(Invited) Spatially Asymmetric Phase Separation of Li4Ti5O12 Visualized by Operando Transmission Electron Microscopy
Authors: Nomura Yuki 
Published: December 2024

The phase-transition kinetics of battery electrodes critically affect battery power output and durability. Li4Ti5O12 exhibits a two-phase reaction between delithiated Li4Ti5O12 and lithiated Li7Ti5O12 and shows high-rate capabilities despite its relatively low lithium diffusion coefficients in the bulk and energy barrier to phase separation1,2. The underlying mechanisms of phase transition and their contribution to high-rate performance remain unclear3–6. To elucidate the kinetics of phase transitions and Li-ion diffusivity, operando analyses are essential for understanding the non-equilibrium phase transition behavior during Li insertion and extraction.

In this study, we observe spatially asymmetric phase separation during the lithiation and delithiation processes using operando scanning transmission electron microscopy (STEM) coupled with electron energy-loss spectroscopy (EELS)7,8. Figure 1 shows the changes in Li distribution during Li insertion and extraction. Figure 1(a) shows an annular dark-field (ADF) STEM image of a Li4Ti5O12 particle. Colored lines are superimposed on the image to indicate the surface or interface categories. Figures 1(b)–(h) show the changes in the Li distribution in Li4Ti5O12 during Li insertion. A core–shell structure consisting of a delithiated Li4Ti5O12 core and a lithiated Li7Ti5O12 shell is formed, although the particles are not completely covered with the solid electrolyte. This is in contrast to the delithiation process (Figures 1(i)–(m)), where a core-shell structure is not observed. Surface ionic diffusion, which demonstrates intermediate diffusion kinetics for the delithiated Li4Ti5O12 and lithiated Li7Ti5O12 phases, is the main driving force for asymmetric phase separation. This phase transition and the difference in ionic diffusivity between the two phases result in asymmetric polarization during the charge and discharge measurements. In our experiment, preferential Li-ion movement along the grain boundaries is also identified. Our findings underscore the potential benefits of optimizing surface and grain boundary designs to enhance ionic diffusion in Li4Ti5O12.

References

(1) Schmidt, W.; Bottke, P.; Sternad, M.; Gollob, P.; Hennige, V.; Wilkening, M. Small Change—Great Effect: Steep Increase of Li Ion Dynamics in Li4Ti5O12 at the Early Stages of Chemical Li Insertion. Chem. Mater.
2015, 27 (5), 1740–1750.

(2) Zhang, W.; Seo, D.-H.; Chen, T.; Wu, L.; Topsakal, M.; Zhu, Y.; Lu, D.; Ceder, G.; Wang, F. Kinetic Pathways of Ionic Transport in Fast-Charging Lithium Titanate. Science
2020, 367 (6481), 1030–1034.

(3) Wagemaker, M.; Simon, D. R.; Kelder, E. M.; Schoonman, J.; Ringpfeil, C.; Haake, U.; Lützenkirchen-Hecht, D.; Frahm, R.; Mulder, F. M. A Kinetic Two-Phase and Equilibrium Solid Solution in Spinel Li4+xTi5O12. Adv. Mater.
2006, 18 (23), 3169–3173.

(4) Wagemaker, M.; van Eck, E. R. H.; Kentgens, A. P. M.; Mulder, F. M. Li-Ion Diffusion in the Equilibrium Nanomorphology of Spinel Li4+xTi5O12. J. Phys. Chem. B
2009, 113 (1), 224–230.

(5) Schmidt, W.; Wilkening, M. Discriminating the Mobile Ions from the Immobile Ones in Li4+xTi5O12: 6Li NMR Reveals the Main Li+ Diffusion Pathway and Proposes a Refined Lithiation Mechanism. J. Phys. Chem. C
2016, 120 (21), 11372–11381.

(6) Ganapathy, S.; Vasileiadis, A.; Heringa, J. R.; Wagemaker, M. The Fine Line between a Two-Phase and Solid-Solution Phase Transformation and Highly Mobile Phase Interfaces in Spinel Li4+xTi5O12. Adv. Energy Mater.
2017, 7 (9), 1601781.

(7) Nomura, Y.; Yamamoto, K.; Yamagishi, Y.; Igaki, E. Lithium Transport Pathways Guided by Grain Architectures in Ni-Rich Layered Cathodes. ACS Nano
2021, 15 (12), 19806–19814.

(8) Nomura, Y.; Yamamoto, K.; Hirayama, T. Visualizing Asymmetric Phase Separation Driven by Surface Ionic Diffusion in Lithium Titanate. J. Mater. Chem. A
2023, 11 (43), 23243.

Acknowledgements

This study was supported by the Innovative Science and Technology Initiative for Security (Grant Number JPJ004596) of the Acquisition, Technology, and Logistics Agency of Japan. The research was also supported by Grants-in-Aid for Scientific Research KAKENHI (19H05814, 23K13837, 23H00241, and 23H01858) from the Japan Society for the Promotion of Science and by the Kao Foundation for Arts and Sciences and the Murata Science Foundation.

Figure 1