The mass market penetration of electric vehicles demand lithium-ion batteries (LIBs) that deliver higher energy density, longer cycle life and better safety. [1] Layered lithium transition-metal oxides, particularly Ni-rich lithium nickel manganese cobalt oxides (LiNi_xMn_yCo_1-x-yO₂, x ≥ 0.5, referred to as NMC), are currently considered most promising cathode materials in meeting these requirements. The instability of NMC-based electrode surface, however, prevents them from stable high-voltage operation which is needed for achieving high energy density. [2,3] Insights into the mechanisms governing these instabilities are elusive due to the complex nature of these oxides.

Cathode materials are traditionally prepared by high-temperature ceramic methods which produce aggregated particles with significant inhomogeneity that can contribute to additional challenges in diagnostic studies and data interpretation. In our research, crystal samples with excellent uniformity and well-controlled physical properties are prepared. [4] For example, Figure 1 shows a series of NMC crystal samples with various morphologies. This provides a model platform to investigate the relationships among specific properties, surface chemistry and electrochemical performance without ambiguity. In this presentation, we show the specific role of chemical composition, elemental segregation, particle size and morphology on surface reactivity. Design strategies for better performing NMC cathode materials will also be discussed.

References

1. S.-T. Myung; F. Maglia; K.-J. Park; C. S. Yoon; P. Lamp; S.-J. Kim; Y.-K. Sun, ACS Energy Lett. 2017, 2, 196.
2. F. Schipper; E. M. Erickson; C. Erk; J.-Y. Shin; F. F. Chesneau; D. Aurbach, J. Electrochem. Soc. 2017, 164, A6220.
3. W. Li; B. Song; A. Manthiram, Chem. Soc. Rev. 2017, 46, 3006.
4. J. Zhu and G. Chen, J. Mater. Chem. A 2019, 7 (10), 5463.

Figure 1. SEM images of synthesized NMC crystal samples with four differet morphologies: a) octahedron, b) truncated-octahedron, c) polyhedron and d) platelet.