Enhancing Sodium-Ion Batteries with High-Entropy Layered Cathode Materials | Nature Energy

Sodium-ion batteries (NIBs) are considered to be an ideal stationary energy storage solution due to their affordability and abundance of raw materials. The key to improving their performance lies in the development of efficient cathode materials. Layered transition metal oxides (Na_xTMO₂) are favored due to their high theoretical specific capacity and ease of scalable production, but they often encounter structural degradation and specific capacity decline during charge and discharge cycles. During the charge and discharge process of O₃-type Na_xTMO₂ cathode materials, the oxygen-oxygen repulsion caused by the deintercalation of sodium ions and the oxidation of active metal ions will lead to changes in lattice parameters and structural transformations, which in turn cause microcracks, oxygen loss, and cation migration, all of which will affect the durability of the material. In order to enhance the stability of the cathode material, it is necessary to construct a transition metal oxide layer that is both strong and flexible to provide ion and electron transmission channels.

High-entropy oxide cathode materials have shown excellent electrochemical performance, thermal stability, and structural stability through the synergistic effect of multiple metal ions, but there is currently a lack of systematic design principles. Although the capacity utilization and stability are balanced by moderately reducing the configurational entropy, it may cause lattice distortion or strain, which in turn affects the performance of the material. Current research shows that lattice distortion can significantly change the thermal and electrical conductivity of materials, but the impact on layered cathode structures has not been fully studied. Therefore, developing a design strategy that can regulate lattice distortion and stabilize the high-entropy phase is of great significance for promoting the development of high-entropy cathode materials .

Research content

A research team led by Prof. Yongsheng Hu, Prof. Yaxiang Lu, Prof. Dong Su from the Institute of Physics, Chinese Academy of Sciences, and Dr. Huican Mao from the University of Science and Technology Beijing published a research paper titled "Tailoring planar strain for robust structural stability in high-entropy layered sodium oxide cathode materials" in Nature Energy.

The study developed an all-3D transition metal O₃-type oxide cathode material NaNi_0.3Cu_0.1Fe_0.2Mn_0.3Ti_0.1O₂ (NCFMT), which exhibited improved reversible specific capacity and excellent cycling stability. Replacing Ti⁴⁺ with Sn⁴⁺ (NaNi_0.3Cu_0.1Fe_0.2Mn_0.3Ti_0.1O₂ ; NCFMS) resulted in poor structural reversibility and decreased cycling stability. The study found that the structural integrity of the layered cathode is affected by the compatibility of the constituent elements in the transition metal layer (TMO₂). In NCFMS, the plane strain caused by the displacement of metal ions can cause element separation and crack formation over multiple cycles. In contrast, NCFMT exhibits a robust structural framework for stable sodium ion storage due to the high mechanochemical compatibility between its constituent elements. This finding provides important insights into the design of high-performance layered high-entropy cathode materials.

Figure 1. Differences in atomic structure between NCFMT and NCFMS

Figure 2. Superior electrochemical performance of NCFMT compared to NCFMS

Figure 3. Structural characterization of NCFMS and NCFMT cathodes after cycling

Figure 4. Full cell performance of NCFMT//HC in different voltage ranges.

Summary

This study reveals the key role of high compatibility between constituent elements in improving structural reversibility and energy retention in high-entropy layered cathode materials. By comparing materials with different lattice microstrains, it is found that the significant planar microstrain in the NCFMS cathode is caused by the differences in atomic size, mass and valence electron configuration between Sn⁴⁺ and other 3d transition metal ions, which leads to metal ion migration, Sn separation and metal ion dissolution, and ultimately leads to cathode failure. NCFMT, on the other hand, exhibits excellent electrochemical performance and cycle stability, which is attributed to the high mechanochemical compatibility of its elements in the TMO₂ layer. The study emphasizes the importance of careful selection of elements for high-entropy cathode design, provides new ideas for optimizing high-entropy oxide cathode materials, and provides design guidance for the development of long-term stable commercial sodium-ion battery cathode materials.

Original link: Ding, F., Ji, P., Han, Z. et al. Tailoring planar strain for robust structural stability in high-entropy layered sodium oxide cathode materials. Nat Energy (2024). https://doi.org/10.1038/s41560-024-01616-5.

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