Layered compound
The layered structure of lithium transition metal oxide LiMO2 has strong ionic characteristics and the most densely packed crystal structure. Oxygen ion has the largest ion radius among the three constituent element ions of lithium ion, transition metal ion and oxygen ion, and it first forms a densely packed layer. Then lithium ions and transition metal ions are filled in the gaps between oxygen ions, thus increasing the bulk density of the material. As shown in Figure 1, the closest packing of oxygen ions can be obtained by Hexagonal Close Packed (HCP) and Cubie Close Packed (CCP). Both stacked structures can achieve a packing density of 0.7405. In these two structures, the tetrahedral and octahedral gaps between oxygen ions are covered by 3d transition metals with ionic radius between 0.680 and 0.885A. When CN=6, Co3+: 0.685Å, Ni3+: 0.700Å, Mn3+: 0.720Å, Fe3+: 0.690Å) and potassium ions (0.900Å when CN=6; 0.730Å when CN=4).
If there are n oxygens in a unit cell, there are 2n tetrahedral positions and n octahedral positions (see Figure 2). Therefore, the layered LiMO2 has 4 tetrahedrons and 2 octahedral positions. Taking into account the geometric size of the cation, the tetrahedral position is occupied by ions with an ion radius ratio of 0.225≤r/R<0.414, while the octahedral position is occupied by ions with an ion radius ratio of 0.414≤r/R<0.732. occupy. Since the radius ratio of 3d transition metal ions is between 0.5397≤r(M3+)/R(02-)≤0.7024, and for lithium ions it is r(Li+)/R(02-)=0.7143, so they occupy LiMO₂ The 2 octahedral positions. Since the octahedron and the tetrahedron are very close to each other, it is difficult to further insert more lithium ions.
In addition, the common angle in the two-dimensional direction is not only conducive to the stability of the layered structure, but also improves the conductivity through direct M-M over-interaction, thus reducing the volume change during charging and discharging. Therefore, the compound represented by LiMO2. adopts a layered structure, and lithium, transition metals and oxygen are regularly arranged as O-Li-O-M-O-Li-O-M-O along the [111] plane of the rock salt structure. In other words, lithium ions and transition metal ions each occupy 50% of the octahedral position in the ABCABC face-centered cubic structure. Corresponding to three repeated MO2 layers in one unit cell, lithium occupies one octahedral position. Since O is usually used to represent lithium occupies one octahedral position, the commonly used O3 structure refers to three repeating units.
(When the tetrahedron is regarded as a triangular pyramid, the vertices T, and T, are in opposite directions)
The structure of the layered LiMO2 cathode material is shown in Figure 3.The metal oxide layer composed of transition metal and oxygen and the lithium oxygen octahedron are alternately arranged, and strong ionic bonds are formed in the MO2 layer. The Coulomb repulsion between the MO2 layers allows the insertion/extraction of lithium ions, and ion diffusion along the two-dimensional plane Produces high ion conductivity.
Lithium ions on the surface of the particles are released during the charging process, forming empty octahedral positions, so that adjacent lithium ions can be sequentially diffused and released. During discharge, lithium ions are inserted into empty octahedral positions on the surface of the particles. If lithium is to move between layers, it needs to pass MO, and the hollow tetrahedron position of the layer reaches the hollow octahedral position of another lithium layer. However, the tetrahedron and the transition metal octahedron in the MO2 layer are coplanar. Therefore, lithium ions are required to diffuse through the electrostatic repulsion zone, and higher activation energy is required, resulting in the inability of lithium ions to move between layers. Since the ion conductance is less than the electronic conductance during the initial discharge stage, lithium ions accumulate on the surface of the active material and need to be balanced by diffusion. This results in a potential difference between open circuit and closed circuit conditions during charging and discharging.
Lithium is released during the charging process, and the oxygen atoms in the MO2 layer repel each other, causing the lattice to expand. When the lithium is completely released, a significant change in the structure of the layered LiMO2 active material can be observed, that is, a significant contraction of the c-axis. The change in the lithium content during the charging process causes the positive electrode material to reconstruct and form a stable crystal structure. At this time, the active area of the material should be a single phase. Figure 4 shows the phase transition of lithium transition metal oxide during charging/discharging.
When the lithium content in LiCoO2 is less than 0.5, the structure will change from O3 type to P3 type.Figure 5 shows the structure of various forms of LiMO2 layered oxides, from which it can be seen that O3 and P3 are completely different structures, which are the structural changes caused by the reduction of the lithium content in LiCoO2. As shown in Figure 4, the lithium content has a much smaller effect on the O3 structure of Li[Ni,Mn]O2. It can be expected that the characteristics of this material will be maintained even if a higher charging voltage is applied.
