Structure and electrochemical performance of LiCoO2 electrode material

Structure and electrochemical performance of LiCoO2 electrode material

The difference in the synthesis method creates LiCoO2. The different phases, although all embedded compounds, can be used as cathode materials for lithium-ion batteries, but the difference in crystal structure leads to differences in its electrochemical performance. The layered LiCoO2 (HT-LiCoO2) synthesized at high temperature has higher specific capacity, better cycle performance and safety, and is easier to prepare, thus becoming a large-scale cathode material for lithium-ion batteries currently used in production. Another phase of LiCoO2, the spinel LiCoO2 (LT-LiCoO2) synthesized at a lower temperature (400°C), has been commercialized due to its sharp-angled particles, low bulk density and controversial cycle performance. cold.

HT-LiCoO2 has a-NaFeO2 type crystal structure, R-3m space group, and belongs to the hexagonal crystal system. The trivalent cobalt occupies the 3a position of the octahedron, the lithium ion occupies the 3b position, and the oxygen ion occupies the 6c position. Lithium atoms, cobalt atoms, and oxygen atoms occupy three different positions of the octahedron and are cubic close-packed to form a layered structure. The oxygen atoms of the layered LiCoO2 are cubic close-packed (ABCABC.), and the drill atoms and lithium atoms are alternately arranged on the (111) crystal plane in an orderly manner. This orderly arrangement of the (111) crystal plane causes the lattice Slightly distorted to become a trigonal crystal system, so that the (111) plane becomes the (001) plane, and its space group is R-3m. This structure is called the CuPt structure.

There are three phase change processes in LiCoO2 charging and discharging. The main discharge platform is located near 3.94V, which corresponds to the coexistence of the lithium-rich H1 phase and the lithium-poor H2 phase. The two small platforms located at 4.05V and 4.17V correspond to the other two A phase transition process, its structure changes reversibly between the two phases of the trigonal crystal system and the monoclinic crystal element, and the cycle performance is better. Its actual capacity is about 156mA.h.g-1. When more than 0.5 lithium ions are released, due to C The deformation in the axial direction will cause the lattice constant to change drastically, the crystal lattice loses oxygen, and because the high-valence Co has strong oxidizing properties, it will cause the electrolyte to be oxidized, causing the structural stability and cycle performance of the material to decrease. Therefore, the charging cut-off voltage of LiCoO2 is usually limited to 4.20v in the commercial industry, and its actual specific capacity is only 140-150mA.h.g-1.

When LiCoO2/C batteries are charged and discharged, lithium ions can undergo reversible deintercalation/intercalation reactions in the plane where they are located.

When the battery is charged, part of the lithium ions in the positive electrode active material are separated from the LiCoO2 crystal lattice, and the lithium ions are inserted into the crystal lattice of the negative electrode active material C through the electrolyte to generate LixC6 compound. The negative electrode is in a lithium-rich state, and the positive electrode is in a lithium-poor state. At the same time, electrons are compensated from the positive electrode to the negative electrode through an external circuit to maintain the balance of charge; on the contrary, when discharging, lithium ions are extracted from LixC6 and embedded in the positive electrode lattice through the electrolyte. At the same time, electrons flow from the negative electrode to the positive electrode through the external circuit for charge compensation . The charging and discharging process is to repeat the above process continuously to realize the insertion and extraction of lithium ions in the positive and negative electrode materials. This charging and discharging mechanism is the “rocking chair” battery mechanism mentioned in the previous article. From the perspective of charge and discharge reactions, lithium ions themselves do not participate in the redox reaction during the migration process, so the lithium ion battery reaction is an ideal reversible reaction.

In order to understand the structural change of the material under the overcharged state and realize the LiCoO2 working at a high working potential, researchers have made a lot of efforts. In 1992, Dabn used the in-situ XRD method for the first time to study the structural changes of electrode materials during the charging process. The literature pointed out that during the charging process of LixCoO2 (x>0.75), as the lithium ions between the two main layers are released, the c-axis expands and the distance between the layers continues to expand. This is because as the lithium ion content decreases, the electrostatic attraction between the main laminate and the lithium ion decreases, and the interlayer repulsion increases. In 1994, Ohzuku et al. gave the XRD spectrum of LixCoO2 during charging from 3V (vs.Li+/Li) to 4.8V (vs.Li+/Li), and pointed out that LixCoO2 (x>0,75) is in the charging process. From the H1 phase to the H2 phase, but only the change of the unit cell parameters, it is still a hexagonal crystal system with an O3 structure. And when x=0.55, LixCoO2 transforms into monoclinic phase M, but the phase is unstable and quickly transforms to the hexagonal phase of O3 structure. As the charging continues, the O3 structure transforms into a new phase when x<0.25 . Subsequently, Amatuc-cilao et al. studied the XRD spectra of the charging and discharging process of LixCoO2 material in the voltage range of 3~5.2V (vs.Li+/Li), and they came to the same conclusion as Ohzuku and Ueda. In addition, they observed that O3-LixCoO2 was transformed into CoO2 and its structure was O1. Following the above discussion, Van and Ceder used the first law to calculate the phase diagram of LiCoO2. They found that in the charging process of LixCoO2 (o<x<0.5), there are two phases in addition to the O3 phase, namely the OI phase and the stag phase. And they provided XRD spectra of the two phases, which are similar to the results of Ohzuku and Ueda’s experiments. In short, during the charging process of LiCoO2, as the lithium ions are released, the material changes from O3 phase to M phase, and then quickly to O3 phase, and then further generates HI-3 phase, and finally generates limit structure OI phase. The change in the value of c is as follows: the value of c increases first, the laminate expands, then decreases, and the laminate collapses.