School of New Materials, Peking University: Discovering how spin-electron superexchange interactions regulate lithium battery cathode materials

**[Introduction]** Professor Pan Feng from the School of New Materials at Peking University Shenzhen Graduate School conducted a study using first-principles calculations to explore the role of "spin-electron superexchange" between transition metal ions in ternary layered cathode materials. The research was carried out in collaboration with Associate Professor Zheng Jiaxin, who guided master student Teng Gaozhen, Dr. Xin Chao, and doctoral student Zhuo Zengqing. The findings were published under the title “Role of Superexchange Interaction on Tuning of Ni/Li Disordering in Layered Li(NixMnyCoz)O2” in *J. Phys. Chem. Lett.* Professor Yang Wanli from Berkeley National Laboratory contributed to the experimental measurements and mechanism discussions involving soft X-ray spectroscopy. This work was supported by several key projects, including the National Materials Genomics Major Project (2016YFB0700600), the National Natural Science Foundation of China (Grants 21603007 and 51672012), and the Shenzhen Science and Technology Innovation Committee (Grants JCYJ20150729111733470 and JCYJ20151015162256516). **[Graphic Introduction]** Figure 1: Three-dimensional structure of lithium battery layered cathode material.  (A) Structure of the layered ternary lithium battery positive electrode; (b) Ni/Li inversion motif (TM)6–O3–Ni–O3–Li(TM)5; (c) Ionic environment of reversed Li in the transition metal layer. Figure 2: 180° superexchange interaction after Ni/Li inversion in ternary layered material.  **[Research Content]** Lithium-ion batteries are essential clean energy sources used in various cutting-edge technologies, including consumer electronics, artificial intelligence, electric vehicles, and drones. As the core component of these batteries, the cathode material directly affects energy density, cycle life, safety, and cost. Among the most promising cathode materials, ternary layered oxides such as Li(NixMnyCoz)O2 stand out for their high energy density and widespread use—like in Tesla electric vehicles. Understanding the relationship between structure and performance is crucial for both industrial applications and future material development. In layered cathode materials, transition metal layers and lithium layers are alternated with oxygen in between. It has been observed that Ni/Li disorder is common in these structures, which can impact properties like ion diffusion, capacity retention, and structural stability. While some degree of disorder may enhance stability during cycling, controlling this phenomenon remains a key challenge. Traditional theories suggest that Ni²⁺ and Li⁺ have similar ionic radii, leading to disorder. However, this explanation falls short when considering high-Ni materials where Ni³⁺ is present but still exhibits more disorder. Therefore, re-examining the underlying mechanisms is essential for both theoretical and practical progress. Through first-principles calculations, Professor Pan Feng’s team uncovered the critical role of "spin-electron superexchange" between transition metal ions. This interaction occurs via oxygen atoms acting as a bridge, enabling strong magnetic coupling. After Ni/Li inversion, the reversed Ni²⁺ undergoes spin reversal and forms a 180° superexchange with nearby transition metals like Ni²⁺, Ni³⁺, and Mn⁴⁺. The strong σ bond between Ni²⁺’s 3d orbitals and O²⁻’s 2p orbitals enhances this effect significantly compared to the weaker 90° interactions in the original structure. Among the possible configurations, the Ni²⁺–O²⁻–Ni²⁺ pathway is the strongest, making Ni/Li inversion more likely. This explains why high-Ni materials exhibit greater disorder, while Co-containing systems tend to suppress it. Furthermore, the team found that in high-Ni materials with mixed valence states (Ni²⁺/Ni³⁺), Ni³⁺ tends to migrate into the Li layer, reducing to Ni²⁺ and enhancing the linear Ni²⁺–O²⁻–Ni²⁺ superexchange. Simultaneously, charge compensation leads to Co³⁺ transforming into Co⁴⁺. This was the first prediction of Co⁴⁺ in high-Ni ternary materials, later confirmed by soft X-ray absorption spectroscopy at Berkeley National Laboratory. These findings not only clarify the long-standing issue of Ni/Li disorder but also offer new insights for controlling defects and designing next-generation cathode materials. They open up possibilities for replacing costly elements like Co with more affordable alternatives, advancing the future of lithium-ion battery technology.

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