The structure of the cathode material

What is the structure of the cathode material? Next, the structure of the cathode material will be described in detail.

Most of the cathode materials of lithium-ion batteries have a layered structure, and lithium ions can diffuse in a two-dimensional space; or have a spinel structure, and lithium ions can diffuse in a three-stack space.

Figure 1.1 The relationship between the rhombohedral unit cell and the hexagonal unit cell
Figure 1.1 The relationship between the rhombohedral unit cell and the hexagonal unit cell

Compounds with layered structure mainly include LiCoO2, LiNiO2, LiCrO2, Li2MoO3 and Li0.7rMnO2. The first three compounds have a rhombohedral structure with symmetry in the R 3m space group. As shown in Figure 1.1, the rhombohedral unit cell has the same geometric characteristics as the three axes and the same angle between any two axes. Since this unit cell structure is difficult to visually describe, it is usually represented by a hexahedral structure with three times the volume of a single unit cell (as shown by the thick line in Figure 1.1). In this structure, Li or transition metal ions (M) are respectively located between the two O2 planes in the octahedral structure, thereby forming the Li layer and the M layer.

The structure of Li2MnO3 is slightly different from the above structure, that is, M ions are replaced by Li1/3Mn2/3. Electrochemically active materials with this structure include Li2 RuO3, Li2IrO3, Li2PtO3 and Li1.8 Ru00.6Fe0.6O3, but these compound crystals contain precious metals and are not suitable for practical applications. Many layered electrochemically active materials are made by doping lithium and other metal ions. These compounds can be divided into two categories: one is substituted products, and the other is solid solution compounds. The quasi-ternary phase diagram of LiNiO2-LiMnO2 -Li2MnO3 (see Figure 1.2) can be used to illustrate the difference between these two types of compounds, that is, the three corners of the triangle represent the pure phase, and the midpoint of the LiNiO2-LiMnO2 line shows the composition of LiNi1/2Mn1/2 ,through the XANES spectrum analysis of the K-band edge absorption of Ni and Mn, it is inferred that the valences of Ni and Mn are +2 and +4, respectively. Therefore, the reaction of 1/2Ni2+ + 1/2Mn4+ instead of Ni3+ On this straight line, all Ni3+ is consumed at the midpoint. On the basis of the LiNi02-Li2MnO3 and LiNi1/2 Mn1/2O2– Li2MnO3 components, it is easy to synthesize a series of single-phase products. These products are usually called “Soluble Solids”. The valences of all ions in the solid solution are equivalent to them. The values ​​of the two apex components in the range of all components, the shaded part in Figure 1.1 is the solid solution between Li2MnO3 and LiNiχ Mn1-χO2 (χ≥0.5).

Li2 MnO3-LiNixMn1-xO2 solid solution (shaded area) in the quasi-ternary phase diagram
Figure 1.2 Li2 MnO3-LiNixMn1-xO2 solid solution (shaded area) in the quasi-ternary phase diagram

Among the cathode materials for lithium-ion batteries, two types of solid solutions are well known. One type of its two apex components have electrochemical activity, such as LiCoχ Ni1-χO2, which is a solid solution made of LiCoO2 and LiNiO2, and the other type of which only one of the two apex components has electrochemical activity, such as LiNiO2– Li2MnO3 Solid solution. LiCoO2, LiNin0.5 Mn0.5O2, LiCrO2, LiMnO2 and LiFeO2 are electrochemically active apex components, while Li2MnO3, Li2TiO3 and LiAIO2 are electrochemically degradable apex components. Two different types of apex components are combined to form a variety of lithium-ion battery cathode materials, and the more complex combined solid solution formed by apex components is LiNi0.8Co0.15 Al0.05 O2 or Li1+χNiyCOzMn1-y-zO2+δ .

Among the manganese-based layered compounds, LiMnO2 with a layered zigzag orthorhombic crystal structure and layered LiMnO2 with R –3m symmetry are both electrochemically active materials. The latter is prepared by ion exchange method or liquid phase synthesis method. Both the orthorhombic crystal structure and the layered structure of LiMnO2 are unstable during the electrochemical deintercalation and intercalation of lithium ions, and will transform into a spinel structure when charging and discharging in the 3~4V range. The rhombohedral LiMnO2 synthesized at high temperature requires multiple charge and discharge cycles before it can be transformed into an electrochemically active spinel structure. However, when it is pulverized into nanoparticles, its output specific capacity at the first cycle is close to 200mA·h/g, and its high-temperature cycle performance is superior. Therefore, at this level, it can be practically used as a cathode material.

Using the ion exchange method, a series of stoichiometric lithium-deficient compounds can be synthesized. Li0.7MnO2 analogues are synthesized by ion exchange between Na0.7MnO2 analogues and lithium ions in molten lithium salts. Na0.7MnO2 has deposit defects in the oxygen ion layer, and these defects are partially inherited into Li0.7MnO2 after ion exchange.

The position of the oxygen atom in the CCP structure
Figure 1.3 The position of the oxygen atom in the CCP structure

Oxygen ions usually form a cubic structure or a close-packed hexagonal structure (HCP) in the metal oxide, and all ions are located in the pores of the tetrahedron or octahedron. In the close-packed cubic structure (CCP), the oxygen ion layer can be divided into three types (A, B, C) with different phases, and these layers are in accordance with (ABC). The patterns are neatly stacked (see Figure 1.3). The stacking mode of HCP uses (AB)n. Although the stacking of two oxygen layers in metal oxides is very typical, the stacking order of Na0.7MnO2 is different, which is (ABBA)n. Six oxygen ions form two different shapes of holes: octahedral and prismatic structures (triangular cylinders). Manganese ions are located at the position C in the octahedral structure (between the oxygen layers A and B), which are the same position on the two-dimensional plane of the oxygen layer C, and the sodium ion is located at the edge position between the A layer and the B layer.

O2-Li0.7MnO2 is made from P2-Na0.7MnO2 through ion exchange reaction; 03-LiχMnO2 is made from P3- or O3-Na0.7MnO2. O3-LiχMnO2 with R3m symmetric structure will be transformed into a spinel structure, but the structure of O2-Li0.7MnO2 will not change during charge and discharge cycles. Under the 2.5~4.0V voltage platform, the charge and discharge capacity of O2-Li0.7MnO2 can be as high as 150mA·h/g, and the content is about half in the 3V voltage range, so its energy density is slightly worse than that of the 4V material. Researchers are currently experimenting with the use of metals such as diamonds and nickel to partially replace manganese in compounds, some of which have a higher capacity than the original. However, because of their poor rate performance, they are currently only a research direction and cannot be widely used. For example, Li0.7Mn2/3M1/3O2 (M=Ni, Co) has only a small specific capacity at a low rate of C/20. 100mA·h/g.

Some research reports indicate that the cobalt-substituted compound (O3-LiCo1/2Mn1/2O2) prepared by the ion exchange method can be used as a 5V cathode material, but this material still has some problems, such as its high irreversible capacity.

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