Driven by the increasing need of energy storage for electric vehicles and electronic devices, the technological advancements of lithium-ion batteries (LIBs) are urgently in demand to increase energy density while reducing cost. LiNi_xMn_yCo_1-x-yO₂ (NMC) is among the most promising low-cost, high-energy-density cathodes for next-generation LIBs.[i]^,[ii] NMC cathode materials are generally synthesized through high-temperature calcination of NMC and lithium hydroxides. Control of the calcination conditions has been found crucial to obtaining high-performance NMC cathodes, wherein structural ordering and morphology of the final products are finely tuned to maximize capacity while minimizing surface areas exposed to the electrolyte.[iii]^,[iv] However, making NMC cathode materials of desired structural properties has been challenging due to the complexity of the calcination process, involving phase transformation accompanied with non-equilibrium crystallization of intermediates prior to forming the layered oxides.[v]^,[vi]

In this work, a combination of in situ X-ray spectroscopy and microscopy techniques is applied to track the calcination process during synthesis of the practical system LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) and the archetypal LiNiO₂ (LNO). The reaction pathways and structural evolution of the involved phases were identified, revealing the strong compositional dependence of phase progression, structural ordering, and crystal growth during the calcination process. A strong correlation was found between the degree of lithiation in the rocksalt phase and the progression of the layered phase (see figure 1). In NMC811, the lithiation is facilitated by Co/Mn leading to the highly lithiated rocksalt and resulting in rapid layering at low temperatures. The nucleation of the layered phase is predominately driven by lithiation, while the growth process is kinetically limited by the sluggish Ni diffusion in LNO and further hindered in NMC811 due to the presence of Co/Mn. The findings from this study, with insights into crystallization thermodynamics and kinetics, may provide guidance to the design and synthesis of high-performance Ni-based NMC cathode materials.

[i] N. Nitta, F. Wu, J. T. Lee, and G. Yushin, Materials today, 2015, 18(5), 252-264.

[ii] M. S. Whittingham, Chemical reviews, 2014, 114(23), 11414-11443.

[iii] J. C. Garcia, J. Bareño, J. Yan, G. Chen, A. Hauser, J. R. Croy, H. Iddir, The Journal of Physical Chemistry C, 2017, 121(15), 8290-8299.

[iv] Y. Duan, L. Yang, M. J. Zhang, Z. Chen, J. Bai, K. Amine, F. Pan, F. Wang, Journal of Materials Chemistry A, 2019, 7(2), 513-519.

[v] M. Bianchini, F. Fauth, P. Hartmann, T. Brezesinski, J. Janek, Journal of materials Chemistry A, 2020, 8(4), 1808-1820.

[vi] F. Wang, J. Bai, Batteries & Supercaps, 2021. (https://doi.org/10.1002/batt.202100174)