[Introduction]

Professor Pan Feng of the School of New Materials, Peking University Shenzhen Graduate School, through the first-principles calculation, found the role of “spin-electron super-exchange” between transition metal ions in the ternary layered cathode material. This work was completed by Professor Pan Feng of the New Materials College and Associate Professor Zheng Jiaxin, who supervised the master student Teng Gaozhen, Dr. Xin Chao, and the doctoral student Zhuo Zengqing. The research results were “Role of Superexchange Interaction on Tuning of Ni/Li Disordering in Layered Li (NixMnyCoz ) O2" for the title of J. Phys. Chem. Lett. on. Professor Yang Wanli from Berkeley National Laboratory participated in the experimental measurement and mechanism discussion of soft X-rays. The above work has been supported by the National Materials Genomics Major Project (2016YFB0700600), the National Natural Science Foundation of China (Nos. 21603007 and 51672012), and the Shenzhen Science and Technology Innovation Committee (Nos.JCYJ20150729111733470 and JCYJ20151015162256516).

[Graphic introduction]

Figure 1 Three-dimensional layered cathode material structure of lithium battery

(A) Layered Ternary lithium battery positive electrode material structure;

(b) Ni/Li inversion (TM) 6?O3?Ni?O3?Li(TM)5 structural motif;

(c) The ionic environment of the reverse Li in the transition metal layer.

Fig. 2 180° super-exchange interaction after ternary layered material Ni/Li reversed

【research content】

As a clean energy source, lithium-ion batteries are widely used in cutting-edge technology fields such as everyday electronic products, artificial intelligence, electric vehicles , and drones. The positive electrode material is the core part of the lithium ion battery, which directly determines the energy density, charge and discharge cycle performance, safety, cost and the like of the lithium battery . The positive electrode materials currently participating in the meaning are lithium iron phosphate (LiFePO4) and ternary layered material (Li(NixMnyCoz)O2), among which the ternary layered material has high energy density, and is a positive electrode material widely used in lithium ion batteries. (such as the cathode material used in Tesla electric vehicles), it is also the most widely studied material in the field of lithium-ion batteries. In-depth study of the correlation between structure and performance of such materials is not only important for industrial applications, but also lays a foundation for exploring and finding better cathode materials.

In such a layered material, the transition metal ion layer and the lithium layer are alternately arranged, spaced apart by an oxygen layer. It is found that the Ni/Li inversion is easy to occur in the ternary layered material (see Figure 1), which has an impact on its performance, such as the influence of lithium ion diffusion rate, capacity development and structural phase transition. It is pointed out that an appropriate amount of Ni/Li reverse position is beneficial to structural stability during electrochemical cycling. Therefore, the influence of Ni/Li reverse position on electrochemical performance and how to regulate Ni/Li reverse position have become an important topic of general concern and research. The traditional view is that the Ni/Li inversion is due to the similar ionic radius of Ni2+ and Li+, and Ni2+ is easily reversed to the position of Li(3b), but it is difficult to explain that Ni3+ is contained in the high Ni layered material, but Ni /Li reverse position is more likely to happen. Therefore, re-study and deep understanding of the mechanism behind it has the basic theory and industrial application significance.

Professor Pan Feng from Peking University Shenzhen Graduate School of New Materials, through the first-principles calculation, found the role of “spin-electron super-exchange” between transition metal ions in ternary layered cathode materials (two transition metals (TM) The spintronics interact as electrons through the electrons of the commonly linked oxygen atom (O) as a bridge, as shown in Figure 2, which plays a key role in the modulation of the Ni/Li inversion. After the Ni/Li is reversed, the reverse Ni2+ undergoes spin reversal, which forms a 180° super-exchange with the transition metal ions (Ni2+, Ni3+, Mn4+) of the transition metal layer. Since the 3d orbital of the reverse Ni2+ forms a strong σ bond with the 2p orbital of O2-, the 180° super-exchange effect is much stronger than the 90° super-exchange interaction in the original transition metal layer. In the 180° super-exchange interaction between the reverse Ni2+ and the transition metal ion transition metal ions, Ni2+-O2-Ni2+ is the strongest and Ni2+-O2-Co3+ is the weakest, so the Ni/Li reverse position is most likely to occur after the reverse position. It can form more linear Ni2+-O2-Ni2+ sites. This also explains why in the previous experiments, it was found that high Ni, especially the ternary layered material containing more Ni2+, contained more Ni/Li inversion, while in the "Ni=Mn" ternary layered cathode material. In the middle, Co can suppress the Ni/Li inversion. Based on the super-exchange interaction model, the group also found that in the layered material with high Ni-containing Ni2+/Ni3+ mixed valence state, Ni3+ preferentially reverses to the Li layer to form Ni2+, and spin reversal forms more linear Ni2+-O2. -Ni2+ super exchange effect. At the same time, due to the charge compensation, Co3+ near the original Ni3+ will change to Co4+. This is the first time in the world that the research group predicts the presence of Co4+ in the high Ni ternary layered material. The prediction was also confirmed by the synchrotron radiation soft X-ray absorption spectrum of the Berkeley National Laboratory. The above findings not only provide a good mechanism explanation for the long-term Ni/Li inversion phenomenon of the ternary layered positive electrode material, but also provide a controllable modulation of the anti-position defect of the ternary positive electrode material and the design of the new ternary material. Important clues, such as finding cheaper metal ions that can replace Co.