microcracking; consequently, NCMA cathodes outperform both NCM and NCA cathodes with the same Ni contents, in terms of cycle life, without sacrificing energy density. However, as the Ni contents of the cathodes exceed 90%, a simple, compositionally engineered NCMA cathode lacks the chemical and mechanical stability to suppress surface degradation and the development of permanent microcracks due to the high incidence of highly reactive Ni4+ species on its surface.1,2 Considering the limitations of NCMA cathodes, a combination strategy involving microstructural engineering to inhibit microcracking, by enabling the efficient dissipation of local strain,3 and the modification of the exposed cathode surface, to protect it against deleterious electrolyte attack, is expected to afford superior Ni-rich layered cathodes. In this study, we improved the cycling stability of a conventional NCMA cathode with a Ni content of 93% (a NCMA93 cathode) using a combination strategy involving microstructural engineering and surface modification. To this end, we prepared a Sb-doped NCMA93 (denoted as Sb-NCMA93) cathode; Sb was incorporated to refine its microstructure and radially align and elongate its primary particles. Furthermore, we generated a protective F layer on the cathode surface through a chemical reaction between residual lithium compounds and a NH4F coating agent. To comprehensively investigate the synergetic effect of this combination strategy, conventional and Sb-doped NCMA93 cathodes with and without F coatings were synthesized and characterized and their fundamental properties and electrochemical performances compared. Post-mortem analyses were performed to elucidate the origins of the remarkable long-term cycling stability of the optimized cathodes. Reference s : [1] H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources, 2013, 233, 121. [2] H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155. [3] U.-H. Kim, H.-H. Ryu, J.-H. Kim, R. Mücke, P. Kaghazchi, C. S. Yoon, Y.-K. Sun, Adv. Energy Mater. 2019, 9, 1803902.