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.

Compounds with layered structure mainly include LiCoO₂, LiNiO₂, LiCrO₂, Li₂MoO₃ and Li_0.7rMnO₂. 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 O^–₂ 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 Li_1/3Mn_2/3. Electrochemically active materials with this structure include Li₂ RuO₃, Li₂IrO₃, Li₂PtO₃ and Li_1.8 Ru0_0.6Fe_0.6O₃, 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 LiNi_1/2Mn_1/2 ,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 LiNi_1/2 Mn_1/2O₂– 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 LiNi_χ Mn_1-χO₂ (χ≥0.5).

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 LiCo_χ Ni_1-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, LiNin_0.5 Mn_0.5O₂, 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 LiNi_0.8Co_0.15 Al_0.05 O₂ or Li_1+χNi_yCO_zMn_1-y-zO_2+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. Li_0.7MnO₂ analogues are synthesized by ion exchange between Na_0.7MnO₂ analogues and lithium ions in molten lithium salts. Na_0.7MnO₂ has deposit defects in the oxygen ion layer, and these defects are partially inherited into Li_0.7MnO₂ after ion exchange.

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 Na_0.7MnO₂ 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-Li_0.7MnO₂ is made from P2-Na_0.7MnO₂ through ion exchange reaction; 03-Li_χMnO₂ is made from P3- or O3-Na_0.7MnO₂. O3-Li_χMnO₂ with R3m symmetric structure will be transformed into a spinel structure, but the structure of O2-Li_0.7MnO₂ will not change during charge and discharge cycles. Under the 2.5~4.0V voltage platform, the charge and discharge capacity of O2-Li_0.7MnO₂ 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, Li_0.7Mn_2/3M_1/3O₂ (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-LiCo_1/2Mn_1/2O₂) 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.