Formation mechanism and influence of directional phase domain in battery cathode materials

On October 24, 2022, the team of Professor Chen Qian and Professor Zuo Jianmin of the University of Illinois at Urbana-Champaign published a new study entitled “Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes” in the journal Nature Materials.

This result reports that although the cathode material of the battery has uniform chemical composition in the solid-solution phase transition, the microstructure will have different crystal orientations. The heterogeneity of crystal orientation leads to mechanical strain and grain structure in the material, which further affects the ion diffusion coefficient in the cathode material by more than 10 times. The associative imaging technique of transmission electron microscopy is used for the first time to analyze the composition and structure of the battery cathode material.

The corresponding authors of the paper are Chen Qian and Zuo Jianmin; The first author is Chen Wenxiang.

Understanding changes in the microstructure of crystals is essential for the development and advancement of materials engineering. The fine microstructure, or “directional phase domain,” formed by the arrangement of phases in a specific direction, controls the physical and mechanical properties of the material. When the symmetry of a substance changes in a phase transition, the phase domain in a particular direction is widely present in the phase change solid. The microstructure in the material directly determines the properties of the material, such as the strengthening of nanoprecipitates in aluminum or other alloys, the shape memory of twin martensite in shape memory alloys, and the switching of orientation in ferroelectrics. Changes in the microstructure are also present in several widely used battery cathode materials. Cathode materials involve a large number of phase transitions and symmetry changes in electrochemical cycles, such as cubic to tetragonal phase transitions in spinel structures, rhombosis-monoclinic transitions in layered oxides, and spinel structures formed in layered oxides. However, the nature of the phase domain and its effect on the electrochemical properties of cathode materials are not well known, although recent studies have shown the presence of ordered phase structures in layered electrode and spinel cathode materials through theoretical calculations, electron diffraction, and high-resolution electron microscopy. Previous electrochemical phase transition studies have focused on the inhomogeneity of nanoscale electrochemical components in cathode materials. Research methods include X-ray microscopy with synchrotron radiation and analytical transmission electron microscopy. This compositional heterogeneity is mainly due to diffusion or reaction-limited mechanisms. In contrast, structural heterogeneity associated with changes in symmetry received little attention.

Recently, the team of Professor Chen Qian and Professor Zuo Jianmin of the University of Illinois at Urbana-Champaign used four-dimensional scanning electron microscopy (4D-STEM) and electron energy loss spectroscopy (EELS) imaging techniques in spherical aberration corrected electron microscope to observe the directional phase domain structure formed by the battery cathode material (λ-MnO2) in the solid-solution phase transition. The strain gradient generated by the heterogeneity of the phase domain structure has a tenfold effect on the ion diffusion coefficient in the material by a factor of more than ten years.

Figure 1: The cathode material of the battery has a relatively uniform chemical composition in the solution phase transition, but the microstructure has different crystal orientation and mechanical strain. (Image source: Nature Materials)

Morphology and origin of the directional phase domain in the cathode material of the battery.Spinel structure λ-MnO2 nanoparticles have an electrode capacity of up to 273mAh/g as a cathode material for magnesium-ion batteries. During discharge, the embedding of Mg2+ causes a phase transition of the spinel structure from cubic to tetragonal. Here, imaging techniques associated with 4D-STEM and EELS are used to study phase transition processes. The results showed that at the end of the discharge, the embedded Mg2+ maintained a relatively uniform chemical distribution in the spinel nanoparticles, but they caused uneven mechanical strain. More in-depth data mining techniques revealed that the mechanical strain came from the directional phase domain structure generated during the Mg2+ embedding process. The directional phase domain structure has similar chemical composition but different grain orientation. This directional phase domain structure is due to the loss of symmetry of the substance during the phase transition.

Figure 2: Morphology and origin of the directional phase domain in the cathode material of the battery.(Image source: Nature Materials)

Formation process and mechanism of directional phase domain in battery cathode material.The imaging results during the Mg2+ embedding process show that the number of directional phase domains per unit area in the cathode material first increases and then decreases during the discharge process. The area occupied by the directional phase domain in the cathode material is constantly increasing. The results show that the directional phase domain structure follows the formation process of nucleation, growth and merging in phase transitions. Further spatial correlation analysis showed that there may be elastic interactions between phase domains, and that it favors the formation of phase domains in the same direction near the phase domain.

Figure 3: Formation of the directional phase domain of the battery cathode material during discharge.(Image source: Nature Materials)

Effect of directional phase domain on ion diffusion coefficient.The use of different electrolytes during the discharge process will cause the directional phase domain structure in the cathode material to form different morphologies (“island” topography and “archipelago topography”). The selection of different electrolytes during discharge can facilitate or hinder the phase domain merging process. In addition, large strain gradients usually occur at the boundaries of the directional phase domain. The strain gradient in the material changes with the topography of the phase domain. The distribution of the strain gradient causes the ion diffusion coefficient in the material to change tenfold or more.

Figure 4: Directional phase domains and strain gradients formed in cathode materials under different electrolytes and their effects on ion diffusion coefficients.(Image source: Nature Materials)

The results of this study provide important insights into the formation mechanism of the microstructure of cathode materials, and elaborate the influence of microstructure on the ion intercalation process, which provides ideas for the development of new energy storage materials. (Source: Web of Science)

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