In recent years, with the development of electric vehicles and power grid energy storage demand, the research and development of rechargeable batteries with high energy density and high safety has become a frontier hotspot in the field of new energy. The most critical process of rechargeable and dischargable batteries in the work is the ion transport in the system and the related charge transfer, but the ion migration phenomenon is very complex, and researchers have conducted a lot of research on the ion transport phenomenon from the aspects of moving ion synergy, lattice spatial configuration and anionic charge.
In the 1980s and 1990s, researchers found that in addition to the above factors, while the movement ions migrate, the anionic groups in the lattice frame may also have an effect on the transport of moving ions through changes in local position, and summarized this phenomenon into two mechanisms: paddle wheel mechanism and percolation mechanism. The former believes that anion group rotation and cationic motion need to occur at the same time, while the latter believes that anion group rotation mainly provides a spatial channel for cationic movement. In recent years, with the emergence of borohydrides and antiperovskite materials, this phenomenon has been paid attention to by everyone, and the coupling mode of moving ions and anion groups has begun to be explored, the effect on ion migration and the specific image of the atomic level during transmission.
Recently, Under the guidance of Associate Researcher Xiao Ruijuan and Researcher Li Hong, Wu Siyuan, a doctoral student in the Clean Energy Laboratory of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, focused on the coupling mechanism between anionic groups and migrating ions and their modulation mechanism on the ion conductivity of the system, studied the motion laws of anionic groups with different symmetries, different components and spatial configurations, and constructed a physical model of the influence of anion group movement patterns on the transport potential and transport path of migrating ions.
The research work extracted the possible local structural characteristics and motion patterns of anion groups from the data of inorganic crystal compound structure and ion transport barrier accumulated in the previous period. The kinematic behavior of the anions found can be summarized into three types: local vibration, rotation, and diffusion of anions, as shown in Figure 1. For the case of local vibration of anions, the simple harmonic vibration model analysis can obtain the necessary conditions for promoting the cation motion, that is, the frequency of the two movements is close; for the rotation mode, the ion migration barrier is changed mainly by changing the shape of the potential energy surface of the cation in three-dimensional space.
Fig. 1 The bond valence method estimates the energy barrier of rotation and migration of various anionic groups in the material.
Based on the motion characteristics of the anion group obtained by the above analysis and its coupling mode with migrating ions, the authors judged that there was a vibration-based and rotation-based interaction in LiBF and LiBF crystals, respectively. Through first-principles molecular dynamics simulation, the characteristic quantities such as the change of B-F rotation angle and the change of Li-F distance during the diffusion of lithium ions were extracted, which confirmed that the small amplitude vibration or contraction of the bond of the [BF] group in LiBF would be conducive to the diffusion of lithium ions, while in LiBF, because of the large Li-Li spacing, the anion group needed to rotate in a larger amplitude to promote cation motion, as shown in Figure 2.
Fig. 2 Comparison of Li/[BF] coupling modes of Li/[BF] in LiBF and LiBF, respectively, and their influence on Li transport.
In this work, we develop a physical model for describing the coupling mechanism between anionic groups and migrating ions, explore the method of modulating the ion conductivity of solid materials by anionic groups, and provide ideas for designing new fast ion conductor materials for the next generation of rechargeable batteries based on this.
The research was published in journal of Materials Chemistry A 10 (2022) 3093 under the title "New insights into the mechanism of cation migration induced by cation–anion dynamic coupling in superionic conductors". This research was supported by the National Natural Science Foundation of China (52022106, 51772321).
Article link: https://doi.org/10.1039/D1TA09466A
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