Dopant Redistribution, Phase Propagation, and Electrochemical Properties of Co-Free Layered Cathodes
Authors: Yang Zhijie , Mu Linqin , Lin Feng 
Published: July 2021

With the increasing demand for higher energy density in Li-ion battery applications, the development of Ni-based layered oxide cathodes has been focused on increasing the Ni content. However, higher Ni content gives rise to structural and thermal instability of the cathodes and potential safety concerns in applications. The battery community has made great efforts to achieve better battery performance, with manipulating the doping chemistry and engineering the microstructure being two of the most popular yet not well-understood methods.[1–6] Recent studies suggest that manipulating the dopant distribution is effective to improve cathode stability and battery performance.[1,7] However, the prediction of dopant distribution patterns primarily relies on thermodynamics-based theoretical calculations. Therefore, there is strong research need to understand the kinetic factors by experimentally investigating the dopant distribution, local chemical and structural changes and their interplay in the practical synthesis. In this presentation, we will first discuss our recent progress in probing dopant redistribution, phase propagation, and local chemical changes in the synthesis of layered oxide battery cathodes. Then we will show that these multiscale doping strategies enhance the electrochemical performance of Co-free layered cathodes by improving the cycling stability, rate capability, thermal stability, and self-discharge resistance.
We investigated the dopant distribution, phase propagation and local chemical changes in the synthesis of layered oxide cathodes at multiple length scales using multielement-doped LiNiO2 as a platform. We observed that dopants Mg and Ti diffuse dynamically from the surface to the bulk of cathode particles below 300 °C long before the formation of layered phase. After calcination, Ti is still enriched at the cathode particle surface while Mg has a relatively uniform distribution throughout cathode particles. We also observed that Ni oxidation completes earlier at the particle surface than in the bulk. Ni oxidation and phase propagation are heterogeneous at the mesoscale and exhibit in two forms- radial heterogeneity and mosaic-like heterogeneity, which vary with calcination conditions. Globally, phase transformation takes place from the initial mixed hydroxide phase to the layered phase through a metal oxide-liked intermediate phase. We probed the layered cathode synthesis at multiple length scales in single particle, as well as at ensemble-averaged electrode scale, which connects the local materials properties with the global electrochemical behavior. These findings enrich the understanding of multiscale layered cathode formation and can potentially inform the cathode design, such as dopant distribution and 3D chemical homogeneity.
Ultimately, the resulting materials deliver a material-level specific energy of ∼780 W h/kg at C/10 with 96% retention after 50 cycles, and ~680 Wh/kg with 77% retention after 300 cycles. The dopants reduce the Li/vacancy ordering and mitigate the irreversible phase transformations. Ni dissolution is suppressed due to enhanced interfacial stability. The multielement-doped LiNiO2 also shows higher thermal stability, increased self-discharge resistance, and enhanced Li+ kinetics. Our promising battery performance suggests that there is a large space to manipulate the three-dimensional distribution of dopants to improve the stability of LiNiO2.

References
[1] L. Mu, R. Zhang, W. H. Kan, Y. Zhang, L. Li, C. Kuai, B. Zydlewski, M. M. Rahman, C.-J. Sun, S. Sainio, M. Avdeev, D. Nordlund, H. L. Xin, F. Lin, Chem. Mater.
2019, 31, 9769.
[2] L. Mu, W. H. Kan, C. Kuai, Z. Yang, L. Li, C. J. Sun, S. Sainio, M. Avdeev, D. Nordlund, F. Lin, ACS Appl. Mater. Interfaces
2020, 12, 12874.
[3] J. D. Steiner, H. Cheng, J. Walsh, Y. Zhang, B. Zydlewski, L. Mu, Z. Xu, M. M. Rahman, H. Sun, F. M. Michel, C.-J. Sun, D. Nordlund, W. Luo, J.-C. Zheng, H. L. Xin, F. Lin, ACS Appl. Mater. Interfaces
2019, 11, 37885.
[4] Z. Yang, L. Mu, D. Hou, M. M. Rahman, Z. Xu, J. Liu, D. Nordlund, C. Sun, X. Xiao, F. Lin, Adv. Energy Mater.
2020, 2002719.
[5] Z. Xu, Z. Jiang, C. Kuai, R. Xu, C. Qin, Y. Zhang, M. M. Rahman, C. Wei, D. Nordlund, C. J. Sun, X. Xiao, X. W. Du, K. Zhao, P. Yan, Y. Liu, F. Lin, Nat. Commun.
2020, 11, 83.
[6] Z. Xu, D. Hou, D. J. Kautz, W. Liu, R. Xu, X. Xiao, F. Lin, Adv. Mater.
2020, 2003417.
[7] J.-N. Zhang, Q. Li, C. Ouyang, X. Yu, M. Ge, X. Huang, E. Hu, C. Ma, S. Li, R. Xiao, W. Yang, Y. Chu, Y. Liu, H. Yu, X.-Q. Yang, X. Huang, L. Chen, H. Li, Nat. Energy
2019, 4, 594.
Acknowledgements
The work is supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the award number: DE-EE0008444.